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SEPTEMBER 2026 ISSN 1030-2662 09 9 771030 266001 $ 00* NZ $1590 The VERY BEST DIY Projects! 15 SEMICONDUCTOR INC GST INC GST ANALYSER Identifies and tests diodes, transistors & thyristors Measures forward & reverse voltage, Vbe, hfe, Vgs etc Improvised Electronics and DIY Components Battery BackPack Provides an uninterrupted power source in place of two AA cells, such as in our GPS Clock Driver from 2022 (shown connected to it) Commodore PET Diagnosing a vintage computer The Display System www.jaycar.com.au Contents Vol.39, No.09 September 2026 12 Improvised Electronics, Part 1 Hobbyists can fabricate valves, transistors and even simple integrated circuits in their own home. So let’s take a look at how people can make basic electronic parts and even advanced electronic components. By Dr David Maddison, VK3DSM DIY components 48 How Induction Motors Work Improvised Electronics Part 1: Page 12 Commodore PET Page 58 Induction motors are used to power many devices such as lathes, fans, pool pumps and more. In this short article we describe how they work and what differences exist between the various subtypes. By Andrew Levido Electric motors 58 The Commodore PET Display It is very difficult to troubleshoot these types of older computers due to the number of logic ICs involved. To make things easier, this article will explore how the Commodore PET’s video hardware works in detail. By Dr Hugo Holden Vintage computers 90 Braybon Bros Voltage Regulator This automatic voltage regulator from the 1940s was used to control alternators to produce a mostly constant AC output voltage. With some original parts on hand, I decided to make a replica. By Fred Lever Vintage electronics 28 Semiconductor Analyser This analyser automatically identifies most two- or three-terminal discrete semiconductor devices (diodes, bipolar transistors, JFETs, Mosfets, IGBTs and thyristors). It automatically measures and displays critical parameters. By Andrew Levido Test & measurement project 52 Stereo FM Transmitter With frequency, gain and output power controls, this low-power, digitallycontrolled FM transmitter is a compact and inexpensive device. It can be powered by one to three AA or rechargeable cells. By Charles Kosina, VK3BAR Radio transmitter project 70 Phenomenal Pinball Machine The Display System Battery BackPack for GPS Clocks and more Page 78 2 Editorial Viewpoint 4 Mailbag 45 Circuit Notebook 83 Subscriptions 84 Serviceman’s Log 1. Remote control tester 2. Bluetooth selfie camera 3. Magnetic power switch This series explains how to design and build every part of your own Pinball Machine. This month we cover building and testing the flippers, bumpers, kickers, ball return mechanism and associated parts. Part 4 by Phil Prosser Gaming project 98 Online Shop 100 Ask Silicon Chip 78 Battery BackPack for GPS Clocks 101 Silicon Chip Kits 103 Market Centre 104 Advertising Index 104 Notes & Errata While you might think this project can only be used for clocks, by providing a regulated output from a rechargeable lithium-ion battery, it can be used whenever you need an uninterrupted low-voltage power source. By Tim Blythman Battery project SILICON SILIC CHIP www.siliconchip.com.au Publisher/Editor Nicholas Vinen Technical Editor John Clarke – B.E.(Elec.) Technical Staff Bao Smith – B.Sc. Tim Blythman – B.E., B.Sc. Advertising Enquiries (02) 9939 3295 adverts<at>siliconchip.com.au Regular Contributors Allan Linton-Smith Dave Thompson David Maddison – B.App.Sc. (Hons 1), PhD, Grad.Dip.Entr.Innov. Geoff Graham Associate Professor Graham Parslow Dr Hugo Holden – B.H.B, MB.ChB., FRANZCO Ian Batty – M.Ed. Phil Prosser – B.Sc., B.E.(Elec.) Cartoonist Louis Decrevel loueee.com Founding Editor (retired) Leo Simpson – B.Bus., FAICD Silicon Chip is published 12 times a year by Silicon Chip Publications Pty Ltd. ACN 626 922 870. ABN 20 880 526 923. All material is copyright ©. No part of this publication may be reproduced without the written consent of the publisher. Subscription rates (Australia only) 6 issues (6 months): $77.50 12 issues (1 year): $145 24 issues (2 years): $270 Online subscription (Worldwide) 6 issues (6 months): $55 12 issues (1 year): $105 24 issues (2 years): $200 For overseas rates, see our website or email silicon<at>siliconchip.com.au * recommended & maximum price only Postal address: PO Box 194, Matraville, NSW 2036. Phone: (02) 9939 3295. ISSN: 1030-2662 Printing and Distribution: 1 Huntingwood Dr, Huntingwood NSW 2148 54 Park St, Sydney NSW 2000 2 Silicon Chip Editorial Viewpoint Crystals: more than meets the eye Similar to ICs, we tend to drop crystals into a circuit and expect them to ‘just work’, without realising the amount of engineering involved. When they were first introduced, crystal resonators were expensive devices; today’s high-precision, low-cost crystals are the culmination of a huge amount of research and manufacturing investment. You may be aware that quartz is a piezoelectric material, meaning that when a voltage is applied to it, it changes shape slightly. Similarly, if you apply mechanical force to a quartz crystal, it generates a small voltage. Essentially, quartz acts as an electromechanical transducer. This property is used in piezo buzzers and force/pressure/acceleration sensors as well as crystal resonators. In use, a quartz crystal resonator acts as a very high-Q mechanical resonator, excited by the surrounding electrical circuit. Very few other materials can do this and remain stable in the long term. At the crystal’s natural resonant frequencies, the motional impedance drops drastically, so the electromechanical conversion becomes extremely efficient. Quartz has very low mechanical damping, minimal dislocation mobility, low internal friction in shear modes, and a stable crystalline lattice with very few slip systems. This means that once energy is put into a shear vibration, the lattice does not readily convert it to heat. Most other common solids dissipate orders of magnitude more energy per cycle. For example, a quartz resonator can have a Q value in the range of 104 to 106, while most metals, ceramics and glass operating as mechanical resonators typically have Q values in the range of 102 to 104. The natural resonant frequency of a crystal fragment depends on its size, thickness and the way it is cut relative to the crystalline structure. A crystal can operate in multiple vibration modes: shear, flexural, tuning-fork mode and others, each resonating over a different frequency range. At resonance, the motional reactances cancel, reducing the impedance of the crystal to a low value, often just a few tens of ohms. Off-resonance, the impedance changes rapidly. This is why a crystal oscillator locks so tightly onto one frequency: the crystal’s mechanical resonance dominates the feedback loop. Creating a modern crystal resonator starts with synthetic quartz grown slowly using hydrothermal processes in autoclaves, rather than by melting (as is used for growing silicon crystals in semiconductor manufacturing). The crystalline structure is analysed, then cuts are made at specific angles to create different crystal types (AT-cut, BT-cut etc). The plates are polished to extremely precise thickness, cut into precisely sized and shaped pieces, and electrodes are added with minimal stress to avoid altering the crystal’s behaviour. They are then mounted on tiny flexible supports at vibration nodes and hermetically sealed in a can to reduce ageing. A crystal actually has two closely spaced resonant frequencies. At its series-resonant frequency, its impedance falls to a minimum. Slightly above this is its parallel or anti-resonant frequency, where the crystal’s motional components interact with its electrode and package capacitance to produce a very high impedance. Many oscillator circuits operate between these points, at a frequency determined partly by the external load capacitance. This is why crystals are specified with a particular load capacitance and using the wrong capacitors can shift the frequency. All this work goes into producing a precision device that you can buy for tens of cents each in volume. So next time you use a crystal, consider the effort and technology that went into it behaving predictably and operating seamlessly in your circuit. by Nicholas Vinen Australia's electronics magazine siliconchip.com.au MAILBAG your feedback Letters and emails should contain complete name, address and daytime phone number. Letters to the Editor are submitted on the condition that Silicon Chip Publications Pty Ltd has the right to edit, reproduce in electronic form, and communicate these letters. This also applies to submissions to “Ask Silicon Chip”, “Circuit Notebook” and “Serviceman’s Log”. Breadboarding with a microcontroller module Regarding the question on using the RP2350 Development Board on a breadboard (June 2026, page 100), the photograph below shows what I did to one of mine many years ago. I cut one section down the middle and moved it across to suit the need at the time. It does limit the use for normal breadboarding on the remaining part of the modified section, but it is a large board. The hardest part, if I recall correctly, was prying the required section off the base and then sticking it down again. Brian Playne, Toowoomba, Qld. Picking up radio waves from the Earth I was prompted to write to you about the Earth Radio project in the December 2025 & January 2026 issues (siliconchip.au/Series/454). For over 40 years, I’ve been monitoring the Earth as well, but slightly differently. I monitor the DC voltage from the Earth to the ionosphere. This is about 500V/m but you need high-impedance gear to detect it. Its frequency is from DC to a few tens of hertz. I started using a 6U7 radio valve, sitting at ±800V DC, with the output from the cathode fed into a surplus paper chart recorder operating at 25mm per minute. I then obtained a digital recorder that could have the speed programmed to a more reasonable 25mm per hour. I still use the same instrument, but use a DATAQ data logger and notebook PC running Windows XP. Monitoring the DC conditions, one can see both positive and negative lightning discharges as well as the buildup to severe weather, meteor showers (which ionise the atmosphere) and even earthquakes (detecting the piezo effect as the rocks let go). On extreme sensitivity, people and animals can also be detected walking around as we disturb the field. The sense wire is about 20m long, 10m high and supported at each end with ceramic insulators to keep its impedance high, matching the very high input impedance of the 6U7 (which is wired as a triode). To prevent secondary emission, the filament is under-run at 5V DC. The instrument has been in continuous operation for over 40 years, survived many storms and several direct hits. The lightning arrester is a spark plug mounted on the aerial input, and in severe storms it is fascinating to see the fireworks from the gap. The fine weather potential of 500V/m is also monitored and can give hours, or sometimes days, of warning that a storm is building. I’m sure there exists better technology in semiconductors these days that can handle such voltages, but 40+ years ago, to an impoverished electrical engineer, an old radio valve from 1944 was it! Dick Powell, via email. Confusion over Pico vs Pico 2 .uf2 files I have assembled the March 2025 Pico 2 Audio Analyser project (siliconchip.au/Article/17795). All voltages mentioned in the article appear correct, including the 3.3V regulator output on the OLED screen (MOD2) and the lines to/from the Raspberry Pi Pico module. When I download the UF2 file to the Pico, it appeared in the root directory but disappeared after power cycling the Raspberry Pi. However, nothing ever appeared on the OLED. I’m not sure if this suggests it was not correctly booting from the UF2 and therefore not sending anything to the OLED to display. 4 Silicon Chip Australia's electronics magazine siliconchip.com.au I eventually sorted the problem out – I hadn’t realised there were two different versions of the project and was using the UF2 file from 2023. Having updated with the correct firmware, as you can see in the photo above, it is all working fine. If any other customers struggle to get the ‘Jiffy box’ (they don’t appear to exist in the UK), drop me a line. I have a Bambu X1C and created my own case with all the slots and holes printed as part of the design. This means you don’t need to cut anything out afterwards. I have also printed a case for the Pico Gamer, having ‘sliced off’ the embossed print. If someone is using a plastics-­based printer, rather than resin, they will have problems with that embossed print as it is on the bottom of the base (ie, the surface on the print bed). My case is identical but without that embossed print, so it will print fine. I’m happy to share the STL/STEP files with any interested readers. Paul Gamble, Ashford Hill, UK. Clarification over title for previous letter Thank you for publishing my letter in the Mailbag section of the June 2026 issue. I was dismayed by the title you had put over it, “Transistors should be matched on Vbe not hfe”. That could be interpreted to mean I recommend against matching for hfe. However, that is not what I wrote, and I do not recommend it. I would appreciate it if you published a clarification on this point. Ideally, both parameters should be matched, although doing so could be difficult and tedious. Matching the hfe values of the pair of input transistors in an audio amplifier reduces input current imbalance and hence the offset voltage that appears at the output, which is permanently impressed on the loudspeakers. I checked Douglas Self’s “Audio Power Amplifier Design Handbook” and “Self on Audio” and every circuit I saw of a differential front end displayed lower input resistances and a lower operating current than specified for the Calliope. Reducing these parameters’ values reduces the offset voltage. This offset voltage can be minimised or eliminated by design; for example, by specifying transistors with a high hfe, including an offset null control, or an input DC voltage cancelling circuit. There is no advantage I can see in matching the hfe of the current mirror transistors. A Vbe mismatch increases the distortion at a point in the circuit where negative feedback is least capable of limiting it. It also contributes to the offset voltage, as does any 6 Silicon Chip current mirror Vbe difference. The distortion caused cannot be reduced with an offset control. I think it is unlikely that many hobbyists have access to equipment that will easily measure hfe, whereas Vbe can be directly read using the diode range, which is a feature of just about every DMM these days. I purchased 10 BC558 and 10 BC549 small-signal transistors from Jaycar and measured their parameters with a Peak Atlas DCA75 transistor tester. I also used the diode range of my U1242C multimeter to check the Vbe values. The Vbes varied over a range of 48mV, so the likely spread of values is going to be significantly more than your generous assessment, and for this aspect, I think one should consider the worst case. In summary, both hfe and Vbe in the input transistors, and the current mirror transistors’ Vbe should be matched. It is surprising that few designs include the simple solution of specifying matched pairs for the input and current mirror transistors. They are not cheap, but their contribution to the overall cost of the whole amplifier would be small. The effects of hfe mismatch can be reduced by design without the need for matching; the effects of Vbe mismatch cannot (except by using matched pairs). Phil Denniss, Darlington NSW Comment: regarding the ability to measure a transistor’s hfe, we have an ideal project in this issue that can do that and more (starting on page 28). It also measures Vbe, so it is perfect for performing the type of transistor matching that Phil is describing in his letter. It’s available as a kit and is easy to assemble and program, with the PCB being supplied with almost all the components pre-soldered and the microcontroller pre-programmed (it’s also easy to reprogram as it has a USB bootloader). We like the idea of using matched pairs of transistors, and we did so in the Ultra-LD Mk.4 amplifier from the July-­ October 2015 issues (siliconchip.au/Series/289). Unfortunately, the pairs we used (HN3A51F/HN3C51F), while being ideal in many ways, were discontinued shortly after. That’s one of the reasons we think many designers avoid them. Contrast that with the BC556, which has been available since Adam was a boy. Another alternative for USB Power Monitor regulator I have found a suitable alternative voltage regulator for the Simple USB Power Monitor (June 2026 issue; siliconchip.au/Article/20365) that is better than the original MIC5233 in every way, including operating voltage and price. I have tested the Diotec LDI54-3.3EN (DigiKey Cat 4878-LDI54-3.3ENTR-ND) at $0.44 (unit pricing). It is rated to handle 45V at its input, increasing the power measurement range. I don’t know if other customers have experienced problems with the original regulator, but this worked immediately on both boards I built. Harry Kranendonk, Mount Evelyn, Vic. Comment: see the Notes & Errata on page 104 for another alternative that has been tested and found to work well. The MIC5233 regulator has been troublesome, with multiple faulty units reported. Suggestion for repairing solar cells Here’s a quick note that your readers might find helpful. In the Serviceman’s Log section (July 2026; siliconchip.au/ Australia's electronics magazine siliconchip.com.au ATEM Mini Pro The compact television studio that lets you create presentation videos and live streams! Now you don’t need to use a webcam for important presentations or workshops. ATEM Mini is a tiny video switcher that’s similar to the professional gear broadcasters use to create television shows! Simply plug in multiple cameras and a computer for your slides, then cut between them at the push of a button! It even has a built in streaming engine so you can live stream to popular social media platforms without any extra streaming software required! Live Stream to a Global Audience Easy to Learn and Use Includes Free ATEM Software Control Panel There’s never been a solution that’s professional but also easy to use. Simply press any of the input buttons on the front panel to cut between video sources. You can select from exciting transitions such as dissolve, or more dramatic effects such as dip to color, DVE squeeze and DVE push. You can even add a DVE for picture in picture effects with customized graphics. ATEM Mini is a full broadcast television switcher, so it has hidden power that’s unlocked using the free ATEM Software Control app. This means if you want to go further, you can start using features such as chroma keying for green screens, media players for graphics and the multiview for monitoring all cameras on a single monitor. There’s even a professional audio mixer! Use Any Software that Supports a USB Webcam You can use any video software with ATEM Mini Pro because the USB connection will emulate a webcam! The software thinks the ATEM Mini is a common webcam, but it’s really a live production switcher. That guarantees full compatibility with any video software and in full resolution 1080HD quality. Imagine doing a presentation from a professional broadcast studio to software such as Zoom or Microsoft Teams! www.blackmagicdesign.com/au ATEM Mini Pro has a built in hardware streaming engine for live streaming to a global audience! That means you can live stream training and presentation videos direct to colleagues all over the world in better video quality with smoother motion. Streaming uses the Ethernet connection to the internet, or you can even connect a smartphone to use mobile data! ATEM Mini Pro .................................................................$475 ATEM Mini Pro ISO......................................................$795 ATEM Mini Extreme ISO G2 ........................$3,109 Learn More! Article/20469), a contributor made some repairs to numerous solar lights. I’m not sure if he had any problems with the solar cells themselves; I’ve repaired solar cells in the past – the problem I find is the plastic coating they have goes cloudy with exposure to UV light from the sun. Luckily, it’s only a thin outer layer that goes cloudy, so the solution is fairly quick: sand it with fine wet and dry (eg, 1000 grit) sandpaper, then clean it with a rag dipped in methylated spirits, and finally give it a quick coating of automotive clear coat. The clear coat includes UV protection; without that, it will go cloudy again very quickly. It works like a charm and lasts for a few years. You can do the same thing to the plastic lenses of car headlights when they go cloudy, too. D. T., Sylvania, NSW. Simple LC Meter works well My Simple LC Meter kit (May 2026 issue; siliconchip. au/Article/20235) arrived yesterday safely and is up and going. Beautiful board, many thanks. Bob Grant, St Helens, Tas. Other reasons for smartphone wastage In reference to the editorial in the July 2026 issue on “Looming smartphone obsolescence”, it’s not only the lack of operating system updates and apps only supporting newer operating systems that is causing smartphones to become useless junk. When 3G was turned off, thousands of phones no longer worked. We had a perfectly good, functioning iPhone 5S that we had to replace because it did not support LTE. I’d already replaced my iPhone 4S with an iPhone SE 3rd generation phone, so we used my wife’s iPhone 6S to replace the 5S, and we got her a new iPhone SE 3rd generation phone. The SE phones can run the latest iOS, being a budget iPhone 13, so they should be OK for quite a few years. I wonder how long the iPhone 6S will still be functional before it too needs to be replaced. It never ends! Bruce Pierson, Dundathu, Qld. Comment: you are right. This problem was mentioned in the January 2025 editorial (“As expected, the 3G shutdown was messy”). iPhones appear to be, in general, supported longer than Android phones, although there is significant variation in the length of Android support from manufacturers too. Alternative uses for obsolete smartphones I am writing regarding your July editorial (siliconchip. au/Article/20447). I agree that the lack of vendor support means that perfectly serviceable smartphones get relegated to e-waste. Unfortunately, vendors have no incentive to extend support because obsoleting old phones forces people to buy new phones. On the positive side, in some cases, it’s possible to unlock the Android bootloader (I’m not sure about iPhones) and install an alternative operating system. The difficulty is finding an alternative. Maybe Silicon Chip could do some projects that use old phones. Whilst the phone will no longer get security updates, apps will continue to work unless they are Back Issues The UK ’s Circui t Pr Electractical onics fT sTa r TE r premier electron ics so and Underst Surgery using anding and gyrato rs Make computin g ma ker ma Mrom Finishin h Mic E Th g gazine light con the PicoMite ite E ‘sP La troller T’ aT Audio softwar smart e Designi Out sw iTc h- on discreteng a practic ! al audio op am a Mi p GPS-Sy Analog nchronise ue Cloc d k it wit Ta 6- d Ec WIN M mTo ICRO A CH u Dev ch AR IP elo 10 adE pm 00 LLionPico r Es Kit ent PrEc MisE VaLU ite smis Ta n c Es Tolight E b ox finECont art YoUr TUnErolle dEsiG r ns WIN! Microch ip Inte grat Graphic ed sE s and Touch M CuriTE osity sT – Pa luation bUEva iLd Kit an rT 3 d Us siL ic E oU on ch r ULTiM Ec kE aT E r Jump start Mini LE Driver D Egg Tim er – eg to pe gcellen rfectio t brea n! kf ast, tim Compl PLUS! APRIL 13 Cover.in dd 1 ractica lelec practic ed etin g WideTechno range the Tal inter Cool Bea k – My tru face, Ohmmet th, you ne r truth techn Net Wo ns – Arduin er and AI o tal t work, cir o Boo rk – Rou k, pic ters, pow tcamp: new n’ mix cuit Surg www.e bo er sup Sep 202 ery, re lectron plies, TEM ards update 3 £5.9 adout, publish 9 ! U and ing.co 09 more m 9 7726 <at>p 32 5730 30 alelec APRIL 201 3 £4.40 tronics Practical Electronics is the UK’s premier electronics, computing & maker magazine. Each month has a wide variety of electronics projects suiting beginners and experts alike. It also includes many different features on topics like audio, radio, computers and more. 14/02/2 013 10:33:4 7 24 YEAR COLLECTION OF PRACTICAL ELECTRONICS Every issue of Practical Electronics published from January 2000 to December 2023. Covering 288 magazines & over 20,000 pages of content; see siliconchip.com.au/Shop/3 PDF Download SC7650 ▸ $165 The articles can be downloaded per month, year or the entire block. Some of the excellent columns of Practical Electronics include Audio Out, Circuit Surgery, Techno Talk and more. The download size is approximately 5.4GB. 8 Silicon Chip PDFs on USB SC7645 ▸ $180 plus postage cost Supplied on a 32GB Silicon Chip branded USB flash drive. Purchasing this also gives you access to the download version shown opposite. Australia's electronics magazine siliconchip.com.au EXCLUSIVE TO $4,099 SAVE $366 USE CODE: SIC2608 PORTABLE CNC FOR EVERYONE Order Code: P8990 The ArcDroid CNC plasma cutter delivers precise, automated metal cutting in a compact, user-friendly package. Compatible with many plasma cutters, it produces clean, accurate cuts on steel, stainless steel, and aluminum. Ideal for DIY fabricators, and small workshops, ArcDroid makes professional-quality CNC plasma cutting simple, efficient, and affordable. BONUS Laser Trace Stylus Order Code: P8992 BONUS Marker Holder BONUS Machine Dust Cover Order Code: P8993 Order Code: P8995 (08) 9373 9999 ADELAIDE MOVING* BRISBANE 4 Abbotts Rd, Dandenong 11 Valentine St, Kewdale 11/20 Cheltenham Pde, Woodville 625 Boundary Rd, Coopers Plains MELBOURNE 1/2 Windsor Rd, Northmead (02) 9890 9111 siliconchip.com.au (03) 9212 4422 PERTH SYDNEY (08) 9373 9969 Australia's electronics magazine Specifications & Prices are subject to change without notification. (07) 3715 2200 September 2026  9 NEW 07_SIC_240826 View and purchase these items online: www.machineryhouse.com.au/SIC2608 ADDRESS *OUR BRISBANE BRANCH IS MOVING TO 680 BOUNDARY ROAD, RICHLANDS, 4077 (8.8KMS AWAY) FROM LATE SEPTEMBER updated to use functionality not available in the base Android (or iOS). The combination of a relatively powerful CPU with a good amount of RAM and a reasonably high-resolution touchscreen sounds like a great basis for a user interface to control something. Peter Jeremy, Killarney Heights, NSW. Comment: we thought about mentioning custom ROMs in the editorial as a way to get around this problem but concluded that most smartphone users would not have the knowledge or desire to go through the process. We were also running out of space. Unfortunately, many Android phones these days have locked bootloaders, but for those that don’t, installing something like Graphene OS would be a great idea. Obsolete smartphones do have a variety of uses: as a portable music player (no network connection required), a video playback device, as a WiFi camera on a LAN behind a firewall etc. It seems that Google is doing its best to make it difficult for hobbyists to install software on Android phones, limiting their possible uses in our projects, but it’s something we keep in mind. LED globes are not as reliable as promised LED globes are promoted as having a lifespan of 10 years and 15,000 hours plus, and were supposed to outlast incandescent globes by a long way. However, in reality, LED globes have a much shorter lifespan than incandescent globes. Many years ago, when we used to use incandescent globes, we rarely had to change the globes, which lasted around 10 years or more. Then came CFL globes, which were promoted as long-lasting, but they only lasted a very short time before expiring. Supposedly, LED globes would be the best ever. But I replaced an LED globe in a standing lamp just over a year ago, and I have estimated that it would have only lasted around 2000 hours. The globe was in the open with no shade or enclosure, so it could not have overheated. It’s not like this globe was some unknown cheap garbage; it was an Osram globe, which should be good quality. So much for “new improved technology”. Bruce Pierson, Dundathu, Qld. Comment: it’s true that some LED globes can fall far short of their rated lifespan, perhaps due to poor design or lower-­quality components, especially electrolytic capacitors. However, others can meet or exceed their ratings. Perhaps you should try another brand, such as Philips. Osram is generally reputable, but the quality and longevity may vary between models. Our experience is different from yours. While we’ve had a few LED globes fail prematurely, others are still working after being used daily for ten years, often for many hours per day. So a long lifespan is certainly possible if LED globes are properly designed and manufactured. Smart Media Card giveaway If anybody wants an unopened 128MB Smart Media Card, I’m willing to send it for the cost of postage within Australia. Ric Mabury, Melville, WA. Comment: if anyone is interested, email us and we’ll SC pass it on. 10 Silicon Chip Australia's electronics magazine siliconchip.com.au Improvised Electronics and DIY Components Part 1 by Dr David Maddison, VK3DSM Image source: www.pexels.com/photo/tools-on-a-desktop-7286026 During World War II, prisoners of war scavenged parts such as razor blades and lengths of wire to build clandestine radios. These allowed them to hear news from home and keep track of the progress of the war. These days, hobbyists in their garages fabricate electronic components such as valves, transistors and even simple integrated circuits. T hese activities fall into the category of improvised electronics. In the hobbyist context, improvised devices are made from scratch, from repurposed parts or from limited resources for fun, education or survival. Sometimes it’s simply a thought experiment: how would you make today’s electronic components if commercial parts weren’t available? Who knows — one day we might need to rebuild society from scratch! Improvised electronics showcases human ingenuity, democratises technology and teaches fundamental electronics principles. It can even have survival value in certain situations. Following the pioneers When the pioneers of electricity and electronics (Faraday, Maxwell, Edison, Marconi etc) made their groundbreaking discoveries and inventions, there were no commercial electronic 12 Silicon Chip components available. They were forced to fabricate everything from raw materials – the same challenge faced by modern-day experimenters and improvisers. The fundamental components needed to make almost any electronic circuit are resistors, capacitors, inductors, diodes, transistors or valves, switches and transformers. Some sort of power supply is also needed, such as a solar panel or battery, along with wire and a means of connecting the components together. Most of these components and techniques can be improvised at a basic level. It wasn’t just those early pioneers who used such techniques; from the 1910s through the 1920s, and for many years afterwards, people built crystal radios. Unlike the expensive valve radios that became available from the 1920s, a crystal set could be built with just a coil of wire, a capacitor, a crystal Australia's electronics magazine detector (often galena) and earphones. In the 1920s, newspapers, magazines and the US Bureau of Standards published guides explaining how to build them. They were also widely built for educational purposes. In Australia, The Sunday Mail (Brisbane) published plans for the “Mystery” crystal set on 3rd July 1932 (see Fig.1). Another early Australian crystal radio is shown in Fig.2. The purpose of this article is not so much instructional but to demonstrate what is possible in the absence of commercially available components (for fun, education or survival). We will look at how these components can be made with relatively simple materials and tools. If you wish to make any of these, you will need to obtain detailed instructions from books or online sources, formulate your plans and apply appropriate safety precautions. Some of the siliconchip.com.au chemicals or techniques involved can be hazardous. This article will concentrate on basic components and techniques. The second and final part, published next month, will look at more advanced DIY techniques, such as IC fabrication. Basic components Basic electronic components that can be made at home include antennas, cells & batteries, capacitors, motors, diodes, sound transducers, inductors, LEDs, magnets, magnetic cores, memristors, resistors, solder, transistors, switches, wire and tunnelling diodes. They can be improvised as follows. Antennas Improvised antennas can be made from a length of wire, say 10-20m, for basic reception. For lower frequencies where more inductance is needed, the wire can be wound around a core (we’ll look at DIY inductors and cores later). Using the improvised magnetic core material described later, something like a ferrite rod antenna is possible. The core should be 10-12cm long and 1-2cm in diameter, then wound with about 30-100 turns of insulated wire (0.32-0.64mm in diameter). This can be coupled with a variable capacitor of around 150-500pF, or a fixed capacitor to receive a specific AM broadcast band frequency (by trial and error if necessary). This creates an LC tank circuit. More rods can be bundled for greater sensitivity. More turns are used for lower frequencies, or fewer for higher frequencies. Thicker wire can also be used, reducing the resistance and improving the Q-factor (improving selectivity by rejecting nearby stations but making precise tuning more difficult). Such tuned antennas can be used for crystal radios. If building an improvised low-power transceiver, a magnetic loop antenna can be very useful to obtain good gain and directivity without needing to construct a complex Yagi or erect a tall mast (although a straight wire antenna can also be used). A small magnetic loop offers reasonable efficiency and sharp nulls for rejecting interference. Peter Parker, VK3YE, has videos on making these at: • https://youtu.be/cg1AXQb3VGM • https://youtu.be/Cv_RnLpZ9gw • https://youtu.be/gRRtlarJnKg Capacitors Capacitors consist of two conductive plates separated by an insulating dielectric material. The simplest possible improvised capacitor uses two pieces of aluminium foil with a sheet of paper, plastic kitchen wrap (cling film), waxed paper, or a thin plastic sheet as the dielectric. Capacitance increases with a larger plate area, closer plate spacing and higher dielectric constant (waxed paper or plastic is better than dry paper). Capacitance is given by the formula C = (εA)/d where A is the area of the plates, d is the distance between them, and ε is the absolute permittivity of the insulator. For improvised radios or tuned circuits, the foil-and-paper or plastic wrap version is simplest, but a variable capacitor will be needed for tuning a radio as well. A simple variable capacitor can be made using overlapping Fig.3: some improvised paper capacitors. Source: www. instructables.com/PaperCapacitor aluminium plates or copper-­laminated fibreglass (ie, PCB substrate) separated by a thin insulator (possibly air). The capacitance is adjusted by changing the amount of overlapping surface area. It is also possible to use two cylindrical pieces of metal (eg, a bolt and a metal tube) with a plastic tube separator between them. Another improvised variable capacitor is essentially two pieces of timber hinged on one side, with their inner surfaces covered in aluminium foil. The gap is adjusted to alter the capacitance. For a crystal radio to tune into the AM broadcast band, a capacitor value between about 150pF to 500pF is needed, depending on the value of the air coil or inductor and the frequency to be tuned. This range works well with common improvised air-core inductors. For example, 77 turns of wire on a 10cm-long, 5cm-diameter former gives Fig.1: the “Mystery” crystal set published in The Sunday Mail, Brisbane, 3rd of July 1932. No newspaper would publish such a thing today! Source: https://trove.nla.gov.au/ newspaper/page/10204138 Fig.2: an early Australian crystal set, circa 1925, made by Fred Smithson in Melbourne. Source: https:// collections.museumsvictoria.com.au/ items/404398 siliconchip.com.au Australia's electronics magazine September 2026  13 Fig.4: after making the ‘sandwich’ for a commercial electrolytic capacitor, the foil is wound into a cylinder and inserted into a can. Fig.5: a Leyden jar capacitor. Source: www.instructables.com/ Leyden-Jar-1 Fig.6: a cross-section of one layer of an electrolytic capacitor. A thin layer of aluminium oxide is formed on the anode, which acts as the dielectric layer. The conductive electrolyte allows electrons to flow between the cathode right up against that oxide layer, maximising capacitance per area. Fig.7: charging a homemade supercapacitor. Source: https://youtu.be/9DZtpBa_GnA 14 Silicon Chip Australia's electronics magazine a 66µH inductance. The resonant frequency formula is f =1 ÷ (2π√LC). With 66µH and 150-500pF, the tuneable range is 876-1600kHz. Other improvised capacitor types are as follows. A common variation of the aluminium foil capacitor is to roll the assembly tightly into a cylinder (rolled capacitor) instead of keeping it flat. This increases capacitance in a compact space and it’s how commercial electrolytic capacitors are made. See Fig.3, Fig.4 and the video at https:// youtu.be/n1hP3HKje3k A Leyden jar is a classic design using a glass jar or bottle (some have used plastic bottles) as the dielectric, with aluminium foil (or copper tape/foil) coating the inside and outside surfaces – see Fig.5. The jar can be partially filled with water or a salt solution to improve contact. It can store a much higher voltage and charge than foil-­paper versions. Capacitors can be made from a length of coaxial cable in which the inner conductor and outer shield are used as the plates, with the plastic insulation as the dielectric. They are suitable for HF and VHF applications. For higher capacitance in a compact size, an improvised electrolytic capacitor can be made using aluminium foil plates, a formed oxide layer on one of the plates, a liquid electrolyte like a saturated solution of baking soda in distilled water, and a porous separator like paper towel or coffee filter (plus a container and wire terminals) – see Fig.6. Once the capacitor is assembled as either a layered or rolled structure, the oxide dielectric has to be formed. The positive terminal of a DC supply is connected to one plate, the anode, and the negative to the other plate (the actual cathode is the electrolyte). The current drops as the oxide layer ‘forms’ over several minutes. There is already a natural oxide layer on aluminium, but this enhances it. The capacitor is then ready to use. Work in a well-ventilated area, as hydrogen and oxygen gases can be produced during forming. Use a low current (limit with a resistor if needed) and avoid short circuits or overvoltage. Supercapacitors can also be improvised. The electrodes can be made from copper, nickel, stainless steel, carbon, or titanium coated with a high-surface-area material like activated carbon or charcoal, separated siliconchip.com.au by a porous material (such as a coffee filter or porous plastic). The porous material is soaked in an electrolyte like lemon juice, salt water, sodium hydroxide (drain cleaner, NaOH), potassium hydroxide (KOH) or dilute sulfuric acid (H2SO4). This creates an electrochemical double-layer capacitor with significant energy storage for a homemade device. A homemade supercapacitor is shown being charged in Fig.7. It uses copper electrodes, a paper separator and NaOH, which is OK for shortterm use, but over the longer term, the copper will corrode and the paper will degrade. The author of the video claims a capacitance of 200F – see the video https://youtu.be/9DZtpBa_GnA Such capacitors can only be charged up to about 1.5V, as water decomposition occurs above that. Strong electrolytes (NaOH, KOH or H2SO4) must be handled with extreme care as they are corrosive and can cause burns. Safer options are sodium sulfate (Na2SO4) or Epsom salts (MgSO4). Some selected capacitance values are shown in Table 1. Carbon rods Carbon rods can be extracted from zinc-carbon (non-alkaline) “heavy duty” D cells or non-alkaline square 6V lantern batteries – see Fig.8. They are useful for a variety of applications in improvised electronics, such as: • electrolysis (eg, splitting water into H2 and O2) • producing intense arcs when connected to a high-current source like a car battery or arc-welding transformer • resistive heating elements • the positive electrode in seawater electrodes, paired with a zinc negative electrode • adjustable resistors • electrical contacts for commutators etc Cells and batteries A battery is a group of cells, so if you can make cells, you can combine them to make batteries. Cell types you can make include: A lemon or potato cell. A piece of copper (from an old coin or copper pipe) and a zinc-coated piece of metal is inserted into a lemon, potato or other acidic fruit or vegetable. The acid acts as an electrolyte. These produce low voltages, are short-lived and mainly useful for science demonstrations. siliconchip.com.au Table 1 – capacitance ranges for improvised capacitors Type Capacitance Notes Coaxial cable 60-100pF/m 50W cables give more capacitance than 75W 500mL Leyden jar 1nF 2L PET bottle filled with salt water 8nF A4 aluminium plates separated by paper 1.1nF; 20nF if rolled into a cylinder Electrolytic using baking soda & water electrolyte 1-100µF Supercapacitor using KOH 1-5F for simple activated Don’t exceed 1.2V-1.5V; charcoal type; up to high ESR 250F Spiral construction gives higher capacitance Fig.8: carbon rods retrieved by the author from non-alkaline D cells and a 6V square lantern battery, which internally had four F cells. Fig.9: an improvised lead-acid battery. Source: https://youtu. be/6VzLatndrPU Alessandro Volta made a copper-­ zinc and saltwater cell into a battery, like a voltaic pile, in 1800. You need an alternating stack of copper and zinc pieces, separated by paper or cardboard soaked in salt water, vinegar or lemon juice as the electrolyte. Copper can be derived from old coins or pipes, while zinc or galvanised steel can be used for the anode. These cells require frequent rewetting. An aluminium-charcoal and saltwater cell is made from aluminium foil or scrap as one electrode and crushed activated charcoal (such as the charcoal from a campfire or BBQ) for the other. Salt water is the electrolyte. These batteries have a long shelf life and are activated only when salt water is added. Commercial versions are used on life rafts. A copper-iron and vinegar cell can be made from copper scrap and steel nails, with vinegar or lemon juice as the electrolyte. They are weak and short-lived. The Daniell Cell is a classic 19th-century improvised cell made with a copper electrode in a copper sulfate solution, a zinc electrode in a zinc sulfate or saltwater solution and both electrodes separated by a porous barrier such as unglazed pottery or Plaster of Paris. Copper sulfate can be improvised by placing copper electrodes in sulfuric acid and passing a current between them. Lead-acid cells can be improvised from lead plates, with one lead plate treated to have a lead dioxide coating, plus separators and a sulfuric acid electrolyte. Access to sulfuric acid is restricted in Australia (even though pretty much everyone owns one or more lead-acid batteries filled with it). The following videos show how to make various kinds of leadacid batteries: Australia's electronics magazine September 2026  15 Table 2 – improvised cells/batteries (voltages are approximate) Cell type Anode Cathode Electrolyte Voltage Current Lemon/ Potato Zinc Copper Acidic fruit/ vegetable 0.7-0.9V up to a few mA Salt water Zinc * Copper Salt water 0.8-1.0V up to tens of mA Aluminium- Aluminium Charcoal charcoal Salt water 0.9-1.2V up to ~100mA Copperiron acid Iron Vinegar 0.6-0.8V up to a few mA Copper Sulfate solutions ~1.1V up to hundreds of mA Copper Daniell Cell Zinc Lead-acid Lead Lead Sulfuric acid ~2.1V fully dioxide charged * galvanised steel can be substituted but the voltage will be lower • https://youtu.be/0zQYJJz89Wo • https://youtu.be/tzyUda3upHI • https://youtu.be/pWO1aNUykq4 • https://youtu.be/6VzLatndrPU (see Fig.9) • https://youtu.be/5B8hL_UNUpk Alum (aluminium sulfate) solution is a much safer electrolyte, but gives a battery with different characteristics and much lower capacity. Lithium batteries can also be improvised. An aluminium electrode is coated with lithium manganese oxide and a copper electrode is coated with graphite. The electrodes are immersed in an organic electrolyte, and a plastic separator is placed between the electrodes. This is shown in the YouTube video https://youtu.be/VZ9KcGG-cps You might have some problems getting the chemicals for this one, and they are hazardous. They would therefore not be suitable in a survival situation. This type of battery is best left for a professional laboratory environment, but we included it to show what can be done. The non-rechargeable batteries mentioned above should work straight away, but rechargeable types need some method of charging them. Key characteristics of the above batteries are summarised in Table 2. DC motors One of the hardest things to improvise is a strong permanent magnet, although we will later describe how to improvise weaker magnets. Strong magnets are required for a high-­performance DC electric motor, although no-magnet DC motor can be made by substituting a fixed magnet with an electromagnet (field windings) powered by the same power supply as the armature (rotor), as shown in Fig.10. This design is known as a woundfield or series-wound motor, and was common in early electric motors. It is still used in many DIY and improvised builds. Diodes There are two main types of diode construction: the earlier point-­contact type (using a metal wire pressed against a crystal) and the more modern junction type (a P-N junction formed within a single semiconductor crystal). The point-contact type can be Fig.10: examples of improvised DC motors that don’t require permanent magnets. Source: https://youtu.be/tBQLGrXohL8 16 Silicon Chip amps improvised with basic materials like a semiconductor crystal and a sharp metal wire (‘cat’s whisker’). These improvised devices are often fragile and have low efficiency, but they do work. For the crystal, galena was common in the past, but silicon, germanium and carborundum are also suitable – see Table 3. A rusty or blued razor blade can function as a crude diode because its surface develops a thin layer of iron oxide that acts as a semiconductor. When a graphite or metal point is brought into contact with this layer, it forms a rudimentary schottky barrier junction that allows current to flow in only one direction. This rectification process can demodulate radio waves, extracting the audio-frequency modulation and converting it into a varying signal that can drive a headphone or earpiece to produce sound. The oxide layer is uneven, so finding the ‘sweet spot’ with the contact point is essential for good performance (see siliconchip.au/ link/acbk for more details). A cuprous oxide (Cu2O) diode can be fabricated by cleaning a piece of copper and heating it to cherry red for 30-60 minutes. A black copper oxide (CuO) insulating layer will grow. Let it cool slowly, then scrape or sand off the black layer to reveal the red/ maroon Cu2O layer underneath. This is a p-type semiconductor material. Make a point contact with a needle or use a blob of solder to make contact (cathode, −) with the red surface and make a connection to the metal base (anode, +). Expect a forward voltage drop of 0.2-0.5V and low current handling. An improvised electrolytic ‘wet’ diode can be made by submerging an aluminium or copper plate (anode, +; aluminium is better) and a lead plate (cathode, –) into a solution of baking soda (sodium bicarbonate) or borax Fig.11: an improvised high-impedance piezoelectric earpiece. Source: https://youtu.be/dWkKFD_yZs8 Australia's electronics magazine siliconchip.com.au Fig.12: a matchbox microphone. Source: www.instructables.com/ Matchbox-Microphone (sodium tetraborate) in water. This was invented in 1902 by Frenchman Albert Nodon and is known as the Nodon valve or rectifier. Another home-brew diode can be made by reacting copper with sulfur. Coat copper with sulfur powder and heat indirectly to melt the sulfur, which will eventually ignite, leaving a dark coating. Sulfur fumes are toxic, so this should be done outdoors with suitable respiratory protection. One end of the circuit is connected to the unreacted copper with a fine point (cat’s whisker) used to find a spot that rectifies. While copper works, galena is superior. Fig.13: an improvised speaker without a permanent magnet. Silicon Chip readers might try improving on this design. One set of terminals is for the electromagnet, the other for the signal. Source: https://youtu.be/ wqGmMFo1jGg Earphones/speakers/microphones Crystal radios require high-impedance earphones because the detector circuit has a very high output impedance and produces extremely little power. A low-impedance earphone would heavily load the detector and collapse the signal voltage, whereas a high-impedance earphone allows the audio signal to develop without significantly disturbing the tuned circuit. The ideal earphone impedance for a crystal set is around 20-100kW. One can alternatively use low-­ impedance earphones with an audio matching transformer having a turns ratio of about 50:1 (a salvaged 230V to 5V transformer would be close). A suitable improvised transformer could be made pretty easily. Improvised high-impedance earpieces can be either piezo-electric or magnetic types. Magnetic earpieces were used with radios but required thousands of turns of wire. A classic high-impedance earphone of 2kW might require 2,000-10,000 turns of 0.1mm diameter or smaller enamelled wire depending on the design – see Fig.11. Note that in general, an earphone or speaker can also be used as a microphone in an emergency. Making a DIY high-impedance earphone is shown in the video titled “Crystal Radio Earphone From Common / Cheap Stuff” at https:// youtu.be/dWkKFD_yZs8 It uses a piezoelectric element salvaged from a beeper commonly found in some microwave ovens or other appliances with a beeper (eg, smoke alarms). Also see the video titled “Make Crystal Earphone/Earpiece for Crystal Radio – Homemade” at https://youtu. be/OVapqz4yBCY Another method is to make your own piezoelectric earpiece or speaker using Rochelle salts, which will be discussed later. A microphone can be improvised from a matchbox or similar small box and some pencil leads – see Fig.12 & www.instructables.com/Matchbox-­ Microphone A modern speaker requires a permanent magnet, but older speakers used electromagnets, and that’s still a valid technique. A basic improvised speaker typically uses two coils: one coil, powered by a DC supply (the electromagnet) creates a steady magnetic field, while the other coil (the voice coil) is attached to the diaphragm and driven by the audio signal, causing it to move in the magnetic field to produce sound. For more details, see Fig.13 and the video at https://youtu. be/wqGmMFo1jGg Inductors and transformers An inductor is a two-terminal device that temporarily stores energy in a magnetic field and resists rapid changes in current. It is usually in the form of a wire coil and is among the easiest devices to improvise. It can have an air, powdered/laminated iron, or ferrite core. A transformer is built similarly, but it has two or more coils on a shared core so that energy can be transferred between them. For an improvised coil former, just about any cylindrical material can be used, such as PVC or other plastic pipe, bamboo, timber dowels, cardboard tubes, or even a bottle – see Fig.14. A Table 3 – suitable materials for making point-contact diodes and transistors Material When used Signal sensitivity Ease of use Suitable for point-contact transistor? Galena (PbS) 1900-1920s High Very finicky Very low gain and stability Silicon (Si) 1920s-1940s+ Good Moderate Less forgiving than germanium Germanium (Ge) 1940s-1960s Very high Moderate Easiest to use Good More stable Not practical due to very low gain and stability Silicon carbide 1900s-1910s (carborundum, SiC) siliconchip.com.au Australia's electronics magazine September 2026  17 Fig.17: a circuit to ‘program’ an improvised memristor with either high or low values. Fig.14: air-cored inductor coils wound on 50mm cardboard tubes. Source: https://analogdial.com/TRFRadio/TRF1.htm Fig.15: an improvised transformer using a nut as the magnetic core. This will have high eddy current losses at any significant frequency. Source: https://youtu.be/ M6D-tZEA4jc Fig.16: an improvised step-up boost converter with the transformer wound on a nut and bolt. Source: https://youtu. be/4bgFEFBHzx0 coil is an essential element of LC tank circuits as used in radio transmitters and receivers, as well as in many other applications, like DC/DC converters. The basic rule for improvised (or any) coils is to use the thickest wire that fits the required number of turns within the available space. The inductance of a coil can be significantly improved by using a core, which concentrates the magnetic field and increases the coil’s effective inductance – more on that later. You can also make a transformer using a nut or a bolt as the magnetic core, as shown in Figs.15 & 16. LEDs A weak, primitive LED can be made by replicating Henry Joseph Round’s 1907 discovery by applying around 18 Silicon Chip 10-100V DC at up to 10mA across silicon carbide (SiC, carborundum) crystals (3-10mm in size) sitting on a conductive foil or in an alligator clip (one electrode) to produce a yellow-­ green glow. The other electrode is a sharp point-contact wire (cat’s whisker) pressed against a crystal. If no glow appears with one polarity, reverse the connections. The junction can work in either direction, although one may produce a stronger glow. This is regarded as the world’s first LED. Modern LEDs use different structures. For more on this, see the video at https:// youtu.be/hUYZfjB3GKs Magnets Magnets can be used in improvised speakers or microphones. An Australia's electronics magazine improvised weak permanent magnet can be made without using another magnet by taking a piece of iron or mild steel (like a large nail), aligning it with magnetic north and at an angle to the horizontal to match the magnetic dip angle in that area (typically 30-70°, depending on the latitude) and repeatedly hammering it. This will align some of the magnetic domains and make a magnet strong enough to pick up paper clips or possibly for use in a speaker or microphone. The magnetism will fade within hours or days, but it can be easily refreshed. If you already have power, say from an improvised battery, you can make an electromagnet. A strong magnetic field produced by a coil carrying sufficient current can permanently magnetise hardened steel. Making a magnetic core For making coils or inductors, a magnetic core will give increased inductance compared to an air core. You can make your own improvised inductor, transformer or antenna core by mixing iron oxide (rust) with a suitable binding material like plaster, epoxy, wax or similar in about a 50:50 volume mix. Solid iron or steel can also be used as an inductor core, but is only suitable for very low frequencies (under 500Hz) due to high losses. Powdered iron cores can be used up to about 10-20MHz and ferrite cores up to around 300MHz. Memristors In our recent articles about Analog Computing (May & June 2026 issues; siliconchip.au/Series/459), we described memristors, devices that can remember the last resistance ‘programmed’ into them. It is possible to improvise one. siliconchip.com.au Nyle Steiner demonstrated memristor behaviour with copper, brass or lead that had been reacted with sulfur, then creating a junction of the metal sulfide coating with aluminium. See Fig.17, http://sparkbangbuzz.com/ memristor/memristor.htm and https:// youtu.be/MlswP_qXbdA at 1kHz, a capacitance of around 150300nF and a leakage resistance of 20MW. Because of their high impedance, they need very little current to drive, but are sensitive to voltage, making an earpiece suitable for weak, high-impedance signals from a crystal radio detector. The energy transfer is efficient in Resistors such a configuration as the impedImprovised resistors can be made by ances match. As they are passive various methods. One is to use a soft devices, no electronics are needed to pencil to scribble a solid pattern on drive them. a piece of paper, as shown in Fig.18. The materials needed are: Electrical connections can be made via • 150g of cream of tartar (potassium paper clips. Another method is to wrap bitartrate; make sure it is real and not copper wire around the ends of a pen- a substitute) cil lead – see Fig.19 and the video at • 50-75g of washing soda (sodium https://youtu.be/kMeR1k1C8_M carbonate), or if not available, baking Another method is to use high-­ soda (sodium bicarbonate) baked at resistance wire, like Nichrome or steel about 200°C for one hour wire, to make a wirewound resistor, • 250mL of distilled water wrapping it around an insulating for• two borosilicate glass (eg, Pyrex) mer (ideally made from a heat-resistant beakers or jars material). The resistor can then be pot• a stirring rod or spoon ted. For more on this, see the video at • a coffee filter and a funnel https://youtu.be/b1VfXZgxI2w • a thermometer Liquid resistors and liquid rheoHeat the distilled water to 80-100°C, stats are simple to make, comprising add 150g of cream of tartar slowly water, a salt solution such as sodium and carefully while stirring, then add carbonate (washing soda), or ideally sodium carbonate gradually in small sodium bicarbonate (baking soda) and amounts – the solution will fizz. Keep two electrodes, perhaps stainless steel or carbon rods. To vary the resistance, the concentration of salt water can be varied, or the amount of immersion of one or both electrodes in the solution can be changed. Such devices have been used commercially in the past, and are even in use today. In DIY applications, use Fig.18: an improvised resistor using them in a well-ventilated area and only a pencil, paper and paper clips. for low-to-moderate power dissipation Source: www.instructables.com/DIYand heat buildup. The potential to Emergency-resistor-an-electronicproduce hazardous electrolysis gases circuit-com like hydrogen/chlorine is an important safety consideration. Rochelle salts Piezoelectric crystals develop an electric charge in response to mechanical stress and can be used for microphones, speakers, earpieces or sensors. Piezoelectric Rochelle salt (also known as potassium sodium tartrate tetrahydrate) crystals are relatively easy to make from common ingredients and can be used for high-­ impedance speakers and microphones. Speakers and microphones based on Rochelle salts can have very high impedances, in the range of 20-25kW siliconchip.com.au adding small amounts until there is no more fizzing. Filter the solution through the coffee filter and funnel it into another glass container to remove any particles. Cover the container and leave it in a cool, quiet spot until the crystals form. Then pour off any remaining liquid. The crystals can then be harvested. Dry the crystals with paper towels. If desired, one of the best crystals can be used as a seed to grow larger crystals by suspending it with fishing line in a fresh solution. The best crystals are the biggest and most regularly shaped with two opposing flat surfaces. Electrical contacts can be made using aluminium foil held in place between the faces of a clamp, with pieces of rubber to distribute the forces evenly to avoid breaking the crystal – see Fig.20. Another mounting method is shown in Fig.21. An article discussing commercial uses of Rochelle salt “reproducers” or speakers from the July 1932 edition of Radio-Craft magazine can be seen at siliconchip.au/link/acbt (Fig.22). “leafcutterjohn” made a sound recording using a homemade Rochelle salt microphone that you can listen to at siliconchip.au/link/acbs Fig.20: a method to hold a Rochelle salt crystal between two electrodes. Normally, the crystal would be mostly clear. Source: https://youtu.be/ RtW277wDrtM Fig.19: an improvised resistor using copper wire and a graphite rod from a pencil. Source: https://youtu.be/ kMeR1k1C8_M Fig.21: a Rochelle salt crystal with wires attached using conductive adhesive. It was successfully used as a guitar pickup. Source: https://youtu. be/8QP7F1VT1rw Australia's electronics magazine September 2026  19 A Rochelle salt microphone was also used as a guitar pickup – see Fig.21 and https://youtu.be/8QP7F1VT1rw Solder Wires can generally be twisted together or held together with some type of mechanical fastener, but soldering is more reliable. Leaded solder is the easiest to use and is made from 60-63% lead and 37-40% tin by weight. Flux is needed to get the solder to wet the surfaces being connected. You can find many simple recipes online, but an effective flux can be made by dissolving citric acid in water to make a concentrated solution. The residue is corrosive to electronics and must be washed away with hot water. A better flux recipe is to dissolve rosin (the purified resin or sap from pine trees) in isopropyl alcohol, producing a non-corrosive, classic flux that’s safer and more reliable, with a residue that doesn’t have to be removed. If isopropyl alcohol is not available, methylated spirits or acetone can be used. However, these may harm some plastics, especially acetone. Fig.24: the structure of the first transistor from Bell Labs. Source: https://w.wiki/KsX5 of transistor construction methods. The first transistor made by Bell Labs in 1947 was a point-contact transistor (see Figs.23 & 24) and this is more amenable to home or improvised construction. The more modern type of transistor is the junction transistor with NPN or PNP construction, which is more reliable and a better performer, but requires high-temperature doping, cleanrooms, photolithography, precise junctions and doping, diffusion and epitaxial growth techniques. This is not impossible at home, but it is significantly harder and less reliable. We will discuss improvised junction transistors later. The first Bell Labs point-contact transistor was built on a piece of n-type germanium. No deliberate doping was done to create a p-type top layer, as shown in Fig. 24. Instead, the emitter point contact injected holes (positive charge carriers) into the n-type base material, creating a localised p-type inversion layer near the surface. The collector point contact then collected these holes. This surface effect effectively formed a PNP transistor structure, while the bulk material remained n-type. The n-type germanium served as the base, while the metal-­semiconductor interfaces under the emitter and collector points created the necessary p-type regions through carrier injection and surface inversion. These surface-effect mechanisms were critical to the point-contact transistor’s operation. However, they also made the device notoriously unstable, noisy and difficult to reproduce consistently. This led to its rapid replacement by the more reliable junction transistors (such as the grown-junction and alloy-junction types), which used intentional doping to create well-­ defined p-n junctions. There’s more on this in a video on how the first transistor worked at https://youtu.be/RdYHljZi7ys Early attempts by amateur experimenters often involved cracking open a point-contact germanium diode, such as the 1N34, to extract the small piece of n-type germanium, which served as the base. Contacts were then made from thin metal foil or phosphor-­ bronze wire. The emitter and collector points had to be extremely close together; some Australia's electronics magazine siliconchip.com.au Transistors (simple) As with diodes, there are two types Fig.22: a schematic of an early commercial Rochelle salt speaker from 1932. A DIY device can be much simpler. Source: https://rfcafe.com/references/ radio-craft/rochelle-salt-crystal-reproducer-july-1932-radio-craft.htm Plastic frame Spring Plastic Emitter wedge lead Germanium Base lead Collector lead Gold foil p-type n-type Metal base Plastic frame Fig.23: a replica of the original Bell Labs transistor. Prototypes can be very messy! Source: https://w.wiki/KsX4 20 Silicon Chip Fig.25: an improvised point-contact transistor attempt with a germanium wafer taken from a diode. Source: https://youtu.be/5o8B0ekgsxY texts suggest as little as 10 microns (0.01mm) to achieve any gain. A critical step was ‘forming’ the collector by applying a brief high-­ current pulse (eg, 200V from a capacitor discharged through a resistor). This locally heats the surface and diffuses dopants (such as phosphorus from the phosphor-bronze) into the germanium, creating a localised p-type region under the contact. Apart from germanium, materials such as galena and pyrite have also been used in similar point-­ contact setups. Disadvantages of these homemade transistors include low gain (often β < 1), instability, and a tendency to operate in the negative-­ resistance regime rather than as reliable linear amplifiers. An improvised point-contact transistor can be made from the salvaged germanium and whisker from a germanium diode with an additional contact made from phosphor-bronze alloy material, as explained in the video at https://youtu.be/vmotkjMSKnI It is important that phosphor-bronze contacts be used; a phosphor-bronze guitar string, heated to create an oxide layer, has been suggested. See https:// youtu.be/5o8B0ekgsxY (Fig.25) for a partially successful attempt at making a germanium point-contact transistor. Also see the video at https://youtu.be/ wjiSzUa0aHs and Fig.26. Some have suggested ‘paper transistors’ can be made with a graphite pencil on paper but these are not real transistors. While they can show basic conductivity, weak rectification or nonlinear behaviour due to contact effects and impurities, they lack controlled doping, current gain and reproducible transistor action. They function more like crude resistors with contact non-linearity or siliconchip.com.au Fig.26: a point-contact assembly for various diodes and transistors with pyrite. It was noted that the contacts had to be extremely close together for the device to work. Source: https://youtu.be/wjiSzUa0aHs rectifying junctions than as true amplifying devices. Transistors (advanced) The more advanced approach to transistors requires specialised equipment but is still within the realm of possibility for a determined home fabricator, at least in countries where chemicals like hydrofluoric acid can be obtained by non-commercial users. In Australia, many of the required substances are far more restricted (alternatives to HF acid might be possible). Most transistors created by improvisers are Mosfets (metal-oxide-semiconductor field-effect transistors) rather than BJTs (bipolar junction transistors). We’ll explain why shortly. Essential equipment to make a Mosfet transistor includes: • a high-temperature furnace or diffusion oven • chemicals for etching and diffusion (eg, hydrofluoric acid, phosphoric acid & phosphorus oxychloride) • photolithographic equipment, such as a UV light source, photoresist, developer and photomasks (often printed on transparency film) • cleanroom-like conditions (laminar flow hood or glove box) • deposition tools (eg, thermal evaporator or sputtering system for metal contacts) One simplified technique to make a basic planar n-channel Mosfet is as follows: 1. Start with a p-type silicon wafer 2. Grow a thin oxide layer on the wafer in the furnace (thermal oxidation) 3. Coat the wafer with photoresist 4. Expose the photoresist to UV light through a mask to define the pattern 5. Develop the photoresist and etch the exposed areas 6. Use acids or plasma to etch the silicon dioxide layer in the desired regions 7. Introduce n-type dopants (eg, phosphorus) via high-temperature diffusion to create source and drain regions 8. Deposit metal layers (eg, aluminium) for electrical contacts (source, drain and gate) Fig.27: a homemade n-channel Mosfet transistor. Two devices were fabricated on one piece of silicon in case one device didn’t work. Source: https://youtu.be/ s1MCi7FliVY Australia's electronics magazine September 2026  21 Fig.28: a simple switch made from split-pin paper fasteners (brads), paperclip and cardboard or thin timber. Source: https://redfernelectronics.co.uk/diy-switches Fig.29: Peter Parker’s improvised Morse key, made from simple components. Source: https://youtu.be/bJ0V9VzoNuM This is essentially the classic planar process pioneered in the 1960s by Jean Hoerni at Fairchild Semiconductor, then taken to Intel by Robert Noyce and Gordon Moore. This process is very challenging at home, requiring precision, proper safety precautions and patience. However, it has been successfully demonstrated by dedicated hobbyists like Jeri Ellsworth (www.jeriellsworth. com). She has a video on making an n-channel Mosfet at https://youtu. be/w_znRopGtbE Sam Zeloof (https://sam.zeloof.xyz) also made an n-channel Mosfet, as described in the video at https://youtu. be/s1MCi7FliVY (see Fig.27). Some experimenters have attempted to make transistors from zinc oxide rather than silicon, but they have generally been unsuccessful. Transistors – BJTs Homemade transistors tend to be Mosfets because BJTs are far less forgiving of fabrication imperfections. BJTs require ultra-precise, thin, lightly doped base regions and perfect junction alignment because they don’t use the easier-to-control field-effect mechanism at the surface. BJTs demand atomic-level control over doping profiles and junction depths, which is extremely difficult without professional semiconductor fab equipment. As a result, when hobbyists succeed in making working transistors at home, they are almost always Mosfets (or simple field-effect devices). In detail: • BJTs require extremely thin junction boundaries with very precise doping profiles. Any deviation will cause failure or poor performance. Mosfets need only two doped regions (source 22 Silicon Chip and drain) of the same type in a substrate of the opposite type; the channel forms under the gate oxide layer via the field effect, so no ultra-thin, lightly doped base is required. • In a Mosfet, the gate oxide is forgiving and can be grown thermally. Imperfections are tolerated as long as it is continuous and insulating. • Mosfets can function with relatively large gate lengths (tens or even hundreds of microns) and crude alignment (hand-drawn masks or contact lithography), while BJTs require the emitter and collector to be very closely spaced and precisely aligned to the base. That is extremely hard without professional photolithography and alignment tools. • Early point-contact transistors (1947 Bell Labs) worked via surface inversion layers, but they were unstable and hard to reproduce. Mosfets exploit the same surface inversion/ channel formation, but the gate oxide insulates and controls it reliably, making them more forgiving for crude fabrication techniques. As mentioned earlier, Sam Zeloof has had no trouble building working Mosfets in his home lab but has never succeeded in making a functional BJT. Jeri Ellsworth and other DIY semiconductor YouTubers have also made Mosfets but never reliable BJTs. Most improvised transistor successes shared online are Mosfets or JFET-like structures. Making a simple p-n junction diode ‘just’ involves a single interface that Fig.30: the ‘dead bug’ construction style. This technique is fast and flexible. For more information, see the video at https://youtu.be/pkrp98hXCUs Australia's electronics magazine siliconchip.com.au can form somewhat accidentally or with basic heating or probing, while a BJT needs two precisely aligned and controlled junctions, which is much harder. Switches and Morse keys Simple switches or traditional Morse keys can be improvised, but should only be used for low voltages – see Figs.28 & 29. Making a ‘haptic’ Morse key from a nail file is explained in the video at https://youtu. be/guRh36xGCJ8 Wire Making your own wire is not for most experimenters, and would not even be considered for all but the worst-case scenarios, such as civilisational collapse. You would have to find or smelt copper ore. Once smelted, copper can be hammered and twisted into a wire-like form, a technique that was invented about 4000 years ago in the Middle East. Zinc-oxide tunnelling diodes In 2001, Nyle Steiner K7NS rediscovered and improvised the ingenious improvised electronic device now called the zinc-oxide tunnelling diode (siliconchip.au/link/acbr). It is a quantum mechanical device you can make yourself with ease. This type of diode was originally invented by Russian Oleg Losev in 1923. It exhibits negative differential resistance (NDR), a property where increasing the voltage leads to a decreasing current in a certain range – see Fig.31. This allows it to function like a tunnel diode for oscillators, amplifiers and even simple radio transmitters without needing traditional vacuum tubes, transistors or ICs. A typical junction shows n-type NDR behaviour beginning at 100200mV. The region where current begins to increase again at around 300500mV. So, for stable oscillation, the device is biased within the 100-400mV range. The NDR provides gain, similar to a tunnel diode, sustaining oscillations in a tuned circuit (eg, an LC tank). Unlike commercial components, finding the ‘sweet spot’ for the device is finicky and it may need to be readjusted regularly. The negative resistance region effectively supplies energy to cancel losses in a circuit. This allows a simple LC tank to sustain oscillations (as in RF or siliconchip.com.au audio oscillators). It can amplify small AC signals by reflecting and boosting them, a process called negative-­ resistance amplification. That can provide RF gain in radios, with energy coming from the DC bias voltage. It can also generate modulated carriers for low-power transmission if the bias is varied. Unlike ordinary diodes that only rectify, this NDR gives the two-­ terminal device the inherent gain and instability needed for oscillation, or conditionally needed for amplification. This mimics the active role of a transistor or valve in simple improvised configurations. All that’s needed to make this device is a piece of galvanised (zinc-coated) sheet, heat-treated with a blowtorch to make a thick oxide layer, and a cat’s whisker of steel or copper wire. The zinc oxide layer creates an n-type semiconductor layer. When the cat’s whisker is brought into contact with the zinc oxide, a metal-oxide-­ metal junction is created; the zinc oxide layer creates a barrier similar to that in a schottky diode. A device is shown in Fig.32. We will discuss radios recently built with this device in the following issue. Improvised construction methods Circuits can be built on traditional timber breadboards, hence the term “breadboarding”. Modern plastic cutting boards can be used instead of timber. There is also the ‘dead bug’ style of construction shown in Fig.30 (on a plastic cutting board, as it happens). It’s also possible to ‘air wire’ components together over a grounded metal plate, as shown in Fig.33. This can work well, using a similar principle to double-sided PCBs with a ground plane. I i1 rdiff < 0 i2 v1 v2 V Fig.31: a current-voltage (I/V) graph illustrating the voltage-controlled negative resistance property of a tunneling diode. Fig.32: a zinc-oxide tunnelling diode made from a simple piece of oxidised, galvanised sheet with a ‘cat’s whisker’. Source: https://ashishrd. com/2024/02/24/the-quantumtunneling-transmitter Salvaging materials If in a survival situation, you can salvage materials from wreckage. For example, wire and solder can be melted off components or circuit boards for reuse. Numerous circuit components can be salvaged, such as resistors, capacitors and transistors, to make simple circuits like a radio. Next month We’ll finish this topic next month when we look at more advanced comSC ponents and techniques. Australia's electronics magazine Fig.33: components air-wired by soldering their leads together over a grounded tin plate. This also gives them some mechanical support. Source: Peter Parker, VK3YE September 2026  23 FROM FIRST LAYER TO FINISHED MASTERPIECE. Explore the full range of ELEGOO® Filament Printers now at Jaycar. Neptune 4 TL4970 Neptune 4 Pro TL4972 EXPLORE THE ELEGOO ® RANGE (AU) EXPLORE THE ELEGOO ® RANGE (NZ) Neptune 4 Plus TL4974 Neptune 4 Max TL4976 Centauri V2 TL5104 Centauri Carbon 2 Combo TL4986 OrangeStorm TL4982 Printer Type Build Volume (mm) Nozzle Size & Type Printing Nozzle Speed (max.) Temp (max.) Bed Temp (max.) 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The displayed information includes the device type, a diagram showing which test lead is connected to which pin, and in most cases, some key electrical parameters. These are shown for 15 seconds, then the analyser switches itself off. While results are displayed, you can press TEST again to initiate another test cycle, or the OFF button to put it to sleep immediately. There is a USB Type-C port on the end of the unit that serves three purposes: • It can be used to charge the internal lithium-polymer (prismatic lithium-­ion) battery. • It provides a simple serial interface that can display the test results in a terminal program running on a computer. • The USB port can also be used for uploading firmware, either programming the chip initially or updating it later. The command-line interface via the serial terminal provides a bit more information about the tests being carried out, so it is useful for debugging or just understanding how the device has come to the decision that it has. Capabilities » Identifies and tests diodes, transistors (bipolar, JFET, Mosfet & IGBT) and thyristors » Diodes supported: standard or schottky (single or pair) plus zeners/TVSs/LEDs up to 10V » Bipolar transistor tests: NPN/PNP, standard or Darlington with or without diodes, hfe: 5-25,000 » Mosfet tests: N/P-channel, enhancement/depletion, Vgs(th) (gate-source threshold voltage) » JFET tests: N/P-channel, pinch-off voltage » IGBT tests: freewheeling diode presence, Vge(th) (gate-emitter threshold voltage) » Thyristor tests: identifies SCRs and Triacs » Other tests: detects short circuits & open circuits in devices » Power supply: 1100mAh rechargeable Li-ion cell giving a runtime of ~24 hours The Semiconductor Analyser works with the following devices: ▶ Diodes – identifies standard diodes, schottky diodes, LEDs and zener (or TVS) diodes with a breakdown voltage between 3V and 10V. It measures the forward voltage of all diodes and the breakdown voltage in the case of zeners/TVSs. The test voltage is limited to 12V, so zener diodes with a breakdown voltage greater than about 10V will be identified as standard diodes. ▶ Diode pairs – identifies common anode, common cathode and series pairs of standard and schottky diodes. It identifies common anode, common cathode and back-to-back LED pairs. The forward voltage of each diode or LED in the pair is displayed along with the test current. ▶ Bipolar transistors – identifies NPN and PNP bipolar transistors and Darlington pairs, with or without integrated freewheel diodes. The Australia's electronics magazine siliconchip.com.au Project by Andrew Levido Features & specifications 28 Silicon Chip base-emitter voltage and the DC gain (hfe) are displayed. DC gains in the range of 5 to 25,000 can be measured. Some Darlington pairs have higher gain than this, and in such cases, the device indicates that the hfe could not be measured. This does not mean the transistor is faulty. ▶ Mosfets – identifies N-channel and P-channel enhancement-mode Mosfets and N-channel depletion-mode Mosfets. (P-channel depletion-mode Mosfets can exist in theory, but nobody makes them). For enhancement-mode Mosfets, the gate-source threshold voltage (Vgs(th)) is displayed. This is the gate-source voltage at which the drain current rises to 5mA. For depletion-­ mode Mosfets, the gatesource pinch-off voltage is measured. This is the gate-source voltage at which the drain current falls to 5µA. ▶ JFETs – identifies N-channel and P-channel junction FETs. The pinch-off voltage is measured as for depletion-­mode Mosfets. The Semiconductor Analyser identifies the gate of a JFET, but the drain and source terminals are usually interchangeable. In many small-signal JFETs, they are electrically symmetrical, so the analyser identifies both as “drain/source”. ▶ IGBTs – identifies IGBTs with and without integrated freewheel diodes. The gate-emitter threshold voltage (Vge(th)) is measured in the same way the gate-source voltage is measured for a Mosfet. ▶ Thyristors and Triacs – identifies thyristors and Triacs with a gate sensitivity of 500µA or lower. This includes most small devices, but some high-power devices may not be correctly recognised because the Semiconductor Analyser cannot source enough current to reliably switch them on. ▶ Short and open circuits – a dead short between any two, or all three leads is detected and flagged as an error. A short-circuit is indicated if the voltage drop between the leads is less than 100mV in both directions. An open-circuit error is flagged if there is no conductivity between any of the three leads. If the device does not check out as one of those mentioned above, it is flagged as faulty or unknown. The voltage applied to all devices is limited to 8V until a preliminary identification is made. This is necessary to avoid damage to logic-level Mosfets, siliconchip.com.au which have a very thin gate oxide layer that can be damaged by voltages above this. Mosfets are never tested at higher voltages, but other components may be tested at up to 12V. The test current is at all times limited to around 10mA, so the Semiconductor Analyser is not likely to damage any component you connect, regardless of the lead connections. How it works It is by no means a simple task to positively identify such a wide range of semiconductors given the enormous variation in parameters, but in the end it all comes down to two basic measurement configurations, as shown in Fig.1. At left is the positive drive configuration, where the drive polarity is positive with respect to the power node, and at right is the negative drive configuration, where the drive polarity is negative with respect to the power node. The power node can therefore either be at ground potential or at 12V. The load node consists of a 1kW resistor in series with a voltage source, Vl. The voltage source can be adjusted to put the appropriate bias between the power and load nodes. When the bias voltage is limited to 8V, Vl would be set to 8V in the positive drive configuration and 4V in the negative drive configuration. The load current, Il, is limited by the 1kW load resistor and the on-­ resistances of the analog switches (more on this below) to a maximum of about 10mA. This current can flow in either direction, depending on the configuration. I have used the convention that current flowing out of the load node is positive and current flowing into it is negative. The third terminal is connected to the drive node. This can be a variable voltage source for voltage-driven devices like Mosfets, or a variable current source/sink for current-driven devices like bipolar transistors. The drive node can also be left open or connected to the load or power nodes if required. A 3 × 3 analog switch matrix (Fig.2) allows each of the three test leads to be connected to any of the test nodes. The voltage at each test lead is measured by the microcontroller. We measure the voltages directly at the terminals of the device under test (DUT) rather than at the power, load and drive Australia's electronics magazine Fig.1: all of the tests conducted by the Semiconductor Analyser are based on these two configurations involving a power node, a load node and a drive node. Fig.2: each of the three test nodes can be connected to any of the three test leads (labelled red, green and blue) by a 3 × 3 analog switch matrix. September 2026  29 nodes because the analog switches each have roughly 40W on-resistances and we do not want the voltage drop across them to introduce measurement errors. Identification process You can probably see how we might use the circuits in Fig.1 to characterise components if we knew what they were and which pin is which, but we know neither of these things at the outset. For this reason, the identification process starts with a set of simple continuity tests between each pair of leads. The continuity is measured twice in each direction, once with the positive and once with the negative drive configuration. During these tests, the third lead is driven by a 500µA current of the appropriate polarity. We need to drive the third lead because we would get unreliable readings if it were left open while connected to a Mosfet or IGBT gate lead. Any stray charge on the open gate lead could put the Mosfet/IGBT in an unknown state. The yes/no results of the 12 continuity measurements (3 pairs of leads × 2 directions × 2 drive polarities) are stored as bits in a test flags variable. The hexadecimal value of this register serves as a ‘signature’ that can be used to help identify the device and its connections. Some devices are unambiguously identified and orientated by just these flags. Single diodes and diode pairs connected anode-to-cathode are two such examples. Most, however, need further disambiguation. It might be best to use an example, since it’s not practical to cover the identification process for each device type. An NPN bipolar junction transistor (BJT) or an N-channel junction field-effect transistor (JFET) with the blue lead connected to the base or gate will have test flags of 0x3C5. The four possibilities are shown at the top of Fig.3. The 0x300 bits indicate conductivity from the blue lead to the red lead, but not from red to blue, for both drive polarities. The 0x0C0 bits indicate the same thing from the blue to the green Fig.3: an NPN BJT (bipolar transistor) and an N-channel JFET have the same continuity signature when connected as shown. Test 1 determines what device it is, while Test 2 identifies the emitter and collector if it is a BJT. 30 Silicon Chip Australia's electronics magazine leads. I have shown this symbolically by the grey diodes in the figure. The 0x05 bits indicate conductivity in either direction, but only when the drive is positive (red diodes). The signature for these same components would be 0xC53 if the red lead was connected to the base/gate and 0x53C if the green lead was connected to the gate. The BJT has this continuity signature because of its base-emitter and base-collector PN junctions and because the transistor is biased on by a positive base current. The collector-emitter path conducts both ways when a transistor is switched on because bipolar transistors have an appreciable reverse gain. The reverse gain is a lot lower than the forward gain; a fact we will use to our advantage below. An N-channel JFET has the same signature as an NPN BJT, but for different reasons. The channel is a single piece of N-doped silicon, with the drain and source terminals at either end, so it is conductive in both directions when the JFET is unbiased. The gate region is P-doped, so it forms a PN junction with the channel. This diode can be measured from gate-to-source and from gate-to-drain. The channel does not conduct when the gate-channel junction is reverse-­biased, as the depletion region expands to ‘pinch off’ the channel. This is why we see conductivity in both directions when the gate is positively biased and no conductivity when the gate is negatively biased. At this point, we know that we have an N-channel JFET or an NPN BJT, and we know which test lead is connected to the gate or base. It is the blue one in this 0x3C5 example, but depending on the signature, it may be one of the other leads. We need a further set of tests to fully identify the part and its connections. The next test (Test 1 in the figure) is a simple measurement of load current between the two unknown leads (red and green in our example) with the gate/base lead shorted to the power node. A JFET will conduct regardless of the orientation of the drain and source due to the symmetry of the channel. A BJT will not conduct between collector and emitter when there is no base drive. The above statement is technically correct, but we need to add a caveat. In the configuration where the siliconchip.com.au base-emitter junction is reverse-biased (second from the left), there may be some current flow due to reverse breakdown. A reverse-biased emitter-base junction experiences avalanche breakdown (like a zener diode) at around 7-8V, so this could occur with Vl set at 12V. This neatly illustrates the type of challenges encountered in designing this project. I had to choose a current threshold that would reliably discriminate between a BJT with emitter-base breakdown and a JFET with a high-­ resistance channel. A threshold of 5mA means a BJT with a breakdown voltage of ~6V or above will be correctly identified, as will a JFET with a drain-source on-resistance of ~1.2kW or below. This should work for just about every device out there, but the window is narrow and a good understanding of second-order effects is required. If we find the device is a JFET, we have done all we can to identify it unambiguously. As mentioned above, there is no way to tell the drain from the source as they are electrically identical. If we have a BJT, however, we still have to determine which lead is the emitter and which is the collector. To do this, we use the fact that the reverse gain is always lower than the forward gain. Test 2 consists of two parts, as shown at the bottom of Fig.3. The drive node is set to +500µA and the load current is measured in both directions. If the current in the red-to-green direction is greater than 2.5mA and higher than that in the green-to-red direction, we can assume that the green lead is the emitter. If the current in the green-to-red direction is greater than 2.5mA and larger than that in the red-to-green direction, we can assume that the red lead is the emitter. An internal view of the Semiconductor Analyser, showing the PCB and battery. The battery is mounted using double-sided foam tape. If neither of these is true, the forward gain is less than five times, so the device is probably faulty or it is something unknown. This type of discrimination testing is performed in each case of all ambiguous continuity test results. The tests undertaken are specific to the device in question, but there is only ever a handful of possibilities to sort out, so the complexity of each of these discrimination tests is similar to the one described above. Characterising the device After the part and its connections have been identified down to the Screen 1: this serial console log shows the testing process in a bit more detail, in this case for a BC847 transistor and an MMBFJ112 JFET. In both cases, the initial test flags give the same result. siliconchip.com.au Australia's electronics magazine family level, another set of tests is carried out to further characterise it. To continue with our example, if the DUT is a JFET, the pinch-off voltage is measured. If it is a BJT, its base-emitter voltage and hfe are measured. The base-emitter voltage is used to determine if the DUT is an ordinary BJT or a Darlington pair, since the latter has two base-emitter junctions in series and will thus be over 1V. In the same way, forward voltage is used to discriminate between standard diodes (~0.6-0.7V), schottky diodes (~0.30.5V) and LEDs (>1V). Only the final results are displayed on the LCD screen, but a bit more insight into the process is provided via the serial output. Screen 1 shows the serial output when testing a BC847 BJT and an MMBFJ112 JFET, each connected as per Fig.3. Both have initial test flags of 0x3C5 and you can see them going through the tests described above. The first test with the gate/base unbiased gives an IDUT(0) figure of 3.6mA in the case of the bipolar transistor, and around 10mA in the case of the JFET. The 3.6mA value for the BJT suggests an emitter-base reverse September 2026  31 Parts List – Semiconductor Analyser 1 double-sided PCB coded P9062-1-C, 130.5 × 56.5mm 1 front panel label, 55 × 128.5mm 1 Hammond 1593XBK plastic enclosure, 140 × 66 × 28mm 3 test clips; red, blue and green (CON1-3) [Cal Test CT3180-2, -5, -6] 1 1100mAh Li-Po cell, 51 ×34 × 6mm (BAT1) [Core Electronics CE04377] 1 USB4105-GF-A 16-pin USB-C connector (CON5) 1 JST S2B-PH-K-S 2-pin right-angle header, 2mm pitch (CON6) 1 Littelfuse 1210L075/24PR resettable PTC fuse or equivalent (F1) 1 SMD M3225/1210 6.8µH 1A inductor (L1) [Murata 1276AS-H-6R8M=P2] 1 320 × 240. 2.4-inch TFT LCD with ILI9341 driver and 18-pin, 0.8mm pitch flex cable (LCD1) [AliExpress 1005005796800307 “ILI9341-No Touch”] 2 SMT gull-wing tactile switches, 6.6mm, with 8.5mm actuator (S1, S2) [E-Switch TL3301PF160QG] 1 audio transducer (SPK1) [CMT-0525-75-SMT-TR] 4 4G × 6mm panhead self-tapping screws 3 lengths of hookup wire with red, blue and green insulation, each 350mm long Double-sided 3mm-thick adhesive foam tape Semiconductors 3 DG412 quad NO analog switches, SOIC-16 (IC1-IC3) 3 LMC7101 general-purpose op amps, SOT-23-5 (IC4, IC5 & IC7) 4 TLV2186 dual zero-drift op amps, SOIC-8 (IC6 & IC8-IC10) 1 STM32L433CCT6 LQFP-48 microcontroller (IC11) 1 MAX1555 battery charger, SOT-23-5 (IC13) 1 TLV61046 boost converter, SOT-23-6 (REG14) 2 MCP1711T-33 low-dropout linear regulators, SOT-23-5 (REG15 & REG16) 3 AQ4022-01FTG-C bidirectional 12V TVS diodes, SOD-323 (TVS1-3) 1 SMBJ5.0A unidirectional 5.0V TVS diode, DO-214 (ZD4) 1 3mm yellow through-hole LED (LED1) 2 BSS138K N-channel Mosfets, SOT-23 (Q1, Q2) 4 BAV99 series switching diode pairs, SOT-23 (D1-D4) Resistors (all SMD ±1% M2102/0805 unless noted) 7 100kW 1 47kW ±0.1% 1 43kW 4 36kW ±0.1% 1 30kW 5 27kW ±0.1% 1 27kW 4 20kW ±0.1% 1 20kW 1 15kW 7 10kW ±0.1% 1 10kW 2 5.1kW 1 1.2kW ±0.1% A close-up of the test clips when not 1 1kW ±0.1% in use – the plastic colour matches the wire for easy 3 1kW identification. The test clips included in the kit may not be 1 510W exactly the same but will be similar. 2 33W 1 10W Capacitors (all 50V SMD X7R ceramic M2102/0805 unless noted) 9 100nF 7 10nF 6 10µF 16V Optional parts 1 10-pin 1.27mm pitch SMT header (CON4) [CNC Tech 3220-10-0300-00] 4 small self-adhesive rubber feet breakdown voltage of about 8.4V. In the case of the BJT, the forward and reverse gain is measured. IDUT(1-2) is the lesser of the two, so the emitter must be connected to the red lead. Once the primary identification is finished, the relevant measurements are made. The BJT has a base-emitter voltage of 0.74V at 1mA (so it is not a Darlington) and an hfe of 496 at a collector current of 8.7mA. The JFET has a pinch-off voltage of -3.1V. Implementation Kit (SC7725, $95 + P&P): includes an assembled PCB with all top-side components already fitted, plus all the other non-optional parts except the case, battery and label. The hardware is best understood with reference to the block diagram (Fig.4). The input leads are protected by a circuit that limits the voltage between any two leads to a safe level. This protection is mostly there to protect against ESD (electrostatic discharge) and perhaps a fleeting accidental connection to a low-voltage, low-power circuit. The Semiconductor Analyser is not designed to be used as an in-circuit tester; it may be damaged if connected to a powered circuit. The three input lines are connected to the test nodes by nine analog switches in a 3 × 3 matrix, shown as blue-filled circles in the block diagram. As mentioned above, the DUT voltages are measured directly at the test leads. The Power Node is very simple. It has to provide 12V or 0V to whichever lead it’s connected to and must be able to source and sink at least 10mA. The Load Node is a little more complex. A variable voltage source is created using one of the microcontroller’s two digital-to-analog converter (DAC) channels. This drives the Load Node via a 1kW load resistor. The microcontroller measures the voltages on either side of the 1kW series resistor, then subtracts and scales them to calculate a signed value for the load current. The Drive Node comprises three separate sources: two bipolar current sources and a voltage source. Since only one of them is used at a time, they are all driven from a single DAC output. Two current sources, high range and low range, are needed to measure the DC gain of bipolar transistors over a very wide range. At the lower end, we need to measure hfe in the low single digits (say 2) so we need a gate drive current of around ±5mA. The low current range Australia's electronics magazine siliconchip.com.au 32 Silicon Chip Fig.4: the block diagram of the Semiconductor Analyser. The blue-filled circles are analog switches. has a full-scale current of around 120μA. Circuit details The full circuit is shown in Fig.5 and you should be able to see how it relates to the block diagram. The approximate sections that correspond to the Power Node, Load Node and Drive Node are marked in green text. The input leads are protected by three bidirectional 12V TVS diodes connected between the leads (TVS1TVS3) and three diode pairs (D1-D3) that shunt away any voltages above the 12V rail or below ground. The three input lines are connected to the test nodes by nine analog switches out of 12 in three quad packages (IC1-IC3) in a 3 × 3 matrix. The three remaining analog switches are used in the Drive Node to select one of the three sources. I have used low-cost, industry-standard DG412 quad analog switches here. These have an on-resistance of around 40W, so the DUT voltage is measured directly at the test leads. The lead voltages are buffered by op-amp voltage followers (IC10a/b and IC6b) and reduced to a level suitable for the siliconchip.com.au ADC by simple voltage dividers. I have used ±0.1% resistors here because my design goal was to keep errors for all measurements to ±0.5% or better if I could manage it. The 1kW resistors in series with the buffers’ non-inverting inputs provide an extra layer of protection for the op amp’s internal ESD diodes. The Power Node is very simple. I used an LMC7101 op amp in a non-­ inverting amplifier configuration (IC4), driven from a digital output pin on the microcontroller. The gain Screen 2: the display when a 2N7002K N-channel Mosfet is connected. This is a logic-level device, as confirmed by the low Vgs(on) figure. Screen 3: here a 3.3V 1W zener diode is connected. The reverse breakdown voltage is just 2.5V because the 3.3V specification is at a higher current. Australia's electronics magazine September 2026  33 is set at around 3.9× so that any input above 3.1V will drive the op amp to saturation. If the digital output is at logic zero, the op amp’s output will be close to 0V. The absolute voltages at the Power Node are not critical, but we do want them to be reasonably stable with load. The LMC7101 was chosen for this application (and the Load and voltage Drive Nodes) because its common-­ mode input range includes both power rails and because it can drive to within 100mV of either rail while sourcing or sinking 10mA. The Load Node is a little more complex. A variable voltage source is created by a non-inverting amplifier (IC5) driven by one of the microcontroller’s two DAC channels. The gain-setting resistors are ±0.1% types, not because we need this precision for the load voltage, but because these resistors double as a voltage divider to measure the voltage on the op-amp end of the 1kW resistor. The voltage at the DUT end of this resistor is measured via a buffer op amp (IC6a) and another precision divider. The microcontroller subtracts the two unsigned ADC results corresponding to the voltage at either end to get a signed value for the load current. The Drive Node voltage source is a non-inverting amplifier similar to the one used in the Load Node (IC7), except this time there is no need for high-precision components. The two current sources are identical, based around dual op amps IC8 & IC9, except for the 47kW and 1.2kW current-setting resistors. The 12-bit resolution of the DAC means each current step of the high range (IC8) is about 1.2µA, but to maintain a minimum 1% gain precision, the lowest current we can use is 120µA. This corresponds to a DC gain of around 70×, which is why we need a second current range. With a full-scale current of 127µA, the low range has a resolution of around 31nA. The two ranges give us a DC gain resolution of ±1% or better from hfe values of 5 to around 2700, worsening to about ±9% at the upper limit (25,000) that I have imposed. Howland current sources These two circuits are an interesting configuration known as a Howland current source. They are used 34 Silicon Chip Australia's electronics magazine siliconchip.com.au Fig.5: the full circuit of the Semiconductor Analyser. Refer to the text for a complete explanation of how it works. siliconchip.com.au Australia's electronics magazine September 2026  35 measurements and averaging samples over one mains cycle almost completely eliminates mains interference from the measurements. The ADC and DACs are powered from a special 3.3VA analog supply rail that also feeds the Howland sources’ voltage dividers and the LCD driver chip. This supply is switched off entirely (along with the 12V rail) when the Semiconductor Analyser is off. Note the easy to miss cut-out on the bottom The absolute voltage of this analog of the case for the USB-C socket. The LCD supply rail is calculated each time module is mounted to the PCB using doublesided foam tape. the unit starts by reading the voltage of an internal bandgap reference and diagram, we can use Ohm’s Law to using a stored calibration reading. In derive the simple expression for Iout this way, we can convert the ADC and shown below it. If we substitute the DAC codes to absolute voltages with a difference amplifier gain expression known degree of precision. for Vout, we get the expression for outThe LCD screen is a low-cost put current at lower right. 240×320-pixel (QVGA) TFT available In the Semiconductor Analyser, I from the usual Chinese sources. An SPI have set the ratio of R2:R1 to 3.6 for (serial peripheral interface) interface is both circuits to maximise the voltage used to communicate with the LCD’s swing at the op amp’s output. In both ILI9341 driver chip, with a couple of cases, V2 is set to the midpoint of the additional GPIOs (general purpose I/O 3.3V analog supply by 20kW/20kW pins) required for control signals. The dividers. These are the equivalent integral backlight is PWM-controlled of 1.65V sources in series with 10kW via Mosfet Q1. resistors. Another PWM channel is used to The output current of the high-­ drive the audio transducer via Q2. current source will therefore vary Both of these circuits consume relahere because they can provide a vari- from -4.95mA when the DAC voltage tively large spikes of current, so they able bipolar current programmed by a is zero to +4.95mA when it is 3.3V. are both supplied directly from the unipolar control voltage. The circuit When the DAC output is at half-scale unregulated battery voltage. looks complex, but it is pretty easy to (1.65V), the output current will be Two tactile pushbuttons complete follow because it is based on the classic zero. The low-current circuit can like- the user interface. The TEST button difference amplifier, like that shown wise source or sink between ±126.4µA. is wired to a GPIO pin which has an at the top of Fig.6. additional ‘wake up’ function that is The expression for Vout at lower Control circuitry enabled when the device is switched left in Fig.6 is easy to work out using The Semiconductor Analyser is off. superposition. You just calculate the built around an ST32L433CCT6 microWhen the Semiconductor Analyser expression for Vout for each input sep- controller, IC11. This was chosen is off, the micro is put into its lowest arately (with the other two grounded) because it is a low-cost, low-power power configuration, known as shutand add them together. device but has a powerful M4 Cor- down mode. In this mode, almost With V2 and V3 grounded, the cir- tex core and 256kiB of flash memory. everything in the micro is powered cuit looks like a voltage divider with A good deal of flash is necessary to down, including almost all the periphthe input at V1 followed by a non-in- hold the graphics files to display on erals and even the RAM. The only verting amplifier. With V1 and V3 the LCD screen. things that can wake it are the realgrounded, it looks like an inverting I used two bits per pixel (four alpha/ time clock (which I don’t use) and a amplifier with the input being V2. transparency levels) for the images and few designated ‘wake up’ pins. With V1 and V2 grounded, the circuit font glyphs as a compromise between The measured power consumption looks like another voltage divider, size and image quality. The micro- in this mode is around 1µA, so the with input V3 followed by a non-in- controller comes in a 48-pin leaded battery life when the analyser is off is verting amplifier. Adding these results surface-mounting package, making it virtually unlimited. Because the RAM together gives the expression shown. relatively easy to hand-solder. All but is powered down in shutdown mode, The output is proportional to the two of the pins are used. waking up is the equivalent of restartdifference between V1 and V2, offset The micro includes a 12-bit DAC ing from a reset. by V3. In many difference amplifier and a 12-bit ADC with a built-in overThere is an optional programming circuits, V3 is grounded, and the off- sampler. The ADC oversampler is and debug header (CON4) in case you set term disappears. configured to average 256 individual want to connect a suitable STM32 proIf we add the buffer and sense samples over 20ms for each reading. gramming or debug probe. You don’t resistor, as shown in the right-hand Oversampling reduces noise in the need to load this header if you intend 36 Silicon Chip Australia's electronics magazine siliconchip.com.au to update firmware via the USB port or if you have a pre-programmed microcontroller. The micro’s USB device peripheral is connected to the USB connector via two 33W resistors to provide some basic protection against ESD discharges. The USB control channel pins (CC1 and CC2) are pulled down via 5.1kW resistors to ensure any USB Power Delivery source connected to the unit provides the default 5V bus voltage. The microprocessor senses the bus voltage via a 20kW/30kW voltage divider. If the bus is present, the sleep timer is inhibited, and the device remains on indefinitely unless the OFF button is pressed. TVS diode ZD4 and PPTC fuse F1 protect the device from accidental bus overvoltage or (less likely) reversed polarity. A MAX1555 linear Li-ion charger manages the charging of the single-cell 1100mAh battery. A yellow LED lights up when the cell is charging. The battery voltage is boosted to 12V via switching converter REG14. This is one of the simplest boost converter chips I have ever used, requiring just an inductor and one output capacitor. Two linear regulators provide the 3.3V digital and analog rails. As mentioned above, the 12V rail and the 3.3VA rail are only enabled when the unit is awake. The other 3.3V rail powers only the microcontroller core and is always on. All three regulators were chosen for their very low quiescent currents, which are included in the 1µA off consumption mentioned above. As a happy accident, they all come in hand-soldering-friendly SOT-23-5/6 packages. This is especially pleasing in the case of the boost converter, as most switching regulators seem to come only in tiny leadless packages that are a pain to solder. Device firmware upgrade The STM32L43xxx family of microcontrollers (along with many others from ST) contains a bootloader that can be used to download firmware without the need for a specific programmer or debugger. The bootloader code resides in a special area of flash, separate from the main user memory, and it can’t be modified by the user. Under normal operation, the microcontroller boots into the main flash siliconchip.com.au Fig.6: the Howland current source is a clever circuit that can produce a programmable bipolar current output. It is based on a classic difference amplifier. Fig.7: these overlay diagrams show where parts go on both sides of the PCB. Resistors with red labels are ±0.1% types; the rest can have a ±1% tolerance. The test lead wires loop through the strain-relief holes in the PCB and are soldered from the top side. and executes the user code from there. However, if the main flash is empty or the BOOT0 pin is pulled high when the microcontroller emerges from reset, the bootloader code is executed instead. The bootloader’s role is to download the user code through one of several serial interfaces (USART, CAN, USB, I2C, I3C or SPI) and load it into the flash memory. A specific Australia's electronics magazine communication protocol is defined for each interface. We are only interested in the USB interface, which uses a protocol known as “device firmware upgrade” or DFU. I will explain how to use DFU mode to load or upgrade firmware later. For now, it is enough to know that the BOOT0 pin of the microcontroller is normally pulled down by a resistor, but it can be pulled high by September 2026  37 bridging a pair of pads on the PCB (JP2) to enter DFU mode. Construction All the components except for the battery are mounted on a PCB coded 9062-1-C that measures 56.5 × 130.5mm. Most of the components are mounted on the top side of the board, with the exceptions being the LCD screen, pushbuttons, beeper and the battery charge LED. Start construction by mounting all the components on the top of the board. Make sure you observe polarity where relevant and watch where the ±0.1% resistors are placed. They are marked in red on the overlay diagram (Fig.7). While the parts are all spaced out fairly well and so could be fitted in any order, we suggest you start with the finer-pitch devices or those with many leads, such as the regulators and ICs, Fig.8: drill the front of the enclosure and the end panels according to this diagram. Don’t forget to snip out the two bosses inside the case as shown. then move on to the discrete semiconductors and passives. The advantage of doing it that way is that you have a little more room to work on the more difficult devices. Various soldering methods could be used, including an IR reflow oven, hotplate, hot air wand, or regular soldering iron. If using a regular soldering iron, apply a little flux paste to the pads before placing each part, then check its orientation carefully after placing it and tack-solder one pin. Check the orientation and positioning again and adjust it if necessary. Once you are happy that all leads are centred over the correct pads, tack another pin, then add a little more flux paste on top of the leads. You can then either drag-solder the remaining leads or solder them one at a time with a small amount of solder on the tip of a clean iron. Make sure you don’t touch the two initial joints holding the part in place until more joints have solidified. Also be sure to refresh those initial joints with a little flux paste and some extra heat from the iron to ensure they have flowed correctly. If any pins are bridged during soldering, simply add more flux paste and use a little solder-wicking braid pressed down by the tip of a hot iron to draw the excess solder away. Flux is your friend when soldering finepitch devices. The USB connector has both SMT and through-hole pads. If you solder the through-hole pins first, it helps to locate the SMT pins, making it much easier to solder. It is pretty easy to accidentally create a solder bridge in the microcontroller of USB connector leads, so check these carefully under magnification and clean up any possible bridges with solder wick and plenty of flux. It’s easier to see possible bridges after soldering if you clean away the flux residue (eg, using isopropyl alcohol and a lint-free cloth or nylon brush). Once you have fitted all the components to the top of the board, it’s time to prepare the case. Case preparation Next, prepare the case according to Fig.8. Mark out the top of the case carefully, then remove the two bosses on the inside of the case top as shown 38 Silicon Chip Australia's electronics magazine siliconchip.com.au in the figure. These coincide almost exactly with the locations of the tactile switches, so will interfere with drilling their holes if not removed. It is sufficient to just snip them off with side cutters so they are more-or-less flush with the inside of the case. Drill the holes and cut out the display window. I added a chamfer to the edges of the display opening, but that is purely cosmetic, so it’s entirely optional. The enclosure is supplied with both flat and profiled end plates – we only use the flat ones. I made the three lead holes 2mm in diameter because that suited the test leads I was planning to use. You may have to adjust this to suit the leads you intend to use. The USB slot is best made by drilling two holes as shown and cutting out the material between them with a sharp blade. Once the case is ready, you can fit the components on the bottom side of the PCB. It is best to fit the LCD screen first. Place the LCD face-down on the bottom of the board with the end of the flat flex aligned vertically with the horizontal lines on the overlay. Make sure the contacts are aligned with the pads, and the LCD is parallel with the edges of the PCB. Use a small piece of tape to temporarily hold the flat flex in position, then carefully solder each connection. Once you have finished that, fold the display up into its final position and temporarily secure it to the board with sticky tape. The display will be finally fixed down with double-sided foam-core tape, but I suggest testing everything first. Accessing LCD terminations is difficult to impossible once it is permanently affixed. Now you can add the pushbuttons, making sure to use the marks on the PCB silkscreen to centre the switches’ actuators so they line up with the holes in the enclosure. Solder in the audio transducer next, then thread the LED through the appropriate holes, taking care to get the polarity right (the cathode goes toward the USB connector), but don’t solder it just yet. Fit the PCB assembly into the case and secure with self-tapping screws. You can now push the LED through the hole in the case just far enough that the domed part at the top is just proud of the front panel. When you are happy, you can solder the leads siliconchip.com.au Screen 4: the free STM32CubeProgrammer app can be used to download the firmware to the Semiconductor Analyser via its USB port. Fig.9: a simplified 3D view of the finished board, showing how the wires, LCD screen, LED and other parts are fitted. on the top side of the board and trim off the excess. Testing and programming We are just about ready to fire up the unit and get it working. The best way to start is to connect a current-limited power supply to CON6, set to 4V DC. The correct polarity is marked on the overlay diagram in Fig.7. A current limit of 150mA is a good place to start. Fire up the supply and use a multimeter to measure the 3.3V rail, across C18 (the ceramic chip capacitor between REG16 and REG15) or somewhere else convenient. If the 3.3V rail is good, you can go ahead and connect the battery. Check that the battery charge light (LED1) comes on when the USB port is connected to a power source. If you have a pre-programmed microcontroller, the device should come to life, and you can skip the next section. If you are programming your unit via Australia's electronics magazine the debug header, now is the time to do it. If you plan to use DFU mode, you can follow the steps outlined below. 1. Download the STM32CubeProgrammer application. This is a free download (registration required, unfortunately) from the ST website (www.st.com/en/development-tools/ stm32cubeprog.html). It is available for Windows, macOS and Linux. 2. Unplug the Semiconductor Analyser from the USB cable and disconnect the battery. Solder a short piece of wire or an M2012/0805 0W resistor across the pads labelled “DFU” on the board. This forces the microcontroller to boot into DFU mode. You can skip this if you have an unprogrammed chip – it should boot into DFU mode by default. 3. Reconnect the battery and connect the USB cable to your computer. Open the Programmer application and select USB from the dropdown at the top right of the screen (see Screen 4). September 2026  39 Screen 5 (left): testing a TTC004B 1.5A NPN bipolar junction transistor (BJT). The hfe is specified as 140-280 at 100mA and this one falls close to the middle of that range at nearly 9mA. Screen 6 (middle): the LCD screen shows this display while tests are being run. They take a second or two. Screen 7 (right): this splash screen appears when you press the “TEST” button. Click “Connect”. The programmer should connect and read the device info into the “Target information” panel at the bottom right. 4. Click on the “Open File” tab and navigate to the ELF file containing the code (available as part of the download package from siliconchip.au/ Shop/6/3644). Click on the “Download” button on the top right of this tab and the flash will be erased and programmed. You may get the error messages “Failed to Download Sector[0]” and “failed to Download the File”, but you can ignore them. They appear to be caused by a bug in the programming software that occurs when the code to be flashed occupies more than 50% of the available space. I have not been able to find a fix online, although there are a few posts describing this same problem. 5. Close the programmer, unplug the USB cable and the battery, then remove the shorting link if you used it. Once you plug the battery back in, the device should boot into the uploaded firmware. That’s all there is to it. Finalising assembly Once the device reboots, it should display the version screen briefly, then commence a test cycle. The test should result in an error message saying that all three test leads are open circuit. If everything is satisfactory, you can finish the assembly. Remove the PCB from the case and permanently affix the LCD screen to the PCB using a few pieces of double-­sided adhesive foam tape. The stuff I used is about 3mm thick, and this positions the display firmly up against the front panel. Don’t forget to remove the protective film from the face of the LCD. You can now connect the test leads to the board. These are fed through the end panel, then from the top side of the board, down through the strain-relief holes and soldered in from the top side of the PCB as shown in Fig.9. I made my test leads about 350mm long, although the leads that come pre-attached to the clips supplied in our kits will be closer to 200mm long, which is still plenty. Apply the label to the front panel (Fig.10). The artwork is available for download from siliconchip.au/ Shop/11/3645 I printed mine on self-adhesive glossy paper and covered it with transparent adhesive vinyl film. Use a sharp blade and a straightedge to cut out the display window before applying the label. The label has two rectangles for the display window. Use the outer one if you chamfered the display opening, or the inner one if you did not. The holes for the pushbutton actuators and LED are best made with the label in place. You do not need to cut the label over the hole between the two pushbuttons – the hole in the case is enough for the beeper sound to escape. Finally, you can fix the Li-Po cell to the inside base of the case using double-sided tape. Once you’ve put the case together your Semiconductor Analyser is ready to use. If you run into trouble, you can use the command-line interface for troubleshooting. Connect the USB cable to your computer and fire up a serial terminal program, then connect to the port associated with the USB device. If you type “help” at the “>” prompt, you will see a list of commands you can use to manually control the switch matrix, Power, Load and Drive Nodes, and read the load current or test lead voltages. The CLI is basic, but it has autocomplete for the command (press the tab key) and you can use backspace SC to correct mistakes. Fig.10: the front panel label artwork. This can also be downloaded as a PDF from the Silicon Chip website (the link is in the text). 40 Silicon Chip Australia's electronics magazine siliconchip.com.au TECH & GADGETS Shop 24/7 <at> altronics.com.au Get the latest gear at a great price. C 5065 Gold C 5068 Black NEW! 359 $ Introducing HiFuture® Passion X The premium home speaker you can take on the go! Delivering amazing sound with a touch of luxury - all wrapped in a leather textured finish with metallic trim. 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See latest catalogue for freight rates. *Devices for illustration pursposes only. CIRCUIT NOTEBOOK Interesting circuit ideas which we have checked but not built and tested. Contributions will be paid for at standard rates. All submissions should include full name, address & phone number. Remote Control Tester I occasionally repair electronics for friends, relatives and neighbours. One common problem is malfunctioning remote controls. This simple design is a quick way to test the functioning of all the keys, including intermittent faults. Place it about 30-40cm from the remote and it will flash at roughly 3Hz while a key is pressed. I’m not sure what type of photodiode was used as all the components were salvaged. Probably a BPV10NF or OP999 would do, but there are plenty of options on eBay. The 3.9kW resistor was arrived at empirically, so the value might need a bit of tweaking for different types. When troubleshooting, hook up a 3V source to the remote control using alligator clips. I use a 6mm dowel with a disc of carbon-impregnated anti-static mat offcut glued to the base, but a chopstick and kitchen alfoil would do. Some of the cheap-and-nasty units have an oily layer on the PCB, possibly a breakdown of the keyboard membrane, which should be soaked up with a tissue, then gently clean the contact points and the membrane actuators with cotton buds and alcohol. Another common problem with these units is too much resistance between the contact point of the copper track and the overlying carbon track. 1-2kW is the expected value; 5-6kW is marginal and more than 10kW likely indicates a failure. It can be repaired by scraping the copper track to bare metal next to the carbon contact and applying wire glue or silver conducting paint (available from Jaycar) with a toothpick and a tattooing motion. Note that it will have to dry before it becomes conductive and it may affect more than one key. Frank Murray, Downer, ACT ($45). Editor’s note: most smartphone cameras will also pick up the flashing from an IR transmitter LED. Bluetooth selfie camera using a Raspberry Pi Zero This camera system uses a Raspberry Pi Zero, allowing an image capture to be initiated wirelessly using a compact handheld Bluetooth remote. The system consists of a Raspberry Pi Zero connected to a camera module, an OLED display for status indication, a Bluetooth remote acting as a trigger, a GPIO-based shutdown button and an indicator LED. The Pi Zero can be connected to a WiFi network for downloading the captured images etc (eg, over ssh/ sftp) but the WiFi connection is not a prerequisite for this project. The OLED shows the system status such as readiness, the number of images captured and the IP address (if available). A dedicated button (S1) enables safe system shutdown when held for a specified duration. We have incorporated debounce logic to prevent multiple captures from a single button press. You can see a demonstration of siliconchip.com.au the camera capturing images via remote control at https://youtube. com/shorts/X8CwCNk9Uoc Two different types of Bluetooth ‘selfie buttons’ were evaluated for triggering a Raspberry Pi Zerobased camera system. Although both devices worked seamlessly with mobile phone cameras, their behaviour on Linux differed significantly due to underlying Bluetooth profiles. The smaller remote, mostly supplied with ‘selfie sticks’, operates Australia's electronics magazine as a generic BLE (Bluetooth Low Energy) device using a custom GATT (Generic Attribute Profile) service. Thus, it does not appear as an input device (/dev/input/eventX) and so requires complex BLE characteristic-­ level programming for integration. In contrast, the larger device (AB Shutter3; www.amazon.com.au/dp/ B0G13CLYY5) functioned as a Bluetooth Human Interface Device (HID), effectively behaving like a wireless keyboard. It generated standard key events such as KEY_VOLUMEUP and KEY_VOLUMEDOWN, which were readily detectable via /dev/ input/eventX. This allowed straightforward integration using Python’s evdev library, enabling reliable and low-latency triggering of the camera. While BLE devices may appear similar in function, only those implementing the HID profile offer plug-and-play compatibility with Linux-based embedded systems. September 2026  45 Therefore, HID-based remotes are strongly recommended for such applications. Briefly, to set up such a device on a Raspberry Pi, you can run these commands: $> bluetoothctl [bluetooth]# scan on You will start seeing devices like: [bluetooth]# Device 58:2B:67:FF:6D:77 AB Shutter3 You can then run: [bluetooth]# scan off [bluetooth]# pair 58:2B:67:FF:6D:77 You will see something like: Pairing successful Then run: [bluetooth]# trust 58:2B:67:FF:6D:77 [bluetooth]# connect 58:2B:67:FF:6D:77 This ensures auto-reconnection and no repeated pairing prompts. Then you can run: [bluetooth]# info 58:2B:67:FF:6D:77 Paired: yes Trusted: yes Connected: yes UUID: Human Interface Device (00001812-...) The last line confirms it behaves like a keyboard (HID). Now let’s identify the associated input device: $> cat /proc/bus/input/devices … N: Name=”AB Shutter3 Consumer Control” P: Phys=b8:27:eb:96:29:87 S: Sysfs=/devices/virtual/misc/ uhid/0005:FFFF:0001.0002/input/ input3 U: Uniq=58:2b:67:ff:6d:77 H: Handlers=kbd event1 H: Handlers=sysrq kbd event2 We should use only the kbd interface, which in this case maps to /dev/input/event1. Now that the device is paired, we can run: $> sudo evtest /dev/input/event1 Circuit Ideas Wanted 46 Now press a button on the remote and watch the terminal. Most likely, you will see something like: KEY_VOLUMEDOWN KEY_VOLUMEUP In our Python code, we call the bluetoothctl process to connect to the remote control, then open /dev/input/event1 and look for the KEY_VOLUMEDOWN and KEY_ VOLUMEUP keypress events. This event monitoring is done using the evdev library. Your user needs access to the /dev/input/event1 virtual file to listen for these events, which can be achieved by either changing the ownership of that file to your user, adding your user to the group that owns the file or changing the permission on that file so that all users can read it. When the code detects the button press, it captures an image using the libcamera-still library and stores it in a local file. Visual feedback is provided by flashing the LED and updating the contents of the OLED display. The camera is connected to the Pi Zero with a CSI cable. The 5V power supply can come from a USB power bank or charger. As well as my Python code (which can be downloaded online from siliconchip.au/Shop/6/3642), you also need the following installed on the Pi Zero: Raspberry Pi OS (Lite version recommended), Python 3, OpenCV, NumPy, evdev, luma.oled (to drive the OLED screen) and libcamera utilities. All are open source software. So that the device is ready to go at power-on, we put the following code to the end of the “.profile” file in your main user’s directory (/home/bera/ .profile in my case): if [ -n “$SSH_CLIENT” ] || [ -n “$SSH_TTY” ]; then echo “SSH session detected - skipping autostart” else sleep 5 python3 /home/bera/bluetooth_ shutter.py fi Bera Somnath, Kolkata, India. ($85) Got an interesting original circuit that you have cleverly devised? We will pay good money to feature it in Circuit Notebook. We can pay you by electronic funds transfer, credit or direct to your PayPal account. Or you can use the funds to purchase anything from the Silicon Chip Online Store. Email your circuit and descriptive text to editor<at>siliconchip.com.au Silicon Chip Australia's electronics magazine siliconchip.com.au Magnetic power switch siliconchip.com.au 1 double-sided PCB (see text for source) 1 Li-ion cell with JST-PH connector 1 S2B-PH-SM4-TB JST-PH battery connector 1 AH1392-HK4-7 magnetic sensor, DFN1410 (HS1) 1 MCP73831T-2ACI/OT Li-ion cell charger IC, SOT-23-5 (IC1) 2 BCR402W current regulators, SOT-343 (IC2, IC3) 1 DMP1045UQ P-channel Mosfet, SOT-23 (Q1) 1 BSS138 N-channel Mosfet, SOT-23 (Q2) 2 BAT20JFILM schottky diodes, SOD-323 (D1, D2) 1 red high-brightness LED, M1608/0603 side emitting (LED1) 1 green high-brightness LED, M1608/0603 (LED2) 1 Micro Type-B USB connector [CUI UJ2-MIBH-G-SMT-TR] 3 4.7μF multi-layer ceramic capacitors, M2012/0805 size 1 22pF ceramic capacitor, M2012/0805 size 1 22kW resistor, M1603/0603 size 2 5.1kW resistors, M1603/0603 size the charging current suitable for small cells, below 400mAh. The green LED comes on to indicate charging and goes off when fully charged. The two LEDs have BCR402W current regulators instead of simple resistors. If you want a completely waterproof enclosure with no holes, omit the USB and charging circuit. The maximum voltage for the AH1392 is 6V, so this circuit is not suitable for 2S batteries, which go up to 8.4V during charging. The AH1392 is available in two packages. The largest one is 1.4mm × 1.0mm and it can be soldered with solder paste and hot air. After reflow, check with a magnifier or microscope that the solder has wetted all four pins on the sides of the package. Other variants of this chip hide their connections underneath so they can't be inspected without an X-ray machine. A PCB for this project is available from OSHPark at siliconchip.au/link/ acav – a 0.8mm-thick PCB is best for space-constrained applications. You can view a YouTube video demonstrating this project at https:// youtu.be/WZ68dKLBOh0 It’s ideal to test with a current-limited Australia's electronics magazine Parts list for the magnetic power switch This magnetic switch doesn't require a hole in the side of your project box, so it can be weatherproof, and it’s solid-state so reliable for millions of actuations. The PCB was built to fit into a 29mm diameter model rocket, to switch power to the onboard electronics. It has since been used in a variety of projects that needed a concealed power switch and USB charging. The circuit uses an AH1392 Hall effect sensor from Diodes Inc. to switch the output of a single-cell Li-ion battery. This sensor has two outputs (pins 3 & 4), sensitive to different magnetic orientations. Place a north pole above the sensor pointing down into the PCB and output 1 will go low to switch on Q1, a P-channel power Mosfet. Take away the magnet and it stays latched on due to Q2, an N-channel Mosfet. A south pole applied to the sensor will switch it off by pulling Q2's gate low. The magnetic field around the sides of the magnet can be used, so placing the magnet adjacent to the sensor with north up will give enough field strength at the sensor to switch it on. The capacitor at Q1’s gate ensures that the circuit starts in the off state when a battery is first connected. The parallel resistor keeps it discharged, while the other 5.1kW resistor keeps Q2 on once the switch has been activated. The 22pF capacitor assists in switch-off. Both outputs of the AH1392 are push-pull types, but this circuit prefers an open-drain configuration, since the gate of Q1 must be held low while the pin 4 output is high for ‘no magnet present’. To accomplish this, schottky diodes D3 & D4 prevent the AH1392 outputs from sourcing current. A battery-charging circuit is included so the attached battery may be charged from a USB power supply. This uses an MCP73831 charge controller chip. The 22kW resistor makes power supply set to 20mA and 3V. If you don't have one, connect a small Li-ion cell but be ready to unplug it in case it starts smoking. Double-­double check the battery polarity before plugging it in. Apply the north pole of a magnet to the sensor and the power LED should come on. Apply south and it should switch off. If it doesn't work, start by tracing the signals out of the sensor at the cathodes of D3 and D4 using your multimeter. They should both be high when no magnet is present. Test the USB charging by checking the green charge light comes on when USB is plugged in. It should go off when the battery finishes charging at 4.2V. The continuous output current can be up to 4A, which would discharge a typical 400mAh battery in six minutes. There is no protection for over-­ discharge, so use a cell with built-in protection if that is likely to be a problem for your usage pattern. Current consumption when off is about 10μA, mostly due to the charge circuit sensing the battery. Morgan Sandercock, Minden, Nevada, USA. ($120) September 2026  47 How Induction Motors Work Induction motors power many devices, from industrial applications like lathes and mills to domestic fans, pool pumps and more. We describe their clever design and the differences between them. If you’re considering building the VSD described in late 2024, this article will help determine if it suits your induction motor. By Andrew Levido F ig.1 shows a cross-section through the stator of a simplified threephase induction motor. The stator, made of laminated steel sheets (like a transformer core), is equipped with three axial windings, represented by the red, blue and green circles. The direction of the winding (into or out of the page) is indicated by a cross or a dot, respectively. As current flows in a winding, a magnetic field is produced along the dotted axis, shown in the same colour as the winding. The strength and direction of the field depend on the instantaneous magnitude and polarity of the current. At time A, for example, the red phase current is at its positive peak, while the blue and green phase Fig.1: a rotating magnetic field is produced in the stator of a three-phase induction motor by placing the windings at 60° intervals around the rotor. The black arrow is the vector sum of the fields produced by the windings. that rotates smoothly around the stator once for each mains cycle. The rotational speed of the flux vector in this arrangement of windings (called a two-pole configuration) is 50 revolutions per second, assuming 50Hz mains – equivalent to 3000 RPM. This is known as the motor’s synchronous speed. By interleaving additional sets of three windings, lower synchronous speeds can be achieved, such as 1500 RPM for a four-pole motor or 1000 RPM for a six-pole motor. The rotor of an induction motor is also made of laminated steel, as shown in Photo 1. You can just make out a series of longitudinal slots in its surface, into which aluminium bars have been cast to form the windings. These bars are shorted together at each end of the rotor by thick rings of cast aluminium. This type of motor is sometimes referred to as a ‘squirrel cage’ induction motor since the arrangement of bars and rings resembles a cylindrical cage. The rotor bars are skewed slightly to ensure smooth rotation. Without this skew, the motor would exhibit noticeable ‘cogging’ as it rotates, similar to a stepper motor. The rotating stator field induces a current in these rotor bars by transformer action. In this sense, the induction motor can be considered a kind of rotating transformer with shorted secondary turns. When the rotor is stationary, the current induced in the rotor can be huge, as you might expect with a shorted transformer. The rotor currents create their own magnetic field, which interacts with the rotating stator field to produce a strong torque that sets the rotor moving. Initially, the frequency Australia's electronics magazine siliconchip.com.au 48 Silicon Chip currents are half the maximum magnitude and negative. The magnetic fields these currents produce are shown as vectors (arrows indicating magnitude and direction) of the appropriate colour on the diagram. The net magnetic field, which is the sum of the three coloured vectors, is indicated by the heavy black vector. At point B in the waveform (onesixth of a cycle or 60° later), the green phase current will be at its maximum negative excursion, while the red and blue phase currents will be positive with 50% of the maximum magnitude. This results in a net flux vector, shown at B. It works similarly for point C and so on. The three-phase winding therefore produces a net magnetic field vector of the rotor current is the same as that in the stator. However, as the rotor accelerates, the frequency and level of the rotor current begin to drop because the rotational speed of the stator field, seen from the rotor’s perspective, reduces as the rotor ‘catches up’ to it in speed. If the rotor could reach the synchronous speed, the stator field would appear stationary to the rotor. There would be no induced rotor current and consequently, no torque. The induction motor rotor therefore settles down to a speed just a little lower than the synchronous speed where the diminishing torque produced by the rotor-stator field interaction is balanced by the torque required by the load. Fig.2: the operating point of an induction motor is the intersection between the motor’s torque-speed characteristic (red curve) and that of the load. Slip The difference between the synchronous and rotor speeds is known as the slip. Slip can be described as an absolute frequency (the difference between the rotor and stator current frequencies) or as a percentage of the synchronous frequency. The typical slip for an unloaded three-phase induction motor is just a few percent, so the no-load speed of a typical two-pole motor might be 2900 RPM. That would correspond to a slip frequency of about 1.7Hz. The slip is typically 5-10% at full load, so around 2700 RPM for our example. The red curve in Fig.2 shows the torque-speed characteristic of a typical three-phase induction motor. The blue and green dotted lines represent the torque-speed characteristics of two common types of load – a constant-­ torque load, such as a conveyor, and a square-law load, such as a fan. The motor’s operating point is at the intersection of the motor and load curves. The no-load operating point is also shown for reference. This is close to the synchronous speed, since the only load torque on the motor is produced by the rotor friction and windage. The motor’s speed regulation is defined by the slope of the leading edge of the motor’s torque-speed curve. A stable operating point can only occur on the “leading edge” of the torque speed curve, where the torque provided by the motor is decreasing with increasing shaft speed. If the operating point reaches the crest of the curve (the “pull-out” torque), the motor will stall. The motor’s rated siliconchip.com.au Fig.3: a single-phase induction motor stator has just one winding, so it produces a pulsating rather than rotating magnetic field. Photo 1: the windings on an induction motor’s rotor are aluminium bars cast into slots that run the length of the rotor. These bars are short-circuited at each end by aluminium rings (shown here with cooling ‘studs’). The bars are skewed to ensure smooth torque production. Source: https://w.wiki/AxgX full-load torque is therefore somewhere well below this point. Single-phase induction motors Single-phase induction motors work on the same principle but only Australia's electronics magazine have one stator winding, as shown in Fig.3. During the positive half-cycle of the stator current, the field points to the right and increases from zero to some peak at point A before reducing again to zero (point B). In the negative half-cycle, the same thing September 2026  49 happens but in the opposite direction (point C). Thus, a pulsating rather than rotating magnetic field is produced. While a current can still be induced in the rotor winding by transformer action, there is no rotation of either the field vector or the rotor field at a standstill, so no torque is produced on the rotor. If the rotor is moving, however, the pulsating stator field appears to rotate relative to the rotor, and a torque is produced. This torque-speed characteristic is shown in Fig.4. There is zero torque at the origin, but once the rotor is moving, it will accelerate to some operating point, just like in the three-phase example. It can rotate in either direction – the direction of rotation depends on the direction of the initial starting torque. The diagram shows that the size of the starting ‘kick’ required depends on the load type. The fan requires the rotor to be just barely rotating to generate more torque than the load requires. In the case of the constant-torque load, the rotor must be spinning at almost half the synchronous speed before it becomes self-sustaining. Single-phase induction motors use various techniques to generate this starting torque, as shown in Fig.5. Shaded-pole motors (purple curve) use a shorted turn on the stator to distort the magnetic field to create a modest starting torque. You can see that this is enough to get things moving, but it does not provide much low-speed torque, so these motors are usually limited to easyto-start loads like fans. Shaded pole motors are not very efficient (typically no more than 30%), so they are generally used for motors of just a few hundred watts at most. A separate start winding fed via a capacitor can achieve better starting torque. The capacitor introduces some phase shift in the start winding with respect to the run winding, creating a reasonable start torque. In the ‘permanent split capacitor’ (PSC) motor, the start winding and its series capacitor are permanently connected in parallel with the run winding. This is shown on the orange curve in Fig.5. For loads requiring even higher starting torque, like cement mixers, a large start winding current is necessary. This is supplied through a capacitor, as for the PSC motor. However, since the start winding cannot sustain so much current indefinitely, a centrifugal switch is used to switch it out of circuit once the motor reaches about 70% of full speed. This is called a ‘capacitor-start motor’; its torque curve is shown in dark blue. Not shown on the diagram is a variation on this theme: the capacitor start/ run motor, which has two capacitors and a centrifugal switch. At start-up, both capacitors are connected in parallel to drive the start winding with a very high current. When the centrifugal switch Fig.4: the torque-speed characteristic of a single-phase induction motor shows that there is no torque at a standstill, but as the motor rotates, a torque is produced. Thus, an initial ‘kick’ is required to get the motor moving; the direction of the kick determines the direction of rotation. 50 Silicon Chip Australia's electronics magazine opens, one of the capacitors is disconnected, and the start winding current is reduced to a level that can be sustained indefinitely. Motors with a centrifugal switch are usually not suitable for use with a variable speed drive (VSD), such as the design described in the November & December 2024 issues (siliconchip. au/Series/430). If the motor is run at less than full speed, the centrifugal switch may never open and the start winding can burn out. If the speed of such a motor is to be controlled, the range of possible speeds may need to be limited to those above which the centrifugal switch opens (more on this later). Speed control For many types of motor, such as DC or universal types, varying the speed can be as simple as reducing the voltage. However, that is not very effective for induction motors; Fig.6 shows why. Varying the voltage fed to an induction motor gives a very limited speed control range and poor torque at lower speeds, which is worse for constant torque loads. That makes sense because the synchronous speed is locked to the mains frequency. On the other hand, varying the supply frequency gives a very wide range of speed control and pretty good torque over that range. It should be noted here that we must also reduce the voltage with the frequency to avoid saturating the motor. It turns out we have to reduce the voltage more-or-less linearly with frequency, so a 50Hz 230V motor running at 25Hz requires the application of about 115V. The exception is at very low frequencies, when the fixed voltage drop across the motor winding resistance(s) means we may need to boost the voltage slightly to produce the same flux density and therefore torque. So ideally, an induction motor speed controller should produce a sinusoidal voltage that can vary in frequency from something less than 1Hz to 50Hz at a voltage between 0V and 230V RMS. Most modern solid-state induction motor controllers (including the one described in 2024) approximate that using pulse-width modulation (PWM) to synthesise sinewave(s) from a DC bus derived by rectifying the mains. siliconchip.com.au Controlling a motor with a centrifugal switch While this should be approached with caution, it is possible to control the speed of an induction motor with a centrifugal switch with some provisos. The first approach is to limit the range of the speed control potentiometer so that the motor will always ramp up to the speed range where the start winding is switched out. This could be done by placing a trimpot in series with the track of the speed control pot, between it and ground. This pot would be adjusted so that the centrifugal switch disconnects the start winding even with the speed control pot at its minimum setting. That will, of course, restrict the range of speeds you can run the motor at, but it will at least allow some control, and as long as it’s adjusted properly, there should be no risk of the winding burning out. You will need to check that the minimum speed is sufficient under load, though. The other approach only works if you can access the wiring to the centrifugal switch and involves some extra hardware. Usually, the switch is switching in an external capacitor, so you will usually be able to access the wiring. In this case, you could use a mainsrated relay instead of the centrifugal switch to energise the start winding. The wiring and insulation would need to be done safely for mains voltages. The relay then needs a control circuit, likely a microcontroller. This microcontroller would monitor the motor speed (eg, by measuring the frequency of a reduced voltage version of the AC waveform powering the motor). The microcontroller would energise the start winding when it first detects power being applied to the motor, and it would switch it off once it reached a sufficient speed, or after a set timeout (eg, 30 seconds). The microcontroller should measure the time the start winding is energised, multiply that by a safety factor (eg, five), and refuse to re-energise the start winding for that many seconds after it’s switched off, to give it time to cool down. Note that in this case, if the motor was set to run at a low speed, it’s possible it could stall when the start winding switches off. That’s something the operator would need to be SC aware of. siliconchip.com.au Fig.5: the torque-speed curves of a single-phase induction motor varies depending on its type. Shaded-pole motors are the simplest but are really only suitable for fans. Capacitor start is used when a motor needs lots of starting torque. Fig.6: reducing the voltage is not a very effective way of controlling the speed of an induction motor since its synchronous speed is locked to the mains frequency. Fig.7: varying the mains frequency is far more effective – giving a very wide speed control range. Usually, a VSD will vary both the voltage and frequency applied to the motor together. Australia's electronics magazine September 2026  51 By Charles Kosina, VK3BAR Low-power, digitally-controlled FM TRANSMITTER » Short-range stereo FM transmitter tuneable from 87.5MHz to 108MHz in 100kHz/1MHz steps » Variable RF output in eight steps, under the 25μW legal limit with the specified wire antenna » Variable audio gain in eight steps » Usable range: about 10m » Power: various battery options, 1-3 cells, primary or rechargeable » Current consumption: about 26mA with a 4.2V battery » Optional integrated battery charger » A small OLED screen to show the status » Inexpensive and straightforward to build This small FM broadcast-band transmitter has frequency, gain and output power controls. It shows the current frequency and other settings on a small OLED and runs from a battery of three AA cells or similar. T his FM Transmitter uses just a few parts as it’s based on a small, inexpensive, digitally controlled stereo FM transmitter module. The only major parts required in addition to that are an Atmel microcontroller, a small OLED screen, a small switchmode boost module, a rotary encoder and two potentiometers. Such a module allows you to test FM radios, play music remotely, or use an FM radio as a basic type of intercom. The transmitter module uses a KT0803L chip and is not expensive; you can order it from the AliExpress link in the parts list or our Online Shop. However, it is not a ‘plug and play’ device. It has 18 8-bit registers that need to be set up each time it’s powered on. These are all accessed over an I2C two-wire serial bus. To choose an appropriate microcontroller, we need to consider how many I/O pins are required. The rotary shaft encoder requires three pins, the I2C interface needs two, the power and audio level pots need two analog inputs, plus one for battery voltage measurement. That’s a total of eight I/O pins needed. There are plenty of inexpensive microcontrollers that will handle that. I chose the ATMega328 in a 52 Silicon Chip 32-pin TQFP package as I have several available but the ATMega168 or even ATMega8 can be used as well. They all have more than enough pins for the task. To expand the battery choices, I added a boost converter module, which gives a 5V DC output for an input ranging from 1.5V to 4.5V. This means it can run from two or three AA alkaline cells (one would work, but it would be ‘flat’ very quickly). The advantage of using three over two is that they can be discharged down to 0.5V each. Cells that have been discarded from other equipment can be used. Rechargeable batteries can be used too, either three 1.2V rechargeable NiMH cells or a single Lithium-­ ion cell. In both cases, they can be recharged from an external 5V supply. The current drain with a 4.2V battery is 26mA. As battery voltage goes down, the current drain will rise in proportion. If using alkaline cells, once each gets below 1V, it will be discharged fairly rapidly. This project’s inspiration This project was inspired by a competition being run by the Historical Radio Society of Australia (HRSA) Australia's electronics magazine to build a low-power AM transmitter using valves. The original design was published in Electronics Australia in May 1989. Such a device could be used to connect to a sound source and transmit to nearby AM receivers on an otherwise unused frequency. The May 1989 circuit is shown in Fig.1. It is an extremely simple design: two inputs are mixed and amplified by a triode, which is coupled to a second triode that modulates the third triode acting as an AM broadcast band oscillator. It obviously works, but there are limitations. The antenna is connected to the oscillator coil and would detune the frequency. The oscillator coil has to be wound with about 100 turns. Great care has to be taken working with a circuit running at such a high voltage (250V). I briefly considered building it as I have a sufficient stock of valves in shoe boxes. But these days, I no longer have the high-voltage capacitors and 1-2W resistors, as most of my designs run from low voltages (typically 3.3V or 5V) and use surface-mount components. I thought about making a solid-state transmitter on the FM broadcast band rather than AM. I quickly sketched up siliconchip.com.au Fig.1: the May 1989 low-power AM transmitter circuit that inspired this project. Triodes V2a & V2b are stacked in series; the HT supply passes through the secondary of T1 to the anode of V2a, then through V2 to its cathode and onto V2b’s anode. So V2b’s anode current comes from V2a’s cathode. Thus, the audio signal at V2b’s grid modulates V2a’s supply voltage and so the output amplitude. Fig.2: the analog FM transmitter circuit I originally considered building before I settled on the digitally controlled version. Lacking a stereo modulator, it necessarily mixed the two input channels down to mono and would have been fiddly to set up. such a transmitter using a couple of transistors, which is shown in Fig.2. The oscillator is a JFET and its output is amplified by an NPN transistor. A pi-coupler on the output is connected to an antenna. Two inputs are provided, which could be two channels of a stereo source. They are applied to a varicap to produce the frequency modulation. I went as far as building a prototype of this circuit, but abandoned it for the following reasons. Setting the frequency with a slug in the oscillator coil would have been a fiddly job, and getting the right level of modulation would require a lot of trial-and-error. Also, it would only produce a mono FM signal; one of the benefits of FM radio is that it supports stereo encoding. I then thought about the fact that there are plenty of FM transmitters cheaply available at various shops, designed to plug into a car power socket, for listening to music in a car siliconchip.com.au with an FM radio. I could use one of those ‘off the shelf’. Looking on AliExpress, I came across a very simple module that transmits a stereo FM signal, leading to this project. Circuit details The full circuit is shown in Fig.2. There are three main parts: an FM transmitter module (MOD1), a microcontroller to configure it and provide the user interface (IC1) and a power supply (REG1). Both the FM transmitter (MOD1) and OLED screen (OLED1) are controlled over a shared I2C bus using two wires: SDA (data) and SCL (clock). These are driven by the hardware I2C interface within the microcontroller. An 8MHz crystal has been provided, connected between pins 7 (XTAL1) and 8 (XTAL2), with appropriate load capacitors. This isn’t strictly necessary as the microcontroller has an 8MHz internal RC oscillator and the frequency is not critical. However, if Australia's electronics magazine you set the ATmega328 fuses to use an external crystal and none is present, it’s effectively ‘bricked’, so it’s safer to have one. Rotary encoder RE1 is used to set the frequency. It has two Gray code outputs, A & B, plus an internal switch that’s activated when the knob is pressed in. All three outputs switch to ground when active, so they have 33kW pull-up resistors and 100nF capacitors to ground for debouncing. They are sensed and decoded by IC1’s PD2-PD4 digital inputs. The gain and power output potentiometers (VR1 & VR2) connect across the 5V supply, and their wipers connect to two analog inputs pin on IC1 with 100nF filter capacitors. IC1 uses its internal analog-to-­ digital converter (ADC) to convert the wiper voltages to a number from 0-1023 corresponding to the rotational position of those pots and then sends appropriate commands to the FM transmitter. September 2026  53 Power supply Whatever battery is used, its voltage will typically be in the range of 1.5V to 4.5V. This is fed to the input of switch-mode regulator module REG1 via power switch S1. REG1 produces a 5V output as long as the battery is within the 1.5-4.5V range, which powers IC1, MOD1 and OLED1. CON1, D1, D2 and the 10W resistor provide a means to recharge the battery without removing it. If using a rechargeable battery, you have two options: one Li-ion/LiPo cell (3.7V nominal, 4.2V fully charged) or three NiMH cells in series (3.6V [1.2V per cell] nominal, 4.2V [1.4V per cell] fully charged). In both cases, the 10W resistor limits the charge current while the diodes prevent the battery from being charged above 4.1V – it’s critical that Li-ion/ Table 1: Chip ATmega328PB ATmega168(P) ATmega8 Fuse Extended Byte 0xF5 0xF9 × Fuse High Byte 0xD1 0xD5 0xD1 Fuse Low Byte 0xFF 0xFF 0xFF (Low Byte int. osc.) 0xE2 0xE2 0xE4 LiPo types are not charged above 4.2V. That requires a well-regulated 5V DC supply to be used for charging, as the charge termination voltage depends on the source voltage being close to 5V (5.1V maximum). If a rechargeable battery is not used, these parts can be left off. Alternatively, you could opt for external DC power and forego the battery, powering it via CON1 instead. CON5 provides the optional 38,400 baud serial debugging interface. Only serial output is supported, using Mosfet Q1 with a 1kW pull-up resistor as an inverter and isolator. If you don’t need the debugging interface then CON5, Q1 and the pull-up resistor don’t need to be fitted. Panel preparation Before mounting parts on the PCB, use it as a drilling template. Attach it centrally to the front panel using tape (eg, masking tape). Use a 3mm drill bit to make two diagonal holes and attach the PCB to the panel with 3mm screws and nuts. Fig.3: the circuit of the final FM Transmitter design. IC1 loads the required configuration into MOD1; the audio signal is fed directly into MOD1 from an external source. Rotary encoder is used to change the configured transmission frequency, while VR1 sets the output power level and VR2 adjusts the modulation level. 54 Silicon Chip Australia's electronics magazine siliconchip.com.au Next, use a 1.5/1.6mm drill to mark the position of the four holes for the encoder, potentiometers and switch, and then a 3mm drill for the two remaining corner mounting holes. Remove the PCB and drill out the four 1.5/1.6mm holes to 6.5mm. Another hole has to be drilled in the side to line up with the 3.5mm audio socket on the Elechouse module, plus one for the barrel socket if you are using it. Programming IC1 You have three options for programming the microcontroller, IC1: 1. Purchase a pre-programmed microcontroller from our Online Shop (siliconchip.au/Shop/9/7724) and it will be ready to solder to the board. No further programming will be necessary. 2. Program it before you solder it to the board. You will need an Atmel serial programmer as well as a TQFP32 programming adaptor, such as the one we published in the October 2023 issue (siliconchip.au/Article/15977). The PCB for that adaptor is available from our Online Shop. 3. Program it after soldering it to the board. In this case, you will need a serial programmer with a six-pin socket or ribbon cable. You will also need to solder the six-pin programming header, CON2, to the board. Cheap Atmel programmers are available from AliExpress, such as this one for about $6, including postage: www.aliexpress.com/ item/1005005962442597.html Jaycar also sells a suitable program- Table 2: CKDIV8 1 Do not divide clock frequency by eight CKSEL 1111 Low-power crystal oscillator, 8-16MHz SUT 11 fast rising power, 14 clock + 4.1ms delay EESAVE 0 EEPROM memory is preserved BODLEVEL 101 Brownout detector = 2.7 All fuse field differences from defaults mer, Cat XC4627. Make sure to get the 10-pin to 6-pin adaptor (or you can make one yourself). In both cases, the required software is a free download. Three versions of the HEX file are provided in the download package (siliconchip.au/Shop/6/3643) depending on the exact microcontroller used. Be sure to select the right one. Connect the programmer to the 6-pin header (or if using the TQFP adaptor, wire it up to that board) and use the software to program the HEX file into the chip, then set the fuses to the values provided in Table 1. Note that if you’re using the TQFP adaptor, once you set the fuses, you won’t be able to make any more changes as the chip will need the crystal connected to work. Fuses All three supported chips have at least two fuse bytes (low and high), each with eight bits of configuration data. After you’ve programmed the HEX file into data memory, set them as shown in Table 1. The low byte value for internal oscillator operation is provided in case you haven’t fitted crystal X1. If you have, use the 0xFF value. How you set these depends on the software you’re using, but it should show you the values that will be programmed in hexadecimal, so make sure they match the values given. The differences from the default values are shown in Table 2. Construction The components used are a combination of through-hole and surface-­ mount devices (SMDs). Fit all the SMD devices first; most of them are on the board under the OLED display. Start with the 32-pin microcontroller, IC1, which has pins on all four sides. Make sure the microcontroller has the correct orientation and is positioned centrally on the pads before soldering it. There should be a dot or divot in one corner, indicating where pin 1 is. That corner goes at upper-left, as shown in Fig.4, the PCB overlay diagram. Tack one pin first, then re-check the orientation and placement, verifying the pins on all four sides are correctly centred on their pads. If not, re-melt the joint and gently nudge the chip into position. Repeat until you are happy, then solder the diagonally opposite pin. Proceed to solder the pins on one of the other two sides, first adding a thin layer of flux paste along the pins. With good-quality flux paste on the pins, all you need to do is clean your soldering iron tip, add a bit of solder, then gently drag it along the pins and solder will flow onto them. Another The underside and top of the FM Transmitter PCB. Note that the antenna cable on the FM transmitter module connects to a pad on the underside of the PCB. siliconchip.com.au Australia's electronics magazine September 2026  55 Fig.4: assemble the FM Transmitter by mounting the parts on both sides as shown here. Solder the SMDs first, then the through-hole parts on the same side, then the rest. Note how the OLED screen fits over the top of IC1/X1. The antenna wire from MOD1 goes to a pad on the main PCB just under the lower-right corner. Make sure REG1 is fitted with the orientation shown. Fig.5: this shows where to drill the hole in the right side of the case so you can plug into the audio input socket of MOD1. All dimensions are in millimetres (100% scale). technique is to add a little solder to the tip, then touch the tin to the end of the pin and let the flux draw the solder onto the pin and pad. Repeat until all pins on one side are soldered. Don’t concern yourself too much if you accidentally bridge pins while soldering the first side, since it’s easy enough to fix later. Once one side is soldered, do the other side, then repeat for the other two (where you originally tacked one pin). Check carefully for bridges (eg, using a magnifier) and if you find any, add a bit of extra flux paste, then use solder-wicking braid pressed down with the tip of the soldering iron to draw off the excess solder. Once all bridges are cleared, clean off the flux residue (eg, using alcohol and a lintfree cloth) and then go over the joints again to verify they all look good. After that, use a similar technique to solder Mosfet Q1 (if you are fitting it) but it only has three pins to solder. Once that’s finished, move onto the passives (resistors and capacitors). These are not polarised so you don’t have to worry about their orientations as you solder them. 56 Silicon Chip The resistors should be marked with their values in scientific notation (eg, 33kW = 3302 or 333), while the capacitors will be unmarked but there are only two different values. Ensure the 22pF capacitors go into the right locations as marked. Through-hole parts Now install the through-hole components on the front of the board, using the front panel as a guide to make sure the two potentiometers, encoder and toggle switch are positioned accurately. The OLED screen and transmitter module are plugged into socket strips and attached by screws with standoffs. The OLED can come in slightly different sizes and can have either 2mm or 2.5mm mounting holes. Don’t try to drill out the 2mm holes to the larger size as it would most likely wreck the module. Although four mounting holes are provided for the modules, only two are actually required. Use a short piece of insulated wire to connect the ANT terminal on the transmitter module to the antenna pad on the main PCB, which is near the pads Australia's electronics magazine for the debug header mounted on the other side (CON5). Now that you’ve soldered all the parts, inspect the board for any shorts between pins and bad solder joints, and clean off any remaining flux residue. On my board, I fitted an SMA socket to connect the output to a spectrum analyser (CON3) but it is not necessary. Instead, you can simply solder a wire to the central pad provided from CON3 and attach it to an antenna socket on the back panel of the case. That can be an RCA panel-mounting socket. The external antenna just needs to be a piece of wire about a quarter of a wavelength, which is 75cm at 100MHz. There is no ground plane as such, but the shielded wire to the sound source can be looked upon as the other half of a dipole. We are not after any great efficiency here, as the required range is only some meters. Besides, the maximum allowable radiated power (EIRP) to operate in this band without a license is just 25μW. You don’t need a high-­ efficiency antenna to achieve that. In fact, you don’t want a high-efficiency siliconchip.com.au antenna to ensure you aren’t exceeding the legal limit! Parts List – FM Transmitter Battery selection 1 double-sided PCB coded CSE260501C, 76 × 59.5mm 1 black front panel PCB coded CSE260502, 86.5 × 80mm, 0.8mm thick 1 5V DC regulated plugpack (optional, unless using a rechargeable battery) 1 115 × 90 × 55mm case with clear acrylic lid [AliExpress 1005009896550305] 1 3 AA battery holder OR 1 3.7V Li-ion or LiPo rechargeable cell (and holder if required) 1 panel-mount DC barrel socket (CON1) ● 1 3×2-pin header, 2.54mm pitch (CON2; optional, for programming IC1 in-circuit) 1 4-pin vertical polarised header or two 2-pin types, with matching plugs and pins (CON4) 1 3-pin vertical polarised header (CON5) ◆ 2 4-pin female socket headers (for OLED1 & MOD1) 1 Elechouse FM transmitter module (MOD1) [SC7712 or AliExpress 2840333316] 1 0.96-inch OLED display module with SSD1306 or compatible controller (OLED1) [SC6936 (white) or SC6176 (cyan)] 1 five-pin vertical rotary encoder with a 20mm-long shaft (RE1) 1 ND0205MA 3V to 5V DC step-up converter module (REG1) [SC7713 or AliExpress 1005006176918158] 1 miniature SPDT toggle switch with solder tabs (S1) 2 9mm 10kW linear PCB-mounting vertical potentiometers with 20mm shafts (VR1, VR2) 1 8MHz crystal, HC-49 (X1) 3 small knobs to suit RE1, VR1 & VR2 Hardware 4 M3 × 16mm tapped hex spacers 2 M3 × 10mm tapped hex spacers 2 12mm-long 2-3mm inner diameter untapped spacers 12 M3 × 6mm panhead machine screws 2 M2 × 16mm panhead machine screws 2 M2 hex nuts Semiconductors 1 ATmega328PB-AU microcontroller programmed with CSE0501A.HEX, TQFP-32 (IC1) ■ 1 2N7002 N-channel Mosfet, SOT-23 (Q1) ◆ 1 1N4004 400V 1A diode (D1) ● 1 1N5819 40V 1A schottky diode (D2) ● Capacitors (all SMD M2012/0805 50V X7R MLCC unless noted) 1 4.7μF 6 100nF 2 22pF NP0/C0G Resistors (all SMD M2012/0805 ±1% metal film unless noted) 3 33kW 1 1kW ◆ 1 10W 2W ±5% axial ● ■ ATmega8 or ATmega168 variants are also suitable ◆ optional; for debug interface ● for use with a rechargeable battery (3 NiMH AA cells or one Li-ion/LiPo cell) As mentioned earlier, there is a choice of batteries. In my prototype, I used a triple AA cell holder, which I attached to the inside of the case with double-sided tape. This can take either alkaline non-rechargeable cells or NiMH rechargeable cells, which can be recharged via CON1 if it’s fitted. An alternative is to use an 18650size Lithium-ion cell. These have a nominal voltage of 3.7V and a capacity of up to 3500mAh (although 2-3Ah is more typical). You may find these advertised by overseas suppliers with ludicrous capacities of up to 19,900mAh. This is sheer nonsense, so don’t buy them (if in doubt, stick with a local supplier like Jaycar or Altronics). A single 18650 cell holder can be glued inside the case. Testing If you haven’t already programmed IC1, do it now using in-circuit serial programming header CON2. Refer to the “Programming IC1” section above for instructions. Initial testing is with the board not yet installed in the case. Before connecting power, check with an ohmmeter to make sure there are no short circuits between pin pairs 1 & 2 or 3 & 4 of CON4. Apply power and you should see a splash screen showing FM TX on the top line and the version number on the bottom line. After a second, this is replaced by a four-line display. The top line shows the frequency, line 2 the power level, line 3 the audio gain, and line 4 the battery voltage. Rotating the encoder knob will change the frequency in 100kHz steps. Pressing the knob will toggle to 1MHz steps. Rotating the potentiometers will vary the power and gain levels from 1 to 8. Connect the transmitter to an audio source using a stereo cable with a 3.5mm TRS plug at one end. Find a clear channel on a nearby FM radio and tune the transmitter to that frequency. Try different transmit power settings and use the minimum that gives good results. If you have a spectrum analyser, such as a TinySA, you can fit CON3 and connect it there. You should see a spectrum like the one shown in SC Screen 1. siliconchip.com.au Screen 1: the Transmitter’s output spectrum. The second harmonic is -23dB compared to the fundamental. The EIRP is under 25μW with the specified ¼-wavelength wire antenna. Australia's electronics magazine September 2026  57 By Dr Hugo Holden Commodore PET Diagnosing a Vintage Computer – the Display System Troubleshooting vintage computers that use dozens (if not hundreds!) of separate logic ICs can be very difficult unless you know exactly how they work. This article will explain in detail how the PET’s video hardware works, to make diagnosis and repairs much easier. T here are many model variations of Commodore PET computers (the predecessor to the famous Vic 20 and Commodore 64). This article refers to a type of motherboard known as “The Dynamic PET”, characterised by using 4116 dynamic memory (DRAM) ICs and 2114 static video RAM (SRAM) ICs. Also, it does not contain a CRTC (cathode ray tube controller IC). Therefore, it may or may not exactly match your particular PET. This is the only PET I own, so I am not in a position to perform a similar analysis on the other types/models. The intention of this article is to describe how this PET’s character address generator (CAG) system works, and to provide the operating theory and data on it, to help technicians diagnose and repair it. My unit worked well, so there was no specific fault I needed to fix. But I was curious about the design and wanted to know more about how it works. A detailed operating theory is always helpful in making an accurate diagnosis of a faulty circuit. The worst approach is random and haphazard guessing based on hunches and the absence of test data. Especially if that leads to unnecessarily removing good vintage ICs from the PCB and risking PCB damage. To assist in probing this system, I made a pulse-counting logic The Commodore PET CBM Model 3008, released in 1979, was a successor of the PET 2001 (pictured opposite). 58 Silicon Chip probe to verify that brief pulses on various lines were all present and that no narrow events in a pulse stream had been missed on an oscilloscope screen. This logic probe circuit design was published in Circuit Notebook (December 2025 issue; siliconchip.au/ Article/19378). One reason for using this probe is that some of the pulses in this circuit are extremely narrow, in the region of 100-300ns, and relatively infrequent. This renders them not easily seen on an analog oscilloscope (and maybe even some low-end digital scopes at longer timebase settings). They are either not frequent or long enough to excite the screen phosphor on a typical analog ‘scope, or are missed completely by a too-low sampling rate on some digital ‘scopes. The pulse-counting probe won’t miss them, though. More on the Commodore PET The PET computer, in its various forms, has now become quite a collectible item in the world of vintage computers. There are several reasons why. One is, as the photo shows, it has a fantastic retro look to it. It also has a built-in BASIC interpreter, which is a relatively easy language for a programming novice to learn. The other reason is that the PET was gifted with a very nice cathode ray tube (CRT) video display unit (VDU). The charm of a real CRT seems somewhat unmatched by a modern flat panel display and is inescapably attractive to many. Still, that might depend on your age. In many vintage computer systems like the PET, no raster scan oscillators are present in the VDU; timing signals from the computer take their place. They are then called horizontal and vertical drive pulses rather than sync pulses. This creates the opportunity for malfunction and damage in the VDU, especially if the horizontal drive pulses become abnormal in frequency or duty cycle. That is because the horizontal scan output stage in the VDU also siliconchip.com.au Fig.1: the contents of a typical PET computer’s character ROM. The exact contents vary between models and generations. Source: http://cbmsteve.ca/ cbmchr/index.html generates the EHT (extra high tension) voltage and auxiliary voltages for the CRT. In the Commodore PET VDU, the video signal has no shades of grey; it is simply on or off. The VDU therefore only has a brightness control. A contrast control had no application. In a typical composite VDU, the analog value of the video component of the composite signal controls the CRT’s beam intensity, and the magnitude of those excursions is controlled by a contrast control. A contrast control is essentially a video signal amplitude control. In the non-CRTC (CRT controller) PET discussed here, the horizontal & vertical drive pulses, and the video pulses, are derived from some very creative logic circuits using 74-series TTL ICs. As new PET models emerged, Commodore (as many did) switched to using a CRTC chip. In this case, the generation of horizontal and vertical pulses is performed by one IC, eliminating the complex array of 74-series TTL-based circuits and leaving the one LSI chip to do all the heavy lifting. The PET character address generator (CAG) In the PET, the generation of the horizontal and vertical drive pulses and the video pulses is essentially independent of the 6502 CPU. The CPU simply writes data to a part of memory (video memory), which is then used to generate the necessary signals. The video memory buffer is based on a pair of 2114 1024 × 4-bit SRAM memory ICs. Together, these hold a byte for each character on the screen; the screen has 25 rows of 40 characters, so 1000 total character ‘slots’ Each byte value is the address of the character to be generated in the character ROM, similar to the ASCII scheme. Commodore’s scheme was known as PETSCII or CBM ASCII. The 1000 screen locations (from the perspective of the computer user) are siliconchip.com.au stored at decimal memory addresses 32768 through to 33767. To see characters on the VDU at all possible screen address locations, the CAG needs to scan the addresses of every one of the 1000 screen character locations in the 2114 SRAM several times per screen refresh (eight times, in fact, because the horizontally scanning beam crosses each character eight times). A POKE command can be used to inject a byte value into any specified memory location. In BASIC, if the command “POKE 32768,1” is issued, the letter “A” will appear in the first screen character location, in the upper-left corner of the CRT’s display area. That’s because A is in the second character ROM slot (the first slot is numbered 0). If the 2114 video RAM chips are removed from the PCB and their outputs held to +5V with pull-up resistors, a checkerboard will appear on the screen, because every character address location is seen as having the same binary content of 255 (that is, if the video circuitry beyond the 2114 and the character address generator is working). POKEing a zero value, or tying the 2114 chip output pins low, results in the “<at>” character (stored in character ROM slot zero) instead. These tricks can have some applications in troubleshooting the circuitry. Fig.1 shows the typical set of 256 characters in ROM, although not all PET computers will have identical character sets. While the user accesses the display RAM at addresses 32768 to 33767, the 10-bit counter in the character address generator (CAG) circuit uses addresses from 1 to 1000, represented in binary form. The analysis of the CAG might have been easy if it were just a counter, free to count as a binary counter does, over some range. However, its counting sequences are interrupted, manipulated and controlled by various system pulses, such as the HORZ DISP ON pulse, NEXT pulses and RELOAD pulses. These will be untangled in this article. Untangled, because it is a system of pulse feedback and resets, where the logic conditions of the CAG and some other sub-circuits are detected to create special reset pulses, which after deployment, annihilate the logic conditions that created them, in time frames of 300ns or less. To summarise, the CAG has two roles: first, to generate the control signals that cause the CRT beam to scan the display, and second, to generate the video modulation signal that causes the characters in display RAM The first Commodore PET to be released was the Model 2001 in 1976. Source: Rama – https://w. wiki/Hc9Q (CC-BYSA 2.0 Fr) Australia's electronics magazine 59 to appear on the screen in the correct locations. Getting started It is a good idea to begin with a diagram of the video screen to see how scanning lines and time are allocated (Fig.2). Two very important pulses are the HORZ DISP ON pulse and the VIDEO ON pulses. These define the surface area on the video screen where characters are displayed. The temporal width of a character cell is 1μs as the CRT beam scans the screen from left to right. Characters are made of eight pixels horizontally, with eight of those stacked vertically. The illuminated pixels of a character may not fill the whole cell, because space is often left beside and below it to space the characters apart, as shown in the example A character in Fig.2. However, graphics characters (lines, boxes etc) often use the full width and height. of the active video time (the horizontal retrace time), the CAG is paused and the CRT’s beam current is disabled. So video data only controls the CRT’s beam when the HORZ DISP on pulse is high for 40μs per line. The horizontal scan system The vertical scan system The horizontal scan system causes the electron beam to scan horizontally from left to right, then fly back to the left, at 15.625kHz. As the beam is sweeping the screen, when it reaches the active video area, the HORZ DISP ON signal goes high. During this time, the CAG is active and pixel data is presented in the video signal to render the character cells. The rest of the time, during the horizontal blanking periods on either side The vertical scan system is responsible for the CRT’s beam moving from the top of the screen to the bottom or the horizontal scan lines would simply scan on top of each other. The vertical retrace, where the beam moves from the bottom of the raster scan, up to the top of the raster scan, takes 1.28ms, which is the equivalent of the time taken for 20 horizontal scan lines. The video signal holds the electron beam off for the vertical retrace time and for an additional 20 scan lines time before the active vertical component of the video time, defined by the VIDEO ON pulse. After that, the CRT beam is again extinguished for another 20 lines time before vertical retrace begins. Then the counters are reset and it starts scanning the active video area of 200 scan lines. For each scan line, the CAG is active for 320 clocks, meaning the active area has a resolution of 320 × 200 pixels. Each character to be displayed consists of 8 × 8 pixels, meaning that 320 × 200 pixel area shows 1000 characters (40 × 25). After the 200 active lines are scanned, the electron beam switches off and another 20 lines are scanned (the bottom vertical blanking area). Then vertical retrace is triggered and the beam ‘flies back’ to the top of the screen while off. That consumes another 20 horizontal scan periods. That means that each screen refresh is a total of 260 horizontal scan periods (20 + 200 + 20 + 20), so with a horizontal scan frequency of 15.625kHz, the vertical scan (screen refresh) rate is 60.096Hz (15.625kHz ÷ 260). Fig.2: how the PET’s CRT screen is scanned. During the horizontal and vertical flyback periods, the CRT beam is inactive. In the raster scan zones, the ‘beam’ is scanning the screen, but it is not active as it is outside the display area. Character generation Fig.3: this shows the character addresses and how the lines are counted using the H11 IC. The CAG steps through the character address sequence 1-40 eight times, then 41-80 eight times and so on, until the last row of characters, at addresses 961-1000. During the 200 active scan lines, once the electron beam finishes scanning the left horizontal blanking (raster) area, the CAG becomes active. What it needs to do during this time is determine, for each of the 320 × 200 pixels, whether to drive the CRT beam on or off, making that pixel either bright or dark. The pattern of 8 × 8 bright and dark Australia's electronics magazine siliconchip.com.au 60 Silicon Chip Fig.4: a block diagram of the PET’s character address generator, with some extra components shown that affect its operation. It is responsible for driving the CRT screen, reading characters out of display memory, using those to look up the character ROM and displaying those characters on the screen by modulating the CRT beam. pixels in each of the 40 × 25 cells shows a single character (letter, number, punctuation, graphic pattern etc). Do to this, the CAG must: 1. Keep track of which character cell (1-1000) it is currently scanning. 2. Fetch a byte from SRAM for each character cell that indicates which character to show in this particular cell. 3. Keep track of which line within that cell (1-8) it is currently scanning. 4. Using the byte from SRAM and the line number, read a byte from the character ROM that determines the pattern of eight pixels to show for the current row of the current cell. 5. Send that byte, one bit at a time, to the CRT on/off signal to generate the required pixel pattern for this 8-pixel strip. To achieve this, the character cell address (1-1000) needs to start at 1 in the top-left corner, then increase by one for every eight pixels output, reaching 40 in the top-right corner. Then, for the next line (the second row of these characters), it needs to repeat the 1-40 count. It repeats that count eight times for the eight lines of those characters. On the next line, it counts from 41 to 80; the next row of characters. This pattern repeats, counting the same 40 values for each set of 8 lines, until it reaches the final row siliconchip.com.au of addresses 961-1000. After that, it’s in the vertical blanking period as described above. For each set of eight scan lines, the only thing that changes is the 3-bit counter that cycles through 0...7 to select the appropriate line of each character ‘drawing’ from character ROM. Otherwise, the CAG’s operation is identical for each set of eight scan lines. On the 8th line of each set, the counter re-loading is skipped, so the address starts the next line at a value 40 higher than the previous line (see Fig.3). Character generation That just leaves the question of how the electron beam is modulated on each active scan line. At the start of each character, one byte is read out of the display SRAM at the current character address. That byte forms the upper 8 bits of an 11-bit address into character ROM, with the lower three bits being the current line number, 0-7. That gives a single row of 8 pixels to light for the current character. That byte is loaded into a shift register, and the output of the shift register controls the electron beam. That register is shifted by one bit for each subsequent pixel, at a rate of 8MHz. After eight shifts, the character address is incremented by one, the next 8-pixel Australia's electronics magazine pattern is loaded from character ROM, and that byte is loaded into the shift register. This process repeats for the 40 characters and 320 active pixels of each scan line. The CAG The CAG’s labelled outputs, bits 1 (RA1) to 10 are shown in red in Fig.4. Another box in the diagram is the 4-state machine, which will be examined later. The diagram also indicates where the three signals to drive the VDU originate: VERT(ICAL) DRIVE, HORIZ(ONTAL) DRIVE and VIDEO OUT. This simplified block diagram has been created to show how the 10 bits of the CAG sequentially select a specific ‘cell’ or byte of data at a screen address location in the 2114 video RAM ICs. One line of the corresponding character in character ROM is clocked out of the shift register IC, E11, to create the video signal for the VDU. The purpose of the MUX ICs, F3, F5 & F6, is to allow the CPU to select the addresses of the video RAM on the other half of the clock cycle of CLK1 and therefore be able to ultimately read or write any byte value from the data bus BD0...BD7 into or out of any one of the 1000 screen character locations via the READ and WRITE latches of ICs E7 and E8. September 2026  61 HORZ DISP ON 40μs 1 3 39 Video latch “sample points” 2 4 RA1 40 there are 40 characters per row on the PET’s screen Figs.5 & 6: the left scope shows the video latch acquiring characters from display RAM to be shown on the screen. That happens 40 times for each of the 200 scan lines that cover 25 rows of characters. Right: the same waveforms in Fig.5 zoomed in for a closer look. To display the data represented by memory cells of the 2114 on the VDU screen, it is latched by IC F9 and fed to the character ROM as address values. Only seven bits are latched for the character ROM, because the A10 address input on the character ROM is used to select ‘GRAPHIC’, the alternative lower-case character set that lives in the character ROM. The 8th bit of data (LSD7) out of the F9 latch is used to invert the data clocked out of the shift register E11, so that the characters appear as dark on a light background (‘inverse video’). Therefore, byte values of 128 or over result in the same number or character as those specified by 0 to 127, but they are simply inverted video. Normally, pin 19 (A10) of the character ROM is low, which selects the upper-case characters. If you run the command “POKE 59468,14”, it toggles the pin high, and lower-case characters are used instead, from the upper half of character ROM (as seen in Fig.1). If you run “POKE 59468,12”, it goes back to upper-case. The data is latched by IC F9 at a time late in the high part of the RA1 pulse, which forms the least significant bit of the 10-bit CAG. The reason is to make sure that the output data in the 2114 is stable after it has been presented with the new address of each character cell by the CAG. The scope recording in Fig.5 shows the timing of 8-bit latch F9 latching (effectively sampling) the output data from a memory cell in the 2114s. If the pulse-counting logic probe tip 62 Silicon Chip is connected to RA1 and it is gated by HORZ DISP ON, the probe counts a hexadecimal value of 14 (20 decimal) as it is counting the 20 rising edges of RA1. Using the ‘scope to look at RA1 and the VIDEO LATCH pulses fed to F9 gives a clearer view of that timing (Fig.6). NEXT pulses and digital circuit loops In essence, the four 100ns-wide NEXT pulses are a form of RESET pulse. However, they serve other functions too. These pulses are the key to the operation of the entire CAG circuit, despite being the more difficult pulses to view with a ‘scope than any others in this circuit. These pulses are responsible for switching the logic between the four main states: vertical retrace/flyback, pre-display raster scan, scanning the active video area, and post-display raster scan. Raster scan refers to when the CRT beam scans the part of the display that is outside the display area (perhaps even hidden behind a bezel). No characters are displayed there, but the beam must still traverse those areas. The PET’s NEXT pulses are derived from the pulse streams generated by the 20 LINES and 200 LINES detector circuits. When combined, and with some other logic signals, these detect the end of normal scanning, causing the CRT beam to be switched off, the vertical flyback to start and ultimately the resetting of the CAG, ready to draw the screen all over again. There are four digital reset loops Australia's electronics magazine involved in generating the NEXT pulses. At this stage, we need to refer to the full CAG circuit diagram, Fig.7. The most obvious ‘circuit loop’ in Commodore’s circuit is the CAG’s upper 8 bits (outputs) being fed back to the latch inputs of G3, then those latch outputs being fed to the CAG’s jam load inputs on ICs F2 and F4. This arrangement causes the character cell counter to reset at the start of the first seven of each set of eight scan lines, as noted earlier. The other loops involve the NEXT pulses. These can be seen from looking at where the NEXT pulses are fed, because in each case, the feedback results in a change to the digital logic that generated the NEXT pulses. 1. The NEXT pulses are fed to the CAG’s F2 & F4 clear inputs, clearing the upper eight bits of the CAG when they are triggered. 2. The NEXT pulses are fed via H5’s pins 12 & 11 to latch G3. These make the latch transparent, and it acquires (remembers) the zero condition created by the clearing of F2 and F4 by the NEXT pulse being applied to the F2/F4 counters’ clear inputs. 3. Counter H11 is reset (cleared) by NEXT pulses. RA9 contributes to the pulse stream in the 20 LINES detector and ultimately the NEXT pulse extracted from that pulse stream is a reset pulse, which not only resets the CAG but also the three-bit counter, H11, that created the RA9 pulse. 4. The 20 LINES pulse stream can only exist in the VIDEO OFF time, which is the complement of the VIDEO siliconchip.com.au Fig.7: the main part of the CAG circuit. Signals with numbers in brackets go to another part of the circuit. F2, F4 & H11 are four-bit counters, G3 is a latch while F3, F5 & F6 are quad two-way multiplexers. G6 and G8 are flip-flops, while the functions of the other ICs should be evident from their symbols. The quad NAND gates like H5 may be shown as NAND or NOT-OR gates as those functions are logically equivalent. ON time, created by NEXT pulses controlling the 4-state machine (more on this later). The first NEXT pulse is generated by the 200 LINES detector, and this also controls the state machine to create the VIDEO ON pulse. In summary, the latch G3 and CAG logic states are modified by the NEXT pulses, and these modify the production of all pulses derived from the CAG, including the NEXT pulses themselves. You can think of this as siliconchip.com.au a sort of oscillator, as it goes through the same set of states endlessly while powered. The NEXT pulses get formed by flipflop control into four uniform 100ns pulses. This introduces a small delay between the leading edge of the gated pulses and the leading edge of the NEXT pulses themselves. The result is that the logic state detected by the 200 LINES and 20 LINES detector circuits, when Australia's electronics magazine the precursors of NEXT pulses are detected, can persist a little longer than they otherwise would if they were used the reset the CAG directly. This causes the production of the NEXT pulses to become synchronous with the rising edge of the VIDEO LATCH pulse. This pulse clocks the flip-flop that issues the four NEXT pulses. The NEXT pulses are difficult to see on the ‘scope because they are only 100ns wide and they come in groups September 2026  63 VIDEO ON PULSE high for 12.86ms 3.84ms VIDEO OFF vertical scale 2V/cm both channels The 4 NEXT pulses each 100ns wide Storage scope recording of next pulses TEK 464 scope Fig.8: it’s best to use a high-speed digital ‘scope to probe the NEXT pulses (and some other pulses in this circuit) as they are very brief at ~100ns and can easily be missed on an analog ‘scope. lines are the active video lines where characters are presented on the screen while HORZ DISP ON is high. Although it seems unimportant in the scheme of things, because the NEXT pulses are so narrow compared to the VIDEO ON & OFF timing, it pays to note that the 74LS107 flip-flops are a master/slave type. They change state very shortly after the clock pulse driving them falls low. Fig.10 shows this. This has implications in pulse counting, where the VIDEO ON pulse might be used as a gating signal for the pulse counting logic probe, or where it might be used as an oscilloscope trigger. The pulse-counting probe mentioned earlier will count three NEXT pulses if gated to count in the VIDEO OFF time (when the VIDEO ON pulse is low). This is because the first NEXT pulse rising edge occurs at the trailing end of the VIDEO ON time. If the pulse-counting probe is gated to count for the VIDEO ON time instead, with NEXT pulses feeding the probe tip, it will count one pulse. The CAG in detail Fig.9: this state machine is responsible for disabling the CRT beam and triggering flyback at the end of the vertical scan period. of four at a relatively infrequent interval of around 16.64ms. The energy delivered to the screen phosphor on a standard oscilloscope is barely enough to see them. A storage ‘scope or high sampling rate digital ‘scope is better to view them. A recording is shown in Fig.8. The 4-state machine This circuit, a two-bit counter, is a well-known circuit in the field of amateur radio. It is a quadrature pulse generator, sometimes used for SSB radios. It produces a form of Gray code, where the counter moves through a series of values that differ only by one bit at a time (in this case, 00, 01, 11, 10 and then repeating). 64 Silicon Chip If it was continuously clocked, it would produce pulses in quadrature, but in this case, its operation is interrupted every 16.64ms and it is clocked only by groups of four pulses to produce the VIDEO ON pulse for the CAG and the VERT(ICAL) DRIVE pulse for the VDU. The later VIDEO ON pulse is responsible for gating the second, third and fourth NEXT pulses. That is due to the digital reset loops in the CAG circuit. When the VIDEO ON pulse is low, the four NEXT pulses appear to divide that time into three 20-line or 1.28ms boundaries. From the 4th NEXT pulse to the first one of the following group of four NEXT pulses, it is 12.8ms or 200 horizontal line periods. These 200 Australia's electronics magazine The clear (CLR) inputs of the 74177 counters are active low (they are labelled CLK in the original Commodore diagram for some reason). The jam load control inputs on pin 1 are also active low. While HORZ DISP ON is low, counters F2 and F4 are loaded with whatever values are held in 8-bit latch G3. Each flip-flop within the 74177 is forced to a preset or cleared condition (depending on whether the load bit is high or low). During this ‘load’ time, they cannot count. Latch IC G3 is made transparent when its pin 11 input is high. This means that its outputs simply follow its inputs. It latches (remembers) the current value feeding the latch when pin 11 goes low. Bits 3 to 10 of the CAG loop back to feed the latch inputs. Starting with what could be called line 1 at the top of the screen, the first character location and the start of the first line of that character is character #1. Immediately prior to the first active video scan line, RA1 is high and all the other bits of the CAG, bits 2 to bit 10, are low. This is because when HORZ DISP ON was off (low), this cleared flip-flop G6, making bit 2 low. Bits 3 to 10 are also low at this time because siliconchip.com.au both of the 74177 counters, F2 and F4, were cleared by the 4th NEXT pulse via H5’s pins 12 & 11. At this starting time of the first line of the 200 line character block, the data in the G3 latch is zero, because the 4th NEXT pulse made the latch transparent for 100ns, via H5’s pins 12 & 11, ‘remembering’ the zero condition of the cleared F2 and F4 counters at that time. So, initially: 1. The binary value of the CAG is 1, because RA1/bit 1 is high and all the latches are cleared. 2. The Q output of flip-flop G6 (pin 3), bit 2 of the CAG, is low (0). 3. The value held in 8-bit latch G3 is low for those bits. 4. All outputs of 74177 counters F2 and F4 are low. 5. The 3-bit binary counter H11 (shown added to the CAG circuit) also starts at zero because it was cleared by the NEXT pulse too. This counter is clocked by the HORZ DISPLAY ON pulse when it falls low at the end of active horizontal video time. G11 decodes the 3-bit count of H11 to create a RELOAD pulse that deploys on the 8th repeat of the screen address row, for all 25 rows of characters. When HORZ DISP ON goes high, the CAG then begins counting on negative edges of RA1 pulses, which changes its state every 1μs. Fig.3 shows how the CAG counting This board has the CPU, RAM, ROM and most of the logic ICs that run the computer. it’s laid out in a grid so letter/number pairs are used to refer to specific chips. siliconchip.com.au Australia's electronics magazine Fig.10: the relationship between the very brief (100ns) NEXT pulses and the VIDEO ON signal. starts on the left-hand side of line 1. The counting continues along the first line until the total count is 40 decimal, indicating that one row of all 40 columns of characters has been generated. At that point, HORZ DISP ON goes low, clearing bit 2 of the CAG and activating the jam load inputs on counters F2 and F4, and the counters stop. Since the load value from latch G3 is zero at this time, the CAG total count (the whole 10 bits) returns to 1. The counts on the second line are thus a duplicate of the first. This occurs until 7 lines in total (most of one character row) have been completed. On the 8th line, things change. 3-bit counter H11 is incremented at the end of a line, when HORIZ DISP ON falls low. At the start of the 8th line, H11, has a binary value is 111. To generate a RELOAD pulse, it also requires that the HORZ DISP ON signal is HIGH (due to gate G11). This condition occurs at the start of the 8th line. Therefore, for the whole of the 40μs of the 8th line’s active time, the RELOAD signal is low, making latch G3 transparent so it follows the outputs of counters F2 and F4 over that time. At the end of that 8th line, the HORIZ DISP ON signal goes low, which jam loads the last count value into the counters F2 and F4. The HORZ DISP ON pulse going low clocks the 3-bit counter over to state to 000 and HORZ DISP ON terminates the RELOAD pulse because of gate G11. When latch G3 is released from being transparent, it remembers the final value it had, which was the value of the upper eight bits of the CAG, at the end of the 8th line, ie, 40 decimal. When HORZ DISP ON falls low at the end of that 8th line, this takes the jam load inputs of counters F2 and F4 low; the counters are updated (loaded) with the latched value of 40, hence for the next block of eight lines, lines 9 to line 16, the CAG count starts at 41. September 2026  65 The point of all this is that the CAG has to start at the same value for each set of eight lines as it’s rendering the lines of the same set of characters. It runs from 1 to 40 eight times, then 41 to 80 eight times, then 81 to 120 eight times and so on. While the counts along the first eight lines are being repeated, the output of 3-bit counter, H11 is keeping track of which line within each character is being output (0 to 7), with the three bits being fed into the LSBs of the character ROM, to select the appropriate row of the character to send to the display. The CAG holds the address of a particular screen character cell in video memory. Seven bits of the byte content of that cell are used as the address for the upper seven address lines of the character ROM, A3 to A9. The lower 3 ROM address lines get scanned along the individual eight bytes by 3-bit counter H11, to make up the particular character. Ultimately, one row of each character is presented on the VDU screen via shift register E11 (which serialises it), and it is clocked out along the horizontal scan line to produce the appropriate pixel pattern. Generating the vertical video timing The CAG also produces the vertical video timing via the 4-state machine, which generates the VIDEO ON timing and the VIDEO DRIVE pulse for the VDU. The NEXT pulses are derived from the 200 LINES and 20 LINES pulses. Although the CAG is only able to count when the HORZ DISP ON pulse is high, the CAG is free to keep counting during the vertical interval when the VIDEO ON pulse is low (provided HORZ DISP ON is high). In other words, it still counts in the vertical time window when no characters are being displayed on the CRT. This time window outside the character display time corresponds to the 20 lines of time (1.28ms) prior to the active video area, the 20 lines of time after the active video area and the 20 lines of time for the vertical retrace. During this non-character display part of vertical counting time, which could be called the ‘VIDEO OFF’ time, the same once-per-eight-line RELOAD pulse is applied from the 3-bit counter H11 to the CAG system. It counts in blocks of eight lines, going over the same addresses, just as it does to generate the screen character location addresses in the VIDEO ON time. The 200 LINES pulses are created by gating five of latch G3’s output bits. Since the latch holds character address 961 in the first of the seven lines of the last character block, no 200 LINES pulses occur. Pulses on H5’s pin 6 occur only on the 8th line of a character block, and only on the last row of 40 characters on the 200th line, when the count has reached 968 or over. This is because G2 (pins 9, 10, 12 & 13), I1 (pin 9) and H5 (pins 4 & 5) gate the upper four latched bits and bit 4 of the CAG address – see Fig.11. When the G3 latch is made transparent by the RELOAD pulse, as it is on the 8th line of every block, the latch outputs correspond to the CAG’s upper 8 bits. Bits 4, 7, 8, 9 and 10 of the latch output are fed to the inputs of those gates. The 200 LINES pulse at pin 6 of H5 is generated when all five cited CAG bits are high. Adding their values up, 512 + 256 + 128 + 64 + 8 = 968. So, at character address 968, on the last line of the bottom row of character cells, the 200 LINES line goes low. Pulses then appear there with a width of 8μs on pin 6 of H5 because bit 4 of the CAG address is going low and high every 8μs, while bits 7 to 10 remain high, as shown in Figs.11 & 12. At the end of the 200th line, the CAG does not return to 961; the first NEXT pulse that is generated by the 200 LINES detector results in a reset of the CAG to binary value 0000 0000 01. The NEXT pulse is produced before the ‘time is up’ for the following CAG address value after 1000, which is reset to an address of 1 by the NEXT pulse before a 2μs time frame. The address of 1000 in the CAG at the end of the 200th line is allowed to persist for longer than most of the address states, for around 1.84μs (rather than the usual 1μs) but the CAG address has already been reset to 1 before the end of 2μs corresponding to the next address state for the CAG. The 8μs pulses that lead to the NEXT pulse precursor are fed to the D input of flip-flop G8 from HORZ DISP OFF pulse, 20 LINES and 200 LINES signals via gates H5 & G1. Comparing the last Fig.11: the 200 LINES signal first goes low on character 968, but because RELOAD is low and HORIZ DISPLAY ON is high, it has no effect until just after character 1000 has been fully displayed. 66 Silicon Chip Australia's electronics magazine siliconchip.com.au 1.8μs Pin 6 H5 Pin 6 H5 1.5μs 100ns Pulse terminated by B02H Pin 11 H5 Pin 11 H5 NEXT pulse approx 100ns wide Fig.12: the timing of the NEXT pulse at the end of the main display period relative to pin 6 of IC H5. low-going pulse on 200 LINES at pin 6 of H5 with pin 11 of H5, which controls latch G3, gives even more timing detail – see Fig.12. Not only does the NEXT pulse (inverted by gate H5 at pin 11) allow latch G3 to become momentarily transparent with the new CAG address value, it also clears counters F2 and F4. Fig.13 shows the first 100ns-wide NEXT pulse; it appears to straddle the rising edge of the last 200 LINES pulse. This is because flip-flop G8 is cleared by pulse B02H, which falls low around 40ns afterwards. The leading edge of the first NEXT pulse is created by the leading edge of the VIDEO LATCH pulse, which clocks HIGH data to the Q output of flip-flop G8 at pin 9. The first NEXT pulse going high is the cause of pin 6 of H5, the 200 LINES pulse going high again, because NEXT pulse Fig.13: a zoom-in of Fig.12 so you can see the timing clearly. the logic conditions that caused pin 6 of H5 to be low are eliminated by the reset trigger. It shows a loop propagation delay of something in the order of 50ns after NEXT goes high, or half a NEXT pulse width before the CAG is reset. The end of the NEXT pulse does not extend past what would be the time position for the following address. The address value of 1 would persist for around 160ns before being clocked to 2, 3, 4 etc with the usual 1μs timing stable on each address. The 20 LINES detector The second, third and fourth NEXT pulses are produced by the 20 LINES detector. This is an interesting detector because a unique address did not exist inside the main F2 and F4 counters or the G3 latch system to fully encode it. This is because a 20-line boundary falls inside a zone of repeating CAG addresses. As previously explained, the CAG, regardless of the VIDEO ON or OFF timing, is relentlessly repeating groups of eight-line counts. Therefore, pulse RA9 had to be acquired from the 3-bit counter (H11), which controls the low ROM addresses and generates the RELOAD signal. The 3-bit counter circuit and reload gate are shown in Fig.14. The RO1 and RO2 inputs of 74LS93 counter H11 are active-high to clear the flip-flops within. It was previously noted that the NEXT pulses reset (clear) this 3-bit counter. Scope grab Fig.15 is triggered from VIDEO ON and shows RA9, the MSB of the 3-bit counter (H11, pin 11), and CAG address bit 7, which feeds the 20 LINES gate at pin 1 of G2. For the 20 LINES detector to be 280ns wide pulses not visible on current scope setting RA9 Bit 7 CAG VIDEO ON pulse used to trigger scope Fig.14: how the RELOAD and 20 LINES signals are generated. siliconchip.com.au Australia's electronics magazine Fig.15: the relationship between the MSB of 3-bit counter H11 (RA9) and CAG address bit 7. September 2026  67 Delay timebase recording Hitachi V509 RA9 3.84ms VIDEO ON Pulse Bit 7 CAG 20 LINES pulses G2 pin 6 2V/Div vertical sens. Fig.16: a zoom-in of Fig.15, annotated to make it clearer. operational, it must be outside the VIDEO ON time where characters are presented. Therefore, the VIDEO ON pulse is inverted by gate I1 (pins 5 & 6) to become VIDEO OFF and applied to pin 2 of gate G2. The RA9 pulse stream appears as a chain of two pulses followed by an apparent missing pulse (it is there but not easily seen). This sequence repeats three times during the VIDEO OFF time. Bit 7 of the CAG appears as a chain of seven 17μs-wide high-going pulses Fig.17: like the NEXT pulses, the pulses that trigger them are brief and hard to see on an analog ‘scope. as the count increases, with a broad 8th pulse approximately 256μs wide at the end of the seven pulses. This pulse array again repeats three times in the VIDEO OFF time window. A very narrow 280μs pulse is present in the RA9 pulse stream, noted on the recording with white arrows. This is the precursor pulse to the three remaining NEXT pulses, which reset the 3-bit counter that created the RA9 pulse. This results in rapid termination of the RA9 pulse after it goes high. Fig.16 shows an expanded view of Narrow pulse not seen in this trace Fig.18: this shows how the bit 7 pulses that occur while RA9 is low are ignored; it’s only the four that occur while RA9 is high that trigger the four NEXT pulses. 68 Silicon Chip NEXT pulse precursors 280ns each Australia's electronics magazine the relationship between RA9 and bit 7 of the CAG. Four of the initial pulses of the bit 7 pulse stream occur while RA9 is low. Thus, these four pulses do not make it out of pin 6 G2, into the 20 LINES pulse stream. RA9 falls low again just after the start of the 256μs block of the bit 7 pulse. This shortens that pulse, so that what remains in the 20 LINES pulse stream are three groups of four pulses, each close to 17μs wide, with a gap leading to the difficult-to-see 280ns precursor of a NEXT pulse (Fig.17). The 280ns precursor to a NEXT pulse has its leading edge created by RA9 and its trailing edge terminated by the NEXT pulse it creates, resetting H11 and the CAG; hence, both RA9 and address bit 7 go low. Fig.18 shows how the remaining four pulses leading to each 280μs pulse are eliminated by the HORZ DISP OFF pulse and gate G1. The recording is faint, but when the numbered pulses are high, the HORZ DISP OFF pulses are low, thereby eliminating these 4 pulses at the pin 3 output of G1 feeding flip-flop G8, which issues NEXT pulses. This whole process is summarised in Fig.19. The expected counting probe values are shown. With the probe counting in active video time, by connecting the probe’s gate input to VIDEO ON, and the probe tip is connected to HORZ DISP ON (or HORZ DISP OFF), the count will be hexadecimal C8 (200 decimal) as you would expect given that the active area of the display is 200 lines. If the probe’s GATE is connected to siliconchip.com.au VIDEO ON, it is counting in the VIDEO OFF time, so it will then count to hexadecimal 3C (60 decimal), as there are 60 lines outside the active area. The first NEXT pulse is not counted by the probe in this VIDEO OFF time window because, as previously noted, its rising edge resides inside the VIDEO ON time window. Summary The PET’s character address generator is a master class in glue logic design using 74-series TTL ICs. Due to its complexity and paucity of information provided by Commodore on how it worked, technicians have struggled to repair it. The situation is not helped by the very narrow pulses in parts of the circuit, which are difficult to see with an analog oscilloscope. If you know what pulses are supposed to be there and why, then it makes it much easier to find them. Hopefully, this description of the CAG above will help in fault finding and repairs. Generally, vintage 74-series TTL chips are fairly reliable, but they can occasionally fail. Even one logic gate failing in a circuit like this can result in very complex malfunctions. Also, TTL chips can have various failure modes. It is usually fairly obvious when their output stage fails and the output voltage goes outside the range of standard TTL logic highs and lows. One interesting failure is that sometimes a gate input can go open circuit inside the package. When that happens, the chip’s die assumes the pin is logic high, so a multi-input gate chip can still produce normal-­looking output pulses, but they are the wrong pulses. The only guaranteed method of ensuring a logic chip is working properly is to verify that it is obeying its logic table. Other ICs in the PET are not as reliable as the 74-series TTL parts. In many vintage PET repairs, the 2114 SRAM ICs have been found to be defective. There is also a fairly high failure rate of the 4116 DRAM ICs, and the PIA and VIA chips occasionally fail. The MC3446 GPIB bus driver ICs can also fail occasionally. Mostly, the 6502 CPU remains reliable. I aim to present further articles later to help with PET repairs. One will be on testing the dynamic RAM (DRAM) siliconchip.com.au Fig.19: the CAG address counter continues to run during the vertical retrace and raster blanking periods, but the values are not meaningful. Still, they are shown here, along with the various pulses, to aid in debugging in case something has gone wrong. A missing signal may be the key to diagnosing the fault. memory chips. It uses a diagnostic system based on an added hardware module and some custom firmware held in a ROM that is plugged into the board. Another will be on the Dynamic PET’s 9-inch CRT VDU. This article will describe how to restore Australia's electronics magazine and improve the VDU, including a detailed analysis of flyback transformers and how to test and diagnose them. It will also cover possible non-standard part replacements because the original parts are very SC difficult to find now. September 2026  69 Part 4: plastic & electromechanical parts Phil Prosser’s Phenomenal N ow we are getting to the real ‘meat’ of this project. This article will walk you through the construction of the electromechanical parts and plastic guides. The interplay of light, sound and movement is the essence of a pinball machine. In planning and designing these, we considered what was needed to make a ‘proper’ pinball machine. Asking around, we came up with the following: Launching systems Flippers Reloading systems Kickers and bumpers To make all these parts, we need to combine 3D-printed pieces with solenoids, screws, nuts and other hardware bits. We will explain how these are assembled, keeping the descriptions mostly to exploded diagrams, tips and pointers. Once you have the hang of building, say, the flipper, the kicker is broadly similar in approach. So it gets easier as you go. 📍 📍 📍 📍 Sub-assemblies Pinball Machine Having described and built the electronics and started on the deck layout and cabinet, it’s now time to get to the really fun part: building and testing the flippers, bumpers, kickers, ball return mechanism and related parts. Ladies and gentlemen, fire up your 3D printers! One or two people I spoke to liked the idea of ball traps, where the ball falls into a recess and then the player is given a second ball to use. At some point, the original ball is released, allowing you to play with two balls on the deck at the same time. Still, we had to stop somewhere, so we didn’t include a ball trap in our machine. It probably wouldn’t be hard to make one using the building blocks we supply. There are a couple of unused high-current outputs that could provide that function and the software could be modified to suit. For the various parts (flippers, bumpers, kickers etc), you should be able to build and test them separately without needing a pinball deck. Only the bumper and flipper need to come apart again for final deck assembly. Every part in the Pinball Machine has been sized to work with a 22mm steel pinball. Many professional machines use a larger ball; we wanted to have an authentic pinball experience but really wanted the project to accommodate those with less room. We also didn’t want to make the mechanical aspects of the build too challenging. 22mm ball bearings are readily available. While we’ve done a lot of the hard work and are presenting stuff that we know works, we expect you will have a few ‘go arounds’ on making your first Australia's electronics magazine siliconchip.com.au 70 Silicon Chip Pinball Machine. It’s a big, complex electromechanical device, and no two are likely to be identical. Let’s go over the electromechanical parts in a little more detail than last time before we get to the parts required and assembly instructions. Reload and ball release mechanism The ball reload and release mechanism is automated. This is part of the drama of a pinball machine; when the player loses the game, the system detects this and actuates a solenoid, which propels the ball back into the launch system. This is a simple lever, which allows the solenoid to pull inward, and the lever pushes the ball up the alleyway. Because the table is tilted, once the ball goes over the summit, it rolls down into the release trap. This holds the ball ready for the ‘game start’ signal, which actuates a second solenoid that releases the ball to the launch plunger. The flippers The flippers have a lot of work to do and presented the greatest mechanical challenge. Generating sufficient force to play well is not easy. Good pre-made flipper units are horribly expensive, so we really needed to come up with a more creative solution than throwing money at the problem. Our design was initially very similar to those you will find in an arcade machine. They drive their solenoids with very high voltages and commensurately high currents to get the force required to propel the ball all the way to the top of the table. Our first design followed the oldschool approach, driving 12V solenoids with 48V to get good performance. We could have gone for larger solenoids, but they are very expensive and larger than we wanted. We needed a solution that used cost-effective parts but at lower, safer voltages. One approach we saw on the internet used two solenoids. This is attractive for several reasons. Firstly, we can double the torque on the flippers and get good performance with 24V DC, which is a safe voltage. Secondly, by arranging the solenoids on either side of the flipper axle, we can balance the forces from the solenoids. This increases torque while minimising the overturning moment, which reduces friction in the flipper siliconchip.com.au axle. This is a superior approach to that used in old-school pinball machines, if more complex. We specify 12V, 1.5A solenoids which we overdrive at 24V relative to their 12V specification. Be sure to order the right ones, as there are outwardly similar units that are rated at 12V but operate at a much lower current. These do not generate sufficient force for this application. Our software drives the solenoids at 100% duty cycle for 200ms to generate maximum force for the ball hit, then if the button is held, pulls the duty cycle back to 20%, which reduces dissipation in the flipper significantly while the player holds the flipper ‘open’. This is essential, as the solenoids get very hot when driven at 24V continuously. Bumpers Bumpers sense the ball hitting them and then ‘bump’ the ball away through the action of a circular plunger being pulled down by a solenoid. This is quite a violent action; it is a noisy and interesting aspect of a pinball game. Photo 13 shows how they look on the deck. The forces involved led us to using threaded rod to connect the solenoid to the plunger. this is a strong connection, but somewhat fiddly to set up. We initially tried to 3D-print the whole thing, but it would never have survived long term. Old-school machines used a rather complicated approach to sensing the ball hitting them, which involved the ball rolling onto a disc and tilting it. A finger protrudes through the deck, and this finger actuates a microswitch as its Photo 13: bumpers are one of the most exciting parts of a pinball game. It’s important to include plenty of LEDs to enhance that. Australia's electronics magazine angle changes. We replicated this in an early prototype, but the complexity of this and the fine adjustment required precluded us from proceeding down that path. We have instead taken the approach of integrating a small tactile switch under a similar sense disc and adjusting this to be a ‘hair trigger’ with a jacking screw. This screw serves two purposes. Firstly, it holds the disc above the deck, which the play ball will depress when it rolls onto it. Secondly, it ‘primes’ the tactile switch so that it is on the edge of actuation, making it quite sensitive to the ball rolling onto it. Apart from this, the overall bumper design draws many parallels to those used in conventional pinball machines. One challenge with the bumpers is that there are parts both above and below the play deck, so while you can assemble and adjust the bumper away from the board, you need to separate the upper and lower parts to install it. Old-school mechanisms used a steel plunger. To avoid the complexity of fabricating this, we used 3D-printed parts. When you print the plungers, make sure to set the wall thickness to a minimum of 2.4mm to achieve very thick walls that will better weather the impacts on the ball. We have also used two threaded rods to attach the plunger to the solenoid and made the attachment point of these to the plunger quite thick. This gives us a largely printable bumper that is reliable. To get lights in the top of the plunger, there are holes that run through all parts of the plunger, including the sense disc, allowing the LEDs in the top to connect via flying leads. We have specified two high-brightness LEDs at the top of the bumper and operate them at 30mA to generate bright light effects. Once the holes through the parts are clear (a 2mm drill is your friend), assembly is not that hard. Of all the parts in the machine, the bumpers are definitely the most challenging to assemble, but once you have built one, they are not too bad. Also, they are really important in making the game ‘authentic’! Kickers The kickers operate by sensing the ball hitting a microswitch and then September 2026  71 actuating a solenoid that pushes or ‘kicks’ the ball away. They are not complex, being essentially a curved lever actuated by the solenoid. We have mounted the sensing microswitches via separate brackets, as this allows you to install and adjust these independently. The most important tip we found setting these up is that adjusting the microswitch lever is very important. It needs to be sensitive but not get stuck on. We ended up bending ours with needle-nose pliers to get the sensitivity just right. Because of the way the kicker works, the sensitivity, speed and direction in which the ball is kicked vary a lot. This is all part of the random fun of these in the game. Parts List – Reload Mechanism 1 TAU-0826 12V 1.5A solenoid 1 2-way vertical pluggable terminal block 2 2-way polarised header plugs with matching pins 2 LJ12A3-2 2mm inductive sensors 1 assembled Switch Input PCB (code 08107268) 3D-printed parts (all PLA) 1 Lower Deck Layout L (10% fill, 1.2mm wall, print on raft) 1 Lower Deck Outer Runway L (10% fill, 1.2mm wall) 1 Reload Load Coupling Bushing (30% fill, 2.4mm wall) 1 Reload Load Coupling (30% fill, 2.4mm wall) 1 Reload Load Flipper (30% fill, 2.4mm wall) 1 Reload Load Retaining Washer (solid) Hardware & wire 1 6mm-long self-tapping box screw (4G self-tapper) (to attach the coupling to flipper) 7 6G × 16mm wood screws (to attach the reload mechanism to the deck) 2 M3 × 25mm panhead machine screws (connecting the reload coupling to the solenoid armature and the coupling to the flipper) 2 M3 Nyloc hex nuts 1 M3 × 6mm panhead machine screw (to attach the solenoid to the base) 1 1m length of medium-duty figure-8 speaker wire M3 nut & bolt Self-tapping box screw Fig.22: this shows how the 3D-printed parts of the reload mechanism in the lower-left deck area go together. Parts List – Lower Deck Middle 1 3D-printed Lower Deck Layout M (PLA, 10% fill, 1.2mm wall, print on raft) 4 6G × 16mm wood screws Fig.23: the lower centre deck section is simply this 3D-printed plastic part with guide rails for the ball. 72 Silicon Chip Australia's electronics magazine Rollover sensors There are several rollover sensors, which in our case are inductive sensors. We chose 12mm diameter, 2mm detection range sensors. These mount through the play deck and sense the ball either rolling over them or stationed above them. We use one on the reload mechanism. Most of these are specified to operate from 6-32V DC, so we have designed the controller and breakout board to provide 24V DC to these. Their output is open-collector style, so they pull the relevant input to ground, as do all other inputs (like microswitches). If you choose to use old-school microswitch rollover sensors, they are a direct substitute. We have 3D-printed parts to form under-deck brackets holding the inductive sensors in place; you can use a 3mm screw to hold the sensor tight. We found that drilling a 12mm hole for the 12mm sensors worked well; we kind of ran the drill in and out a ‘few extra times’ to clear the hole, which is terrible form but made the mounting hole that little bit larger so things fit easily. Now that we’ve gotten that all out of the way, let’s build some parts! Reload (lower deck left) construction ■ The reload mechanism is in two parts. The left side of the deck needs the ball sensor installed through the deck, as shown in the deck drawing last month, so the controller ‘sees’ when the game is lost and the ball is ready to reload. This is essential. While part of the deck, it is good to get the marking and drilling for the hole done along with the reload mechanism. We find that marking the exact locations of holes like this using the printed parts reduces errors in the final alignment. Fig.22 gives a sense of what goes where in this part of the deck. Mount the 12V, 1.5A solenoid to the deck section using 6mm M3 screws, with either superglue or Loctite to ensure the solenoid never shakes loose. Then install the bushing onto the axle, which is printed into the base section. It will slip right on and should rotate freely. Install the flipper on top of this. Connect the solenoid armature to the coupling with the 25mm machine screw, but do not over-tighten the nut, siliconchip.com.au as this needs to articulate over the throw of the solenoid. Install the reload flipper on top of the bushing and push the armature into the solenoid, then the coupling onto the flipper. Using the printed washer and self-tapping box screw to secure the coupling to the flipper. We have printed a hole into the flipper axle so the self-tapping box screw will tighten easily. Do not over-tighten this, as you need the coupling to move freely. This screw is just there to stop it from falling off. Make sure to keep that spring with the solenoid, as it returns the flipper to a neutral state when the solenoid is not operating. Take care, because if you drop them, they tend to disappear into another dimension. Lower deck middle construction ■ The lower middle deck section is simply one 3D-printed piece, shown in Fig.23. This fits to the right section with two locating lugs and butts to the left section once installed along the base. Parts List – Ball Release & Launcher 1 TAU-0826 12V 1.5A solenoid 1 2-way vertical pluggable terminal block 3D-printed parts (all PLA) 1 Lower Deck Layout R (10% fill, 1.2mm wall, print on raft) 1 Ball Reload Positioner (30% fill, 2.4mm wall thickness) 1 Launcher 6.6 degree Shim (solid) 1 Lower Deck Outer Runway Right (10% fill, 1.2mm wall) 1 Reload Ball Release Drive (30% fill, 2.4mm wall) 1 Reload Ball Release Lower Washer (30% fill, 2.4mm wall [solid]) 1 Reload Ball Release Slide Coupling (30% fill, 2.4mm wall) 1 Reload Ball Release Slide (30% fill, 2.4mm wall) 1 Reload Ball Release Solenoid Coupling (30% fill, 2.4mm wall) Hardware & wire 5 6mm-long self-tapping box screws (4G self-tappers) (ball positioner fixing, attaching couplings to drive and drive to deck) 5 M3 flat washers (under self-tapping box screws to secure the couplings to the release drive) 7 6G × 16mm wood screws (to attach the reload mechanism to the deck) 1 M3 × 25mm panhead machine screw (to connect the reload coupling to the solenoid armature) 1 M3 Nyloc hex nut 1 1m length of medium-duty figure-8 speaker wire Ball release & launcher (lower deck right) construction ■ The lower right deck section includes the ball release, the launcher and reload adjustment. Take a look at the exploded diagram, Fig.24, to get a sense of what goes where in this part of the deck. Install the solenoid using the M3 × 6mm screws and make sure to glue or Loctite them into place. Then install the large washer and the circular drive mechanism. Take the drive, coupling and slide, and work out which is which. Next, secure the solenoid coupling to the solenoid armature using the 25mm screw and Nyloc nut, then get these assembled to the deck section. Finally, use three 6mm self-tapping box screws to secure the couplings to the drive and the drive to the deck. The ball positioner can be loosely installed now; final adjustment is required once the deck is assembled, to get the ball in the best location for the launcher to propel it up the table. We include a 6.6° launcher shim, as this will be required to get your launcher to fit neatly against a vertical front panel. Make sure to keep that spring with the solenoid, as it returns the release siliconchip.com.au M3 nut & bolt self-tapping box screw & washer Fig.24: this shows how the parts for the ball release, launcher and reloader go together in the lower-right section of the deck. to a neutral state when the solenoid is not operating. If you apply 12V to the solenoid now, it should operate freely. Flipper construction ■ The flipper assembly has quite a few parts, shown in Figs.25 & 26. You need a 23mm or larger hole in the deck. For our prototypes, we simply used a 25mm spade bit to make that hole. The flipper assembly goes through the deck, with the solenoids and drive below and the flipper above. Australia's electronics magazine You should do an initial build off the deck, then pull the flipper off for final installation. We apologise right now that this mechanical part will be fiddly to set up with the multiple solenoids and couplings. The assembly process follows these steps: 01 There are normal and mirrored parts for the flipper base and drive. Make sure you have the right set of parts. If you try fitting the wrong bits together, it will be really confusing. September 2026  73 Parts List – One pair of Flipper Assemblies 4 TAU-0826 12V 1.5A solenoids 4 2-way vertical pluggable terminal blocks 3D-printed parts (all PLA unless noted) 1 Double Drive – Driver (solid) 1 Double Drive – Driver Mirrored (solid) 2 Double Drive – Limiter (40% fill, 2.4mm wall) 1 Double Drive 12mm Deck (30% fill, 2.4mm wall, PLA or ABS [preferred], print on raft) 1 Double Drive 12mm Deck Mirrored (30% fill, 2.4mm wall, PLA or ABS [preferred], print on raft) 2 Drive Arm Washer (solid) 2 Flipper 90 plus 4 (40% fill, 5mm wall, ABS) ■ 4 Solenoid Coupling 38mm (solid) ▲ 2 Washer Lower (solid) 2 Washer Upper (solid) ■ “plus 4” is the shaft length, which is 1mm more than a 12mm deck needs. Other lengths are available if you need them. ▲ other lengths are available if you need them. Hardware & wire 4 6mm-long self-tapping box screws (4G self-tappers) (to retain the flipper drive to the flipper axle) 4 M3 flat washers (under self-tapping box screws) 6 6G × 16mm wood screws (to attach the flipper base to the deck) 4 M3 × 25mm panhead machine screws (to connect the couplings to the solenoid armatures) 8 M3 × 20mm panhead machine screws (to connect the drive limiter to the base and the solenoid couplings to the plungers) 4 M3 × 16mm panhead machine screws (through the drive mechanism to secure the couplings) 8 M3 × 6mm panhead machine screws (to attach the solenoids to the base) 9 M3 Nyloc hex nuts (for the 16mm & 25mm machine screws) 1 1m length of medium-duty figure-8 speaker wire 1 50cm length of 10 × 5mm neoprene rubber band Fig.25: the flipper mechanism is somewhat complex, using two solenoids in a balanced arrangement for more power and stability. Assemble them as shown here. M3 nut & bolt from underneath drive & deck 20mm M3 nut & bolt underneath Fig.26: an exploded view of the flipper assembly. This is ‘upside down’; the solenoids are underneath the deck and the flipper above. The two M3 × 16mm machine screws with nuts connect the drive to the couplings (red and green), with self-tapping screws and washers holding the couplings to the flipper axle. 74 Silicon Chip Australia's electronics magazine 02 Fit the drive limiter to your base section using four M3 × 20mm machine screws and Nyloc nuts. Fit each solenoid to the base using two M3 × 6mm machine screws, with Loctite or superglue on the threads. Our design assumes the TAU0826 20N 12V solenoid, which you really need to use for the flippers to get the torque required. These need to go on before installing the base to the pinball deck. 03 Fit the base and bushing from the underside of the deck. The bushing fits through the hole in the deck and ensures that the flipper operates smoothly. You need to use the 12mm drive part for a 12mm deck. Screw the base section to the underside of the deck with the hole lined up. Fix the base section to the underside of the deck using 6G × 16mm wood screws. 04 Take the 10×5mm neoprene rubber and cut one end at 45°. Wrap the rubber around the flipper and work out where to cut the other end so the rubber fits tightly onto the flipper. Use superglue to glue the two ends together. 05 Select your flipper (right or left) and put the 27mm diameter printed washer on the flipper axle. Put this through the deck from the top. 06 Once this is secure, install the flipper and upper 27mm washer from the top of the deck side. 07 Connect the two solenoid drive arms to the flipper drive using M3 × 16mm machine screws. The screw comes through the drive from the rear, and the coupling is secured to the small axle with it, which adds significant mechanical integrity to this coupling. We used the 38mm arms; you should dry-fit yours and choose the right length. 08 Now put the lower 27mm washer on the flipper shaft from the rear and then add the flipper drive coupling. 09 Referring to Fig.25, connect the flipper drive to one of the solenoids using a 25mm-long M3 machine screw and Nyloc nut, then assemble the flipper drive to the flipper and insert the solenoid armature into the solenoid. You should now be able to insert the second solenoid armature into its solenoid and then insert the connecting 25mm M3 machine screw and Nyloc nut to get the assembly together. 10 Secure the flipper drive to the rear of the flipper axle with two 4G × 6mm screws and 3mm ID flat washers. This will hold the flipper drive to the siliconchip.com.au Photos 14 & 15: a completed flipper assembly, viewed from underneath and side-on. flipper axle, and if you ever need to get things apart, you can. 11 At this point the flipper ought to be able to rotate quite freely. If not, is your deck too thick? We have included flipper STL files with extended axle lengths that would allow thicker decks to be accommodated. 12 Connect the solenoid couplings to the solenoid plungers using M3 × 20mm machine screws and Nyloc nuts. You should now have functional flippers. Photos 14 & 15 show the finished flipper assembly. You can test this section by applying 12V to the solenoids. If you choose to do this, make sure there is a reverse diode across the solenoids to stop the back-EMF from damaging your power supply. The flipper action is pretty rapid and will give you a sense of what to expect with the other parts. Bumper construction ■ You’ll probably want to make several bumpers (our machine has three). The assembly process is: 01 Check that the 3D printed pieces fit together and run smoothly. We have designed everything with a good gap to ensure things don’t stick. Check the prints don’t have bumps or dags; if things don’t run well, you can sand parts if needed, though we didn’t with these tolerances. The exploded drawings are in Figs.27 & 28. 02 We put two high-brightness white LEDs in the top of the bumper as shown overleaf. We used parts from the Cree C513A series, which worked a treat. Bend the legs as shown and solder them anode-to-cathode so they are in series. We ran the anode and cathode wires right through the assembly, as shown in Photo 16. 03 Now we need to get the lower base assembly piece and install the ball detection microswitch. The part has been made to accommodate an siliconchip.com.au Parts List – Bumper Assembly 1 TAU-0826 12V 1.5A solenoid 1 2-way vertical pluggable terminal block 2 2-way polarised header plugs with matching pins 1 four-pin SMD tactile switch with short actuator [Altronics S1112A, Jaycar SP0610] 2 Cree C513A series 30mA LEDs (any colour) 3D-printed parts (all PLA) 1 Bumper Ball Detect Larger (solid) 1 Bumper Base Lower (30% fill, 2.4mm wall) 1 Bumper Base (30% fill, 2.4mm wall) 1 Bumper Plunger Coupling (solid) 1 Bumper Plunger (solid) 1 Bumper Top (solid – make this colourful, maybe transparent or translucent; the LEDs light through it) 1 Bumper Under Deck Bracket (30% fill, 2.4mm wall; should not need supports but remove them if used) 1 Bumper Shim for Base to Clear LED Holders (2.4mm wall) Hardware & wire 9 9mm-long self-tapping box screws (4G self-tappers) (attach upper to lower base, attach lower base to deck bracket, secure top of bumper) 2 80mm lengths of M3 threaded rod (cut from a longer piece) 1 M3 × 20mm panhead machine screw (to secure the solenoid armature to the coupling) 1 M3 × 10mm panhead machine screw (as a jack screw for the microswitch) 2 M3 × 6mm panhead machine screws (to secure the solenoid to the under-deck bracket) 9 M3 Nyloc hex nut (for the 20mm machine screw and threaded rods) 1 1m length of medium-duty figure-8 speaker wire 4 1m lengths of light-duty hookup wire (green, green, red & black/white) Fig.27 (above): an exploded diagram of the bumper assembly, with the threaded rods not shown. 9mm jiffy screw Fig.28 (right): joining the upper and lower 3D-printed sections of the bumper. M3 10mm screw Australia's electronics magazine September 2026  75 Photo 16: LEDs installed in the top of the bumper. The recess in the upper base section has been sized to fit a 5mm LED. We chose high-brightness white LEDs, but you could use coloured types. Photo 18: the upper and lower bumper sections with the switch and LEDs installed and secured. Altronics S1112A SMD tactile switch (it has standard dimensions so if you can’t get that one, you’ll be able to find an equivalent). The connections for the wires are as shown in Photo 17. Use a multimeter to check that you have connected to the right terminals, as it is bothersome pulling this apart to fix a silly error. 04 Feed the green wire through the inner holes in the upper and lower base pieces. We ran a 1.5-2mm drill bit through all the holes to clear the printing dags from inside these holes. After that, the light-duty wire fit well (it didn’t at first!). 05 Solder the tactile switch to the 500mm lengths of light-duty hookup wire and get the switch snug in the cavity. This is deep enough that you need to use the M3 adjusting screw to raise the switch when adjusting it later on. Make sure the tactile switch is central to the recess. Adjust the hookup wire if necessary – this is required to get the jacking screw central to the switch. 06 Using two 500mm lengths of light-duty hookup wire, run red and black wires through the lower and upper bumper base pieces. Solder the red wire to the anode (more positive) LED terminal, making sure not to leave a blob of solder, as this needs to fit into a 1.2mm hole. Repeat with the black wire for the cathode. 07 Now fit the ball detection disc to the lower base section as shown in the figures, then feed the LED wires through the holes in both the ball detection disc and the lower base section. 76 Silicon Chip Photo 17: the bumper LED and sensor wiring. We soldered lightduty hookup directly to the LED leads and tactile switch. Take care with soldering to the LED leads to ensure it fits into the 1.2mm hole in the bumper. Make the flying leads at least 500mm long to reach the breakout board. 08 Push 4G 9mm panhead screws into the holes in the lower base section and affix the lower and upper parts (see Fig.28). Once together, this should be as shown in Photo 18. 09 Push a 10mm M3 machine screw into the centre hole. We can screw this in to adjust the sensitivity of the ball detection. This needs to be screwed forward enough to jack up the tactile switch to the point that it lifts the ball detect disc to the upper base assembly. When adjusted properly, the switch is not depressed, but pushing down on any edge of the ball detect mechanism will actuate the switch. This is noticeable when you push the disc with your finger, so adjusting it is not hard. Do not over-tighten this as you will simply force the switch to always be on. 10 For a 12mm deck, cut 80mm lengths of M3 threaded rod and thread them onto the plunger. Remember that the top is the flat side. Use a Nyloc nut at the top so you can get this off later if needed. Run a normal M3 nut up from the bottom of the thread and lock it to the bottom of the plunger bracket. Put a drop of superglue or threadlocker on the bottom nut to ensure it does not come loose. 11 Now present this assembly to the below-deck section. Push the lower base section in until the top of this is about 0.5mm above the deck height, which will need to include the mounting shim, as shown in Photo 19. This prototype used machine screws that were about 6mm too short. This resulted in the top two assembly holes aligning between the under-deck bracket and the lower base piece. These accept 4G × 6mm screws but use the holes which align for you. We have included a whole Australia's electronics magazine range of staggered holes, allowing the bumper to be used with a range of deck thicknesses. 12 Now we need to install the plunger coupling to the solenoid using an M3 × 20 machine screw and Nyloc nut. 13 With the plunger and base assembled to the under-deck section at the right height, we now need to assemble the plunger coupling to the plunger’s M3 threaded rod. With the plunger aligned with the top of the bumper (as high as it will go), put two normal M3 machine screws onto the threaded rod about 15mm from the bottom, then push the plunger coupling on and add two Nyloc nuts from the bottom. You will need to have the solenoid in the base assembly to do this, loosely affixed using two M3 × 6mm screws and lock washers. With the plunger at maximum height, tighten the M3 nuts on the threaded rod. Now push the plunger down and check that it does not hit the ball detect disc. Adjust these until it works, and fix the normal M3 machine screws with superglue, then tighten the solenoid. 14 Make the connections to the solenoid using heavy-duty wire. Solder 500mm lengths to it and insulate the connections. 15 Photo 20 shows the finished bumper. To test this, use a 24V DC supply. On connection, the bumper should compress downward quite Photos 19 & 20: the initial construction of the bumper (left). A completed bumper, ready for installation (right). siliconchip.com.au rapidly, and if you have everything connected to the bumper/kicker board, the LEDs will light. If anything is not running smoothly, address the problem before proceeding. Our prototypes all worked fresh off the printer, so if your printer is adjusted well, you should not need to fiddle too much. Kicker construction ■ You’ll probably want to make several kickers (our machine has two). The overall kicker design is shown in Fig.29. We have made the parts different colours to make them easier to distinguish. Assembling of the kicker is pretty simple, especially compared to the bumper. We have included holes to suit the two main solenoid types that we expect you will find. The assembly process is: 01 Check that all the parts printed OK. 02 Screw the kicker coupling to the solenoid using a 20mm M3 machine screw and Nyloc nut. 03 Connect the kicker coupling to the kicker arm using another 20mm machine screw and Nyloc nut. 04 With all of these connected, push the solenoid into the kicker base and jiggle the kicker arm to align the mounting hole. Use another 20mm machine screw and Nyloc nut to secure these. The complete assembly is shown in Photos 20 & 21. 05 Solder a diode across the solenoid leads if you are testing this, as the back-EMF from these solenoids is extremely powerful. 06 Use a 24V DC supply to test this. Apply voltage and the kicker should kick quite violently, then return to its rest position when disconnected. To assemble the microswitches used to trigger the kicker: 01 Take a kicker microswitch bracket and assemble a 25mm lever microswitch to it. The aim of these is to get the lever end of the microswitch up to the height of the rubber band around the kicker. 02 Use 20mm-long M2.5 machine screws and Nyloc nuts to secure these, as shown in Photo 22. 03 Mount the kicker microswitch assembly next to the kicker, under the play deck. Use the adjustment to set the depth so that the kicker rubber band ‘rope’ is close against the microswitch, to ensure this switch is sensitive to the ball hitting the rope. Next month Wow, that is a lot of parts, and in most cases, you need several of each. That is all for this month. Next month, we will finally get to the end of this series. We will show the final assembly steps and how to get your whole Pinball Machine up and running. You will see that we have a custom theme for our machine. We really expect you will have your own theme and approach to decorating your own creation. This is where you get to go wild. Loud colours, light layouts and your artistic creativity can come to SC the fore. Parts List – Kicker Assembly Photos 20 & 21: the assembled kicker when printed using white filament. The final kicker has a narrower ‘paddle’ at the top and you should use Nyloc nuts on all machine screws. 1 TAU-0826 12V 1.5A solenoid 1 2-way vertical pluggable terminal block 2 2-way polarised header plugs with matching pins 2 KW12 25mm roller (straight) lever arm microswitches 1 50cm length of 5 × 5mm neoprene rubber band 3D-printed parts (all PLA) 1 Kicker Arm 12mm deck (solid) 1 Kicker Base (solid) 1 Kicker Coupling (solid) 2 Kicker Microswitch Bracket (solid) Hardware & wire 3 M3 × 20mm panhead machine screws (to secure the solenoid armature to the coupling and then to the kicker) 2 M3 × 6mm panhead machine screws (to secure the solenoid to the under-deck bracket) 3 M3 Nyloc hex nuts 4 M2.5 × 20mm panhead machine screws 4 M2.5 Nyloc hex nuts 1 1m length of medium-duty figure-8 speaker wire 2 1m lengths of green light-duty hookup wire Photo 22: the kicker trigger microswitches attached to their 3D-printed brackets. Fig.29: this shows how the 3D-printed sections of the kicker go together. siliconchip.com.au Australia's electronics magazine September 2026  77 Project by Tim Blythman Battery BackPack for GPS Clocks USB-rechargeable lithium-ion battery LED charge indicator Mid-rail tap for GPS Analog Clock driver Mounts directly to the 2022 GPS Clock Driver PCB Pin headers for general-purpose use Regulated 3.0V/3.3V output at up to 500mA (other voltages possible) While this design sounds like it has a niche application, it can be used in many places that need an uninterrupted power source at a low voltage and with a modest current requirement. It neatly replaces the battery pack in our 2022 GPS Clock Driver, making it a simple upgrade. This add-on PCB (shown attach to the GPS-Synchronised Analog Clock Driver) is much the same size as two AA cell holders and can replace the battery on the Analog Clock Driver. It can also be used as a general-purpose, uninterruptible, low-voltage power supply. I have a GPS-Synchronised Analog Clock (September 2022, siliconchip. au/Article/15466) on my office wall. I originally built it to test my WiFi Time Source for GPS Clocks (June 2023, siliconchip.au/Article/15823) but, perhaps unsurprisingly, I found it quite useful. Unfortunately, I’m finding that non-rechargeable AA batteries don’t last as long as I had hoped. I think I may have a pack of dud cells, or perhaps the current draw of the clock is a bit higher than Geoff’s prototype. Since there are a few steps to reset the clock when the batteries need to be replaced, it is a hassle when the battery goes flat. I figured that a rechargeable battery with a USB socket could replace the two 1.5V cells and would have uses in other places where a simple, rechargeable power supply is needed. The circuit is quite simple, but there is one small catch to make it work with the Clock, which we’ll discuss shortly. Effectively, the Battery BackPack is powered from a mains USB power supply, keeping the onboard lithium-ion cell fully charged. The USB power supply provides power to the connected clock or other device while it is present. If the power source is removed, the lithium-ion battery takes over. Circuit details Fig.1 shows the circuit. You might recognise that this is quite similar to the rechargeable battery circuit we have used in some other projects, such as the August/September 2025 USB-C Power Monitor (siliconchip. com.au/Series/445) and the September 2024 Compact OLED Clock and Timer (siliconchip.au/Article/16570). The main component is IC1, an MCP73831 charge regulator IC. USB power is provided at CON1, a USB-C socket. For USB-C, the two 5.1kW resistors are needed to communicate that this device is a power sink and should be supplied 5V from a power source. CON2 provides a simple pin header that can be used as an alternate connection for an incoming 5V DC supply. The two 10μF capacitors and the 10kW resistor are the minimum data Fig.1: this design provides circuitry to charge a lithium-ion cell and provide a regulated 3.3V output. CON4 matches a header on the GPS-Synchronised Analog Clock Driver, allowing it to replace the existing battery arrangement directly. 78 Silicon Chip Australia's electronics magazine siliconchip.com.au The header on the right connects to the Clock Driver PCB. If you are not using it with a Clock Driver, this tab can be removed to save space. The 470μF capacitor and two 100kW resistors are also not needed in this case. Scope 1: the blue shows the voltage on the 470μF capacitor as the clock advances, while the red trace is the 3.3V supply rail. Even with the deviation seen here, there is more than enough voltage to drive the clock mechanism. sheet requirements needed for the IC1 charge regulator chip to function. The 10μF capacitors provide input bypassing and output filtering, while the 10kW resistor sets the battery charge current to 100mA. IC1 provides a STAT output that is low during charging and high when the battery is fully charged. We have connected bicolour LED1 as shown in Fig.1; the arrangement of the 1kW resistors means that it will show red while charging and green when charging is completed. The resistor chain is powered from the USB supply, so it does not waste battery power, and the LED will be off if USB power is absent. Thus, you can quickly know if the unit is running from USB power or running down the battery. The three schottky diodes allow a seamless transition when USB power is applied or removed. When USB power is present, current flows through D2 and D3. D1 is reverse-­ biased, so no current is drawn from the battery, and the charge controller can accurately sense the battery voltage and current for correct and complete charging. D1 also prevents USB power from being directly fed into the battery. We use D2 and D3 in series from the USB supply to help share some of the dissipation in the downstream regulator. With typical load currents, the extra diode will drop no more than 0.3V, so even at 4.5V, a badly sagging USB supply will still provide power in preference to a fully charged lithium-­ ion cell at 4.2V. REG1 and its two 10μF bypass/filter capacitors provide a regulated 3.3V that is used to power the attached clock circuitry (or other load). The expected voltage from a pair of AA cells is 3V, but it is possible to see over 3.2V from a pair of fresh alkaline cells. So virtually any circuit designed to run from two alkaline cells should work with a 3.3V supply. In any case, the GPS-Synchronised Analog Clock Driver specifically can operate with a supply up to 3.6V, limited mainly by its microcontroller, so 3.3V is suitable and uses a commonly available regulator value. If you plan to power a particularly sensitive circuit, you could swap it for a 3.0V regulator, which is also available. However, the dissipation in the regulator would be a little higher, so the maximum load current will probably be reduced somewhat. The MIC37100 has been specifically chosen for its low dropout voltage, ensuring that it can provide 3.3V (or near enough), even with a fairly flat Li-ion cell. The SOT-223 package allows more dissipation than a smaller SOT-23 part would. Its quiescent current is slightly higher than other similar parts, and there is no low-voltage cutout, so it is important that a protected lithium-­ion cell is used. siliconchip.com.au Australia's electronics magazine The remaining circuitry provides the centre tap that is derived from the midpoint of the two AA cells in the original circuit. This allows the Clock Driver to produce a bipolar drive signal to the clock movement. While there are many possible ways of doing this, the drive signal current is quite modest, and importantly, it should be symmetrical about the supply mid-rail. So we have simply provided a fairly large half-rail bypass capacitor (470μF). It is biased towards the midrail voltage by a 100kW/100kW resistor divider, which draws only a few extra microamps from the rechargeable cell. The time constant of this arrangement is around 23s, so it does take a while to settle. Fortunately, the Clock Driver does not start driving the clock mechanism immediately, giving time for the settling to occur. The Clock Driver output will also tend to pull the capacitor towards mid-rail. Scope 1 shows the voltage on the capacitor as the pulses alternate. You can see that the alternating pulses cancel out their effects on the capacitor, and its average voltage remains near the mid-rail as desired. Performance Figs.2 & 3 show some charts of the output voltage and component dissipation at various output currents. Since REG1 is a linear device, the September 2026  79 input current roughly matches the output current, although there is a small amount of current through its ground pin. The data sheet notes a typical ground current of 11mA for a 1A output; Fig.3 takes this into account by assuming a constant 11mA ground current. We expect it will be lower than this in most cases (around 0.5mA at the current drawn by a typical clock). Fig.2 indicates that the regulated 3.3V output will be maintained at all times with the USB supply or a fully charged cell. We expect that will cover most scenarios, since the device is not intended to be used for long periods without USB power. Even with a discharged Li-ion cell, which could drop as low as 3.3V, the unit still provides 2.5V at a 1A load, well above the 2.25V that causes the Clock Driver to switch to low-power mode. Typically, a device designed to run from two alkaline cells will run down to 2V (1V per cell), although occasionally you will find a device that stops working at a higher voltage. 2.25V should be pretty safe for most such devices. With good output voltage performance, Fig.3 is more critical to checking the range of safe and proper operation. The 1N5819WS data sheet notes a maximum dissipation of 250mW at 25°C; you can see that is reached at around 600mA of current. This part does require derating as the temperature rises, with 40°C ambient reducing that to 200mW, reached at 500mA output current. The SOT-223 package of the regulator is typically rated for over 1W, so it should have no issues with continuous operation up to 500mA. The PCB has large copper areas and thus good thermal mass, so it should be able to handle brief excursions above this. In practice, our clock drew just over 100mA on power-up, and the normal operating draw is around 3.3mA, which appears to be on the higher side of what’s expected (perhaps the cause of our frequently flat battery!). So even a power-hungry clock should be able to operate for a few weeks without USB power. PCB design The Battery BackPack has been designed to replace the pair of AA cells on the GPS-Synchronised Analog Clock Driver and thus the BackPack PCB is the size of two AA cells. So it is a suitable size for other applications that expect a pair of AA cells. If you aren’t using the Battery BackPack with the Clock Driver, power can be supplied via either the CON1 USB-C socket or the CON2 header. To avoid problems with two power supplies feeding each other, we recommend choosing and fitting just one power input socket. If using CON2, the standard 5V USB range of 4.75V to 5.25V is safe. Lower voltages should not cause damage, but IC1 may not be able to charge the cell unless there is at least 0.3V of headroom. IC1 can operate up to 6V, but you will need to derate REG1 if the supply voltage is higher than 5.25V. Half of the PCB is taken up by the single AA (14500-sized) cell holder, while the remainder is covered with the circuitry seen in Fig.1. The CON4 output connection has the same pinout as CON3 on the GPS-Synchronised Analog Clock Driver PCB, allowing direct mounting of the new board on the Clock PCB. CON4 is on a small, protruding tab which can be easily snapped off if CON4 is not needed. The so-called mouse-bites encourage the board to break at the desired location. Breaking the PCB can release fibreglass dust, so we recommend doing so outside wearing a mask. If you wish to remove the tab, do this before assembling the PCB. Carefully run a sharp knife over the traces near the edge of the tab. Doing so will reduce the chance of the traces being torn, which could ruin the PCB. A light scoring on both sides will encourage a clean break. Use pliers to gently flex the tab. It will take some force, but flexing should cause it to break along the line of holes. Clean up the rough edge with a file and ensure any stray fibreglass is removed. Make sure that there are no loose traces or other copper that might cause a short circuit. Once you have broken off the tab, there is no external connection for the mid-rail tap, so the 470μF capacitor and two 100kW resistors are useless and should be omitted, which will also save a small amount of drain on the lithium-ion cell. CON3 is a simple header that breaks out the regulated and unregulated outputs and ground. We expect constructors might use this connection if they are using the BackPack to power some other device that expects around 3V but does not need the mid-rail tap. The PCB also has a pair of pads to suit a AAA or 10440-sized cell holder. Lithium-ion cells of this size are not as common as AA or 14500-sized cells, but might be necessary in tight spaces too, due to the thickness of the resulting assembly. Construction Fig.2: the output of the Battery BackPack is fully regulated while a USB power supply is available or the lithium-ion cell is near capacity. In any case, there is always enough voltage to power the Clock Driver, even with a nearly flat Li-ion cell. 80 Silicon Chip Fig.3: the power dissipation in the individual components under various output loads. The offset from zero at low current draws is due to the quiescent current of the regulator, which we have estimated conservatively. Since we will be fitting SMD parts, you’ll need the usual gear, such as flux paste, tweezers and a magnifier. Refer to the Fig.4 overlay diagram for the component locations. Start by applying flux to the SMD pads on the righthand side of the PCB. Rest IC1 in place; Australia's electronics magazine siliconchip.com.au Fig.4: check the component locations during assembly with this overlay diagram. We recommend fitting IC1 and the USB-C socket (CON1) before the other components, since they have the most closely spaced pins. We used some foam-backed double-sided tape to secure the Battery PCB to the Clock Driver since the header only attaches at one point. Make sure that no other parts of the two PCBs can come into contact. it is asymmetrical, so should only line up one way. Tack one lead and confirm it is flat against the PCB and aligned before soldering the remaining leads. Check for bridged pins and use solder-wicking braid and extra flux to draw out the excess solder from any bridges. Next, solder the USB-C socket (if you are using it). After placing it flat on the board and soldering the signal pins, apply a generous amount of solder to the side pads that secure the shell to the PCB. That will give it good mechanical strength. Work through the smaller passives. Fortunately, all the SMD capacitors are the same value. Note that all three diodes have their cathodes facing the same way. Make sure to get this right, as there is a risk of directly supplying power to the lithium-ion battery if one of the diodes is reversed! Solder REG1 next. It too should only fit one way. After that, use a flux cleaner or solvent such as isopropyl alcohol to clean off the excess flux from the board. Let it dry and inspect it closely for bridges and dry solder joints. If you find any problems, rectify them before continuing. At this stage, you should be able to connect a USB-C power supply for testing. You can probe some voltages with respect to ground; use the middle connector of CON3 or the lower pad of the unfitted 470μF capacitor as the ground connection. You should see between 4V and 5V on the V+ pad of CON3 and 3.3V (3.2-3.4V) on the 3.3V pad of CON3. This should be the same as the top + pad of CON4. The two middle pads of CON4 should show around 1.6V. If your voltages are significantly different, remove power and check the component placements and soldering. Disconnect the power supply and fit the remaining through-hole parts. Make sure that all leads are trimmed as flat as possible to reduce the chance of them contacting the PCB this will mount on. The battery holder pads appear to align with those on the clock driver, but should be kept well clear as they are at different voltages and contact might allow the lithium-ion battery to inadvertently discharge. Bend the leads on the electrolytic capacitor, making sure that it will be fitted with the polarity shown on the PCB, then solder it in place. Follow with the LED. The K cathode marking corresponds to the green element of the LED; you can use a light-emitting diode tester or multimeter in diode test mode to confirm this. When it lights green, the red probe is on the anode and black on the cathode. Finally, fit the cell holder, BAT1, taking care with the polarity. As for the other through-hole components, cut the leads flush so there is no chance of them shorting from the underside of the PCB. If you like, you can fit the cell now and connect a USB-C power supply to charge it. Check that the LED lights up red and then turns green, indicating it is fully charged. If the LED lights up green initially, it may be fitted with siliconchip.com.au Australia's electronics magazine the wrong polarity. Remove the cell before proceeding. Fitting it to the Clock Driver If you are upgrading an existing GPS Clock Driver PCB, you will need to remove the existing cell holders first. The four-way CON4 header should be soldered to the underside of the Battery PCB. The four-way header makes the electrical connections to the Clock Driver and will provide some mechanical strength. It’s a good idea to place a few lengths of thick, foam-backed, double-sided tape between the two PCBs in the area where the AA holders used to sit on the Clock Driver PCB. These will insulate the exposed pads and provide extra support for the added Battery BackPack PCB. The larger pad (located in the centre of the cell holder) is isolated on both the Clock Driver and Battery PCBs, so it could also be used for soldering a sturdy wire between the two. Watch that no wire fouls the cell. Whatever option you use, also be sure to solder the four-way header between the two PCBs and trim the leads short on the underside. You can now start the Clock Driver with our modified procedure to test for correct operation. Manually adjust the clock hands to about 10 seconds before the next half hour and then insert the cell. The LED on the Clock Driver should flash twice to indicate a normal startup. The attached time source (GPS module or WiFi Time Source) should start September 2026  81 Silicon Chip PDFs on USB ¯ A treasure trove of Silicon Chip magazines on a 32GB custom-made USB. ¯ Each USB is filled with a set of issues as PDFs – fully searchable and with a separate index – you just need a PDF viewer. ¯ Ordering the USB also provides you with download access for the relevant PDFs, once your order has been processed ¯ 10% off your order (not including postage cost) if you are currently subscribed to the magazine. ¯ Receive an extra discount If you already own digital copies of the magazine (in the block you are ordering). up and run for maybe 10 seconds or so – long enough to acquire the time. Then the LED on the Clock Driver will switch to a long flash sequence until the next half-hour is reached. Refer to the article in the September 2022 issue if there are problems with the Clock Driver. Powering the time source is probably the greatest load the Battery BackPack will see, so you should be able to identify any problems early on. If the Driver appears to lock up, it may not be able to power the time source, so you should check that the battery is charged. You can then use the S1 button on the Clock Driver to manually advance the clock hands and test that the Battery BackPack can power the clock mechanism. If the 470μF capacitor is still charging, it might take a few ticks before the mechanism starts moving, but after this, it will be charged to the correct bias point. Advance the time (using S1) until it is exactly at the next half-hour. Check that the clock starts as expected; it should take no more than 30 minutes. About 12 hours after being powered on, the Clock Driver will check its time source again. Check that the clock continues to run after this; if so, then all is well. Other uses The Battery BackPack can also be used as a general-purpose replacement for a pair of AA or AAA cells in lightduty applications. It will not have the high current capability of AA cells, with the limit being about 500mA, as noted earlier. This is partly limited by the dissipation and dropout voltage of the regulator, so it will vary with the tolerance of the load to voltage sagging and the state of charge of the lithium-ion cell. Brief bursts of higher current draw may be fine, as long as they do not overheat the regulator or diodes. Refer to Figs.2 & 3 to check the behaviour at your desired operating current. CON3 also provides a connection that is upstream of the regulator, so could be used if your circuit can tolerate voltages above 3.3V. In this case, you might also wish to bridge out one of D2 or D3 to reduce the voltage drop. Modules like the Raspberry Pi Pico include a buck-boost regulator that can operate between 1.8V and 5.5V, so that is an example of a case where feeding the raw battery voltage to the device is ideal and will give the best efficiency. If you don’t need the 3.3V output, you could also omit REG1 and the capacitor for the 3.3V rail (the 10μF part closest to the 470μF capacitor). Removing the regulator will delete the main quiescent current draw, so it might be preferred for extreme lowSC power applications. Parts List – Power Supply for GPS Clocks EACH BLOCK OF ISSUES COSTS $100 NOVEMBER 1987 – DECEMBER 1994 JANUARY 1995 – DECEMBER 1999 JANUARY 2000 – DECEMBER 2004 JANUARY 2005 – DECEMBER 2009 JANUARY 2010 – DECEMBER 2014 JANUARY 2015 – DECEMBER 2019 OUR NEWEST BLOCK COSTS $150 JANUARY 2020 – DECEMBER 2024 OR PAY $650 FOR THEM ALL (+ POST) 1 double-sided 56 × 39mm PCB coded 11105261 1 AA-size (14500) through-hole cell holder (BAT1) 1 14500-sized lithium-ion rechargeable cell with protection circuitry 1 USB-C power-only SMD socket (CON1) 1 2-way 0.1in/2.54mm pitch pin header (CON2; optional, only for non-USB supplies) 1 3-way 0.1in/2.54mm pitch pin header (CON3; optional, for general-purpose use) 1 4-way 0.1in/2.54mm pitch pin header (CON4) electrical tape or foam-backed double-sided tape for insulation Semiconductors 1 MCP73831T-2ACI/OT lithium-ion cell charge controller, SOT-23-5 (IC1) 1 MIC37100-3.3 3.3V LDO linear regulator, SOT-223 (REG1) 3 1N5819WS SMD schottky diodes, SOD-323 (D1-D3) 1 3mm red/green bicolour through-hole LED (LED1) Capacitors 1 470μF 10V radial electrolytic, up to 12mm tall 4 10μF 25V X5R M3216/1206-size SMD MLCC Resistors (all SMD M3216/1206-size ±1%, ⅛W) 2 100kW 1 10kW 2 5.1kW 2 1kW WWW.SILICONCHIP.COM. AU/SHOP/DIGITAL_PDFS Kit (SC7707, $25 + P&P): includes all the parts listed above except the Li-ion cell. 82 Australia's electronics magazine Silicon Chip siliconchip.com.au Subscribe to The VERY BEST DIY Projects ! Model Headboard Destination multiple compatible OLED display sizes; HF, SSB & QRP Display serial input or infrared control; 3.3V operation TRANSCEIVER TEST SET Terahertz Waves how they are used in astron Ultrasonic Cleaner ADJUSTABLE Australia’s top electronics magazine omy, medicine and more Silicon Chip is one of the best DIY electronics magazines in the world. Each month is filled with a variety of projects that you can build yourself, along with features on a wide range of topics from in-depth electronics articles to general tech overviews. If you have an active subscription you receive 10% OFF orders from our Online Shop (siliconchip.com.au/Shop/)* AUGUST 2026 ISSN 1030-2662 08 9 771030 266001 $15 00* NZ $15 90 INC GST INC GST Published in Silicon Chip Destination Display; August 2026 Rest of World New Zealand Australia * does not include the cost of postage Length Print Combined Online 6 months $77.50 $87.50 $55 1 year $145 $165 $105 2 years $270 $305 $200 6 months $95 $105 1 year $180 $200 2 years $335 $370 6 months $115 $125 1 year $220 $240 All prices are in Australian dollars (AUD). Combined subscriptions include both the printed magazine and online access. 2 years $410 $445 Prices are valid for the month of issue. Try our Online Subscription – now with PDF downloads! DCC Accessory Decoders; July 2026 Simple USB Power Monitor; June 2026 Amplifier Clipping Indicator; May 2026 An online issue is perfect for those who don’t want too much clutter around the house and is the same price worldwide. Issues can be viewed online, or downloaded as a PDF. To start your subscription go to siliconchip.com.au/Shop/Subscribe SERVICEMAN’S LOG Soviet PDP-11-40 (SM-4) computer repair Here’s a repair story from the past. After completing my electronics engineering degree in Poland in 1978, I started working at the Institute of Mechanical Engineering at a technical university (called a Polytechnic in Poland). The institute acquired a minicomputer. My duties included maintaining it and assisting scientists with programming it. To learn the internals of this minicomputer, I was sent on a course for a few weeks. The computer was an SM-4, a Soviet copy of the Digital Equipment Corporation (DEC) PDP-11/40. The story was that during the Vietnam War, Russians captured an American frigate with this minicomputer onboard and copied it. It’s hard to tell if those rumours were true, but during the course, the lecturers from the Soviet Union were using the original English documentation of the DEC PDP-11. The shape and position of the switches and lights on the front panel of the SM-4 look exactly like those of the DEC machine. One of my tasks was to regularly run test programs to check the health of the minicomputer. We had a collection of programs to test various components: the main processor board, memory, peripherals etc. Each test program was on punched tape. To run the program, it had to be loaded into the computer’s memory first. The computer did not have the equivalent of ROM for a startup program, so after switching it on, I had to manually enter the loader. Using switches on the front panel, I would set the starting address and the first instruction to Items Covered This Month • An old Soviet computer • A smart bin turned dumb • Repairing a Marantz PM630 amplifier • A broken standing fan • Compaq CQ56 laptop repair Dave Thompson runs PC Anytime in Christchurch, NZ. Website: www.pcanytime.co.nz Email: dave<at>pcanytime.co.nz Cartoonist – Louis Decrevel Website: loueee.com 84 Silicon Chip be stored at this address. After pressing the save switch, the instruction was stored and the address was incremented for the next instruction. For the rest of the loader, I only needed to enter the instruction codes and press the save switch for each one. There were twenty-something instructions to enter. The instruction code was an octal number rather than the hexadecimal we use today. For example, the instruction to clear a memory address (CLR) had code 050 (000101000). After a while, I just remembered those codes, so I could enter the instructions quite quickly. After entering the loader, I would put the tape with the program in the punched tape reader, set the address of the first instruction of the loader and press the run switch. The program was read from the tape into the memory and could then be executed. If there were no problems, the test would run to the end. I remember the first time there was an error: the test of the floating-point module stopped halfway. The address of the instruction that had an error was displayed on the front panel. It was time to apply what I had learned during the course. I switched off the machine, removed the front cover and unplugged the floating-point board. There was a so-called engineering panel, an extension that plugged into the rack in the place of a board and you would then plug the board into it. This way, the board was exposed outside the rack, with both sides fully accessible. It also had switches to set the address to stop the program and others to perform some operations. I plugged the engineering panel into the slot where the floating-point board originally was and connected the floating-­point board to it. The board was now outside the rack. After switching on the computer and loading the floating-­point test, I set the engineering panel break address to stop the program at the instruction where the test failed. I started the test program, and it stopped at the address set on the engineering panel before executing the instruction. At this point, the computer would normally execute this instruction, but using the engineering panel, I could step through the microinstructions that were processed to execute the program instruction. The computer was built with TTL 7400-series ICs (Soviet equivalents, to be specific). Australia's electronics magazine siliconchip.com.au Following the path of the signal at each microinstruction, I checked the inputs and outputs of various ICs. At one of the micro-steps, I found a NAND gate with all of its inputs high and the outputs were also high – all were 5V in the 7400 universe. There were no desoldering tools, so to replace a faulty IC in a DIP through-hole package with 14 pins, I cut all the legs off and removed the chip body. Next, I desoldered and removed from the board all 14 pins one-by-one. After cleaning out the old solder using a wire (we did not have desoldering braid), I soldered in the replacement chip. I cleared the breakpoint address on the engineering panel and executed the floating-point test again. This time, the test ran through without errors. I unplugged the board, took out the engineering panel and plugged the floating-point board back into the rack. I ran the test again just to confirm there were no more errors. Today, we would typically replace the board rather than go to the component level, especially since a single IC performs the work of thousands of individual chips. Only occasionally could we identify the faulty part. Cas Filar, Duncraig, WA. Sensor-triggered rubbish bin repair My youngest son asked if I would look at a rubbish bin with a sensor-actuated lid that had stopped working. The bin uses photo sensors to activate a motor that raises and lowers the lid when a hand is waved over them. My initial thought was that it would not be an economical repair. However, I enjoy a challenge, have spare time, and if it could be fixed cheaply, it would defer the cost of a new bin. That would be particularly helpful with the current cost of living and six mouths to feed in the household. The first step was to check the power supply, which consists of four 1.5V dry cells. All tested OK, so I proceeded with the disassembly. After removing ten self-tapping screws, I separated the lid assembly and quickly identified a likely source of the fault. The motor and gearbox assembly that drives the lid was coated with some kind of black, sticky, corrosive substance. I desoldered the wires from it, removed the motor/gearbox assembly and tested the motor using my bench power supply. There was no movement. I then began dismantling the gearbox, drawing a diagram to record the assembly sequence of the many nylon gears and shafts. All the gears were covered in the same sticky residue. After sliding the final drive gear off the motor shaft and removing two additional screws, the motor came free. Further testing confirmed the motor was lifeless. I carefully bent back the three retaining tabs securing the motor siliconchip.com.au endplate and slipped the rotor out. The brushes are thin strips of flexible metal, and they were bent and split. I tried re-shaping them and re-assembled it, but I found that the rotor would not turn. I suspected that bending the retaining tabs had disturbed the shaft alignment slightly. After some online research, I located a suitable replacement motor: the RF320 rated at 6V, 6000 RPM, 24mm in diameter, with threaded mounting holes in the right position. Fortunately, once received, the motor was a direct drop-in replacement. After thoroughly cleaning the gearbox and gears to remove the residue, I re-assembled the unit. The lid operated perfectly once installed. Another successful repair and one less item to be discarded. Phillip Webb, Hope Valley, SA. Marantz PM630 amplifier repair About eighteen months ago, my wife and I moved from the city to a rural town in the south-western part of the Wheatbelt in Western Australia. Being a collector of classic hifi equipment, I was recently asked if I was interested in acquiring some older Marantz hifi gear for free. I like older Marantz equipment, so my friend and I went over to an old abandoned farmhouse to grab the gear. The whole ‘box and dice’ was there: a genuine Marantz cabinet replete with a PM630 amplifier, TT530 turntable, ST530 tuner, SD530 auto-reverse cassette deck, CD54 CD player and some massive speakers. These had all been sitting in the farmhouse ever since it was abandoned many years ago for new digs. Australia's electronics magazine September 2026  85 Anyway, all the gear was quite dirty, and there was plenty of evidence of a mouse infestation from days gone by, judging from the deposits left behind on some of the gear. The speakers and the hifi cabinet were in too bad shape to bother with, but the rest of it, being Marantz ‘Champagne’ series, was loaded into the boot of the car with great enthusiasm. All of the equipment needed a good cleanup and a good looking at. I set about testing each item after a thorough cleanup, and as is typical of hifi gear of this ilk, the CD player, cassette deck and turntable all needed new belts. The cassette deck was the worst of them, with the main flat drive belt having turned to ‘goo’ over the years, leading to a very messy clean-up job. The turntable was a direct-drive type with linear tracking, utilising two small belts linked to motors and gears, one for the pickup arm tracking ‘sled’ and one for raising and lowering the arm. As for the CD54 CD player, the belt was a little loose, so it could not quite drive the loading tray mechanism in and out properly. New belts were duly ordered from my favourite supplier and fitted to each unit, restoring each of them to working condition, except for the cassette player, which would not auto-reverse reliably, so that was left for another day. Mostly easy fixes so far. Just for interest, the CD54 is essentially a first-generation CD player, employing the famous Philips CDM-1 swinging-­ arm laser mechanism and TDA1540 14-bit DACs. The unit is built like a tank, employing a heavy diecast chassis, and is quite sought after by collectors, me being one! It is CD54 number two for me. The tuner was relegated to the shelf, as I rarely listen to AM and FM broadcasts these days. The next repair candidate was the PM630 amplifier. It proved to be dead on power-up; the display and control panel LEDs did not light up as expected, so off came the cover to have a poke around. I managed to find service manuals for all of the gear on the interweb. Overall, the amplifier was in great physical condition, with no rust, no scratches, and no verdigris on the knobs, the latter being common in poorly stored units. Basic troubleshooting started with checking the main fuse, which was intact. Next, all the internal and onboard fuses were checked and also found to be intact. A check of the power supply voltages turned up no +5V rail (actually nominally around 5.7V), and no ±15V rails; the former revealing why there was no life from the microprocessor-­ controlled front panel. The lack of ±15V was not the best find either. Surprisingly or not, the main power amplifier and auxiliary supplies were all okay, and the speaker outputs had only a few millivolts of DC offset. So at least there seemed to be no faults in the main power amplifier circuitry, which was a welcome discovery. A subsequent visual check of resistors R807 and R808, both 27W fusible resistors for safety reasons, revealed that they looked pretty stressed. In fact, a deeper dive with a multimeter on the ohms range revealed they were both open circuit. Subsequently, I found that the ±V rails were both shorted to ground, explaining the unfortunate demise of the resistors. This did not inspire confidence, though, as there was a great deal of circuitry hanging off these supplies. I decided to turn my attention to the missing +5V rail first, noting that it was not shorted to ground, which was kind of good news. In fact, on closer inspection, the +5V supply output measured near zero, with the ‘input’ side sitting at around the expected voltage. Each of these supplies employs a zener diode ‘boosted’ by a series pass emitter-follower transistor, regulating the voltage to about 0.6V below the zener voltage. Anyway, since F803 and R810, a 10W fusible resistor, were intact, I measured a few voltages around the +5V supply, starting with the zener diode, a 6.2V type. I only measured a few tens of millivolts here, so I thought This section of the Marantz PM630’s circuit shows the power supply. Several of the electrolytic capacitors had failed, and not in the usual ways: with a high ESR or low capacitance. 86 Silicon Chip Australia's electronics magazine siliconchip.com.au that the zener had failed short-circuit, thus not providing any voltage at the base of the booster transistor. To be sure, I desoldered it carefully and measured it out of circuit using a power supply and series resistor, since I don’t have a fancy semiconductor tester. It measured 6.2V! OK, I thought, maybe there’s another culprit pulling down the voltage at the base of the transistor. Sure enough, a resistance measurement between the base of the transistor and ground indicated a near-short-circuit. It turned out that capacitor C818 was almost a dead short. This would be one of the few times I have ever come across a shorted electrolytic capacitor in my lifetime, especially since it seemed to be working well within its ratings. This capacitor was a 47μF 16V unit. With 5.6V across it in normal operation, I had no clue why it had failed. In any case, I was confident that refitting the zener diode and changing the capacitor should restore the +5V supply. This saw the +5V rail come to life along with the pretty lights on the front panel. Progress was being made! Having previously found the ±15V supplies shorted to ground, I now suspected C815 and C816. As it turns out, these were both shorted, which explains the missing voltages and fried resistors. Not having fusible resistors on hand, I had to order the same, along with new capacitors, and wait. Once they arrived, I fitted and changed all fusible resistors and small capacitors around these power supplies, including C813 and C814, for good measure. The failed capacitors were all small types from one particular Japanese manufacturer which, as it turns out, being of this vintage, are notorious for going bad. The good news is that all of this effort was rewarded with a now fully working PM630 to add to my collection of Marantz hifi gear! For reference, the circuit snippet at lower left shows the power supplies and components in question. Richard Kabzinski, Ellenbrook, WA. Standing fan repair It was a hot day, so I needed a standing fan for my work area. I got a spare fan out of the shed but as I was carrying it to my work area, the fan broke into two pieces, with the stand and shaft breaking off the main body and falling on the ground. As I had hold of the main body, the fan itself was not damaged. siliconchip.com.au The plastic had become brittle over time, and it just broke under the weight of the fan while I was carrying it. I went back to the shed and retrieved two dead fan motors with their control panel assemblies still attached. I would use one of these to repair the fan that was still working. I usually keep fan parts for repairs; I recently rebuilt a standing fan that my son picked up from the Op Shop that was missing the blade and guard; it also had no stand. The motor still worked, so I was able to rebuild it using parts from previously failed department store fans. These department store fans have a high mortality rate, as the motor has a non-resetting thermal fuse buried deep in the windings. When it blows, the motor is useless and the fan can’t be repaired, short of replacing the motor. The old fans from well-known brands do not have this thermal fuse, so it’s common for fans over 50 years old to still work. I went to undo the screws in the control panels, and of course they were “tamper-proof” types. These had an indented triangular hole that required a triangular bit. I’ve found these particular screws previously when I repaired a power board some time ago. At the time, I looked through about eight bit sets, and I did not have a bit for this type of screw, so it might not be common yet. Not having a suitable bit for this type of screw, I made a screwdriver to fit them from a piece of thick fencing wire. I used that to open the three control panels, ready for the repair. I checked the two spare control panels and found that one would not fit the fan I was repairing, but the other one fitted nicely. So I would swap out the broken parts with the good parts and get the fan operational again. Both control panels were wired the same way, so I just had to swap the wires from the broken one to the replacement one. The wires are just pushed into the terminals on the switch, and they can be easily removed by inserting a thin tool into the terminal next to the wire. I used the point of a compass for this job. The Neutral wire in the mains cable and the Neutral wire going to the motor were joined in a crimped connector; they were not even twisted together or soldered. That seemed dangerous to me. The blue wire coming from the motor was too short to reach the terminal in the new control panel, so I extended it by soldering on and heat-shrinking a short length of blue wire salvaged from the wiring in the fan that I took the control panel from. I also soldered the Neutral wire before refitting the original crimp connector to it. Australia's electronics magazine September 2026  87 Servicing Stories Wanted Do you have any good servicing stories that you would like to share in The Serviceman column in SILICON CHIP? If so, why not send those stories in to us? It doesn’t matter what the story is about as long as it’s in some way related to the electronics or electrical industries, to computers or even to cars and similar. We pay for all contributions published but please note that your material must be original. Send your contribution by email to: editor<at>siliconchip.com.au Please be sure to include your full name and address details. The disassembled Compaq CQ56 laptop. The very dusty heatsink is shown below, with the reassembled laptop shown next to it, waiting for a new screen & keyboard. While I don’t like these crimp connectors, they serve the purpose of covering the joint and are fine if the joint is soldered first. Reusing one saves using more heat-shrink tubing. With the replacement control panel fitted, I reassembled the fan, and it was ready to use again. It’s very handy having spare parts to be able to effect repairs, so I hang onto these dead department store fans. That was another successful repair that saved this fan from the junk pile. Bruce Pierson, Dundathu, Qld. Compaq CQ56 Laptop repair I was looking through some junk laptops that a friend gave me a while ago, and amongst them was a Compaq CQ56 laptop. It was missing the screen, keyboard, battery, right hinge cover and RAM. Could it work? I got a charger, plugged it in, pressed the power button and the laptop lit up. That was a good sign, but with no screen, I couldn’t be sure it was working. I had wrecked a non-working HP dv6 laptop recently. The Compaq’s screen connector was still there in the empty lid, so I had a look at it and it just happened to match the one in the HP. Its screen had some scratches, but it would do as a test screen. I didn’t have the correct keyboard for this CQ56, but I had a smaller one with the same connector, so that was worth trying. 88 Silicon Chip I disconnected the charger, installed some RAM, connected the screen and keyboard, reconnected the charger and pressed the power button. Holding down the F10 key, the screen lit up, and I was at the settings screen. Amazingly, this stripped-out laptop actually worked. Of course, the CMOS battery (cell) was flat, as expected with a 16-year-old laptop. It was time to dismantle it and start the rebuild. Fortunately, the optical drive was still in place; it’s always difficult to find a correctly fitting front and a rear retainer to suit a particular laptop. I dismantled what was left of the computer and decided I would upgrade the CPU while I had it apart to change the CMOS’s CR2032 cell. I looked online regarding what CPU I could use; because this laptop has a GL40 chipset, it only supports CPUs with a front-side bus (FSB) speed up to 800MHz. This ruled out using an Intel P8600 at 2.4GHz, which has a front-side bus speed of 1066MHz. The CQ56 came with an Intel Celeron T3500 CPU at 2.1GHz. Other supported CPUs include the Intel T9300 at 2.5GHz and the Intel T9500 at 2.6GHz. Unfortunately, I did not have either of these, so I had to settle for an Intel T4500 CPU at 2.3GHz. I had removed this CPU from another laptop where I replaced it with an Intel P8600. The T4500 was still a worthwhile upgrade. Australia's electronics magazine siliconchip.com.au After removing the heatsink and fan from the motherboard, I separated the fan from the heatsink and found the worst blocked-up heatsink that I have ever encountered. I’ve seen some badly blocked-up heatsinks, but this one is top! With the replacement CPU fitted and the heatsink and fan cleaned, I refitted the fan to the heatsink, cleaned off the old heatsink compound, applied a new smear and refitted the heatsink to the motherboard. When I went to replace the CR2032 cell, I noticed that the cell holder was broken, so I would need to replace it. I looked through my dead motherboards that I kept for spare parts when I wrecked non-working laptops and found a similar cell holder. I used my 20W soldering iron to remove it from the motherboard, then removed the cell holder from the CQ56 motherboard and fitted the replacement cell holder. Fortunately, the cell holders had the tabs at the ends, so this was an easy replacement. Next, I detached the lid from the main laptop body so I could rebuild the lid with a replacement screen. I unclipped the front panel from the lid, ready to replace the screen. I don’t know who stripped this laptop, but they didn’t remove the two screws from the bottom of the screen front cover and they just ripped the front cover off. It’s amazing that they didn’t break anything else when they did that. I had a good screen that came from a non-working Toshiba C850 laptop that I’d wrecked a while back. Even though this screen came from a different brand, it was compatible with this CQ56 laptop. All the screws to fit the screen were missing, so I grabbed a loose lid from a laptop I’d wrecked and retrieved all the screws I needed. I reassembled the lid with the replacement screen, reinstalled the motherboard and put the laptop back together again with a brand new keyboard that I’d ordered from eBay. I had several salvaged HP batteries that had come from dead laptops, so I fitted one. A quick test showed that the laptop was now working. I set the time and date and checked other settings in the BIOS, then saved the settings. The first battery I tried didn’t charge, but the third one did. Now it was time to install Linux, which is a good choice for a laptop this old with only 4GB of RAM. It would be a useless snail trying to run Windows 10 or 11 on it. I had downloaded the Pearl OS 8 ISO and burned it to a DVD earlier, so I used that. When I tried to install updates, all I got was an error message that the repositories did not have a release file, so no updates could be installed. Looking online, I saw that version 8 had been discontinued, but versions 12 and 13 had just been released recently. I read some reviews and they were all bad for version 13, but there was a good review for version 12, so I thought I would try it. I downloaded version 12, then burned it to a double-layer DVD, as it was 5.26GB, so it wouldn’t fit on a single-layer DVD. I installed that on the CQ56 laptop. The installation went smoothly, apart from a bug with the scroll direction for the touchpad, but after updating, that was fixed. This once piece of scrap now has a new life. A good result. Whenever I get a chance to buy non-working laptops for peanuts (or free), I pick them up because you never know when the good parts from them will come in handy. SC Bruce Pierson, Dundathu, Qld. siliconchip.com.au Australia's electronics magazine September 2026  89 Vintage Electronics Braybon Bros Automatic Voltage Regulator I have personal experience with this fascinating device that started life in the 1940s but was manufactured into the 1960s. It was used to control alternators to produce a more-or-less constant AC output in the face of varying loads. A production Braybon T3 AVR, serial number 4777. By Fred Lever B efore the Second World War, AC mains power was spreading around Australia. In the main cities, 240/415V three-phase was reticulated with homes utilising 240V single-phase. Appliances were being made to comply with this, using the 3-pin plug and socket system still in use today. Outside of the big cities, factories and farm properties installed their own generators using low-voltage DC or medium-voltage AC. The DC systems usually ran at 32V. Some installations had storage batteries charged by a generator (petrol, diesel or wind-powered). 240V AC systems started displacing DC systems as they could utilise the ever-growing commercial range of appliances and electric motors. Smaller systems were usually under 10kW with a single phase, while more ambitious 10-50kW systems were 3-phase types, allowing larger motors in workshops and homes. Townships installed generator systems large enough to power a local network, including street lighting, shop premises and homes (or domiciles). Examples of a farm/factory set and a town powerhouse installation are shown in Photos 1 & 2. 90 Silicon Chip The typical farm/factory set shown in Photo 1 comprises a Lister two-­cylinder diesel engine coupled to an alternator to produce AC. The alternator set was made by the Sydney firm Braybon Bros, providing around 6kW at 240V AC. Braybon and many other small manufacturers found a growing market to supply generating sets, alternators and control equipment for defence requirements during wartime, and the subsequent post-war boom into the 1960s. The typical town powerhouse shown in Photo 2 has two identical sets with six-cylinder diesel engines, possibly from Blackstone, coupled to brush alternators of around 200kW capacity. The power generated was 3-phase 415V AC. This was sufficient for a small town to run the main street lighting, shop and house power. You can identify the engine prime movers easily. Each drives an alternator that in turn drives a smaller DC generator called an exciter. The exciter and alternator are electrically connected in a series configuration, allowing the AC alternator to have its high-power voltage level controlled by the lower-power DC exciter using Australia's electronics magazine an automatic voltage regulator (AVR). The generating set in Photo 1 has the AVR control system mounted proud at the end. That is the two metal boxes, the top one being the hand rheostat (HR), and the lower one is the AVR. These items are production examples of a Braybon Type B rheostat and a Braybon Type 3 AVR. Out of sight is a gauge to show the output voltage, plus some operator controls, including the AVR hand/auto switch. Braybon Bros invented this type of AVR in 1940 to fill a wartime lack of supply of imported AVRs. The new AVR was very successful, despite its utter simplicity and modest cost in comparison to what had been available. I decided to make a replica of the design to investigate why it worked so well despite its apparent simplicity. AVR operation Fig.1 shows a basic electrical circuit of a generating set with an alternator, exciter and an AVR. The exciter is depicted on the left, with shunt field XF and armature XA. The armature supplies excitation power to the alternator field, AF. The output windings deliver three-phase AC power to the load. siliconchip.com.au Photo 1: a typical alternator that would have been used to power a farm or factory. The exciter generates a DC voltage, adjusted by resistors LR and HR. LR is set to provide a maximum limit and HR, the hand rheostat, is the manual voltage control giving a range of 50% to 150% of the rated voltage. The switch allows either manual control of the voltage by rheostat HR or, when switched to auto, the AVR is introduced to the field circuit. The AVR senses the AC voltage from the alternator and automatically adjusts its internal resistance. When properly set up for automatic operation, the hand rheostat provides an initial voltage of about 70% of the rated voltage on no load. The AVR is then switched on, taking the voltage up to 100%. The AVR then tries to maintain this despite a varying load. Fig.2 includes a typical excitation curve of an alternator, showing the change in AC voltage against DC excitation. With no load, the AVR is adjusted to settle at point A on the curve, the rated voltage. When the Photo 2: large alternators like these were used to power small towns, including street lighting. maximum load is applied, losses increase in the alternator, so higher excitation is required to advance to point B, by AVR action, to restore the voltage to normal. The rise in DC excitation from no load to full load in these alternators was in the range of 2-4 times, depending on many factors. Making a Braybon Type 3 AVR That brings us to the subject of this article: making a replica of the Type 3 vibrating armature point AVR (AVR T3) to glean some insight into how it worked. This AVR type was the third arrangement by the designer, S. C. (Stan) Braybon. Earlier types were the Type 1 solenoid carbon pile and Type 2 oscillating rotor point. There were some variations in the initial units, but once into production, the configuration stabilised and was virtually the same throughout the 40-odd-year life of the design. Fig.1: the basic principle of controlling alternator output by varying the exciter voltage. siliconchip.com.au Australia's electronics magazine The author has the unique position of having worked for Braybon Bros, both making and servicing the Braybon AVRs, as well as developing the subsequent solid-state AVRs for the Braybon set range. Back in the 1970s, I had a toolbox holding all sorts of spares used on service calls. When no longer used, I parked it under a bench in my workshop. It lay almost untouched for decades, to be opened only when searching for some piece of gear related to 1960s gensets, like carbon slip ring brushes or field rectifiers. For this article, tipping all the contents out and sorting through the junk at the bottom revealed some truly unobtainable, crucial parts for a T3, not the least being a couple of sets of tungsten vibrating points and some badly machined metal parts. They could at least be re-machined and put back to use! This enthused me, so I set to work using photographs of a stock AVR and Fig.2: the relationship between the DC excitation voltage and alternator AC output voltage is not completely linear. September 2026  91 a factory AVR test unit in my collection as a guide to make new parts. A production unit In the lead photo we have an AVR T3, like the one mounted on the genset in Photo 1. The top cover is removed, revealing not that much in the way of parts underneath! Nothing is missing. By referring to Fig.3, you can see almost all the active items. Coil M is the red bobbin at the rear of the lead photo, rectifier RB is the selenium plate device and transformer TX the grey object. The ballast resistor (BR) and capacitor (C) are out of sight underneath. The rocking armature sits on the frame of coil M, with the points (PP) on the nose, and the reference spring (S) just visible halfway between the points and the centre pivot. The spring reaches through the baseplate and hooks onto a clever right-­ angle spindle mechanism that appears at the front as the round control knob. This AVR is fitted with the point-­reversing switch, RS, the toggle facing the front of the photo. With reference to Fig.3, a simple explanation of how it works is as follows. Accept that if the points (PP) are closed by the pull of spring S, a field resistor such as HR in Fig.1 connected to the REG terminals will be shorted out, and the AC voltage will rise. When the pull of coil M due to the rising AC voltage exceeds the pull of the spring, the points open and the AC voltage will fall. The points will ‘hunt’ for a balance point, and so the AVR will hunt around a voltage. On M, winding SC creates the pull-in proportional to the AC voltage delivered by TX, BR and RB. Coil CC is connected in anti-phase to SC, and the changing field current tends Photo 3: I started by recreating the magnetic assembly. 92 Silicon Chip Fig.3: the configuration of the voltage regulator. The magnetic field generated from the AC voltage to be regulated is opposed by spring tension to control a set of points. to damp any change of state, adding an ‘anti-hunt’ action. At first sight, that is all you need to know. Like anything else, the actual way it works is more complex. Building one and testing it was the only way to discover more. To do this, all I had to do was draw up a list of parts required, refurbish or make each part, tick them off the list and put it together. Simple! The magnetic path and machined a thread on one end to secure it to the Bakelite base plate. I similarly made the brass post to carry the lower stationary contact from a length of 12.7mm (half-inch) diameter brass rod with one end tapered and tapped for 3mm. The other end was threaded to secure it to the baseplate. I made these parts and confirmed that the dimensions suited each other by a rough mock-up, shown in Photo 3. I made the steel parts for the magnet coil assembly first. I had the subspec armature and new points from my trove of toolbox parts, but needed to machine the L-shaped bracket and the coil core. The easy way to achieve the L shape was to part off a piece of 3 × 2.5 × ¼-inch mild steel angle and shape the top of the L to a 45° chamfer so the corresponding pivot milling in the armature fitted freely. I made the coil core next, machining it from 25.4mm (one inch) diameter mild steel rod. I cut it to length The Bakelite baseboard Photo 4: the voltage adjustment screw mounted on a Bakelite baseboard. Photo 5: the magnetic assembly and spring are mounted on the baseboard. Australia's electronics magazine For the baseboard, I cut a 200 × 200mm piece from 9.5mm (3/8-inch) thick Bakelite sheet. The voltage-­ control spindle mechanism casting mounts on a centreline, and this determines the exact position of the coil assembly as the reference spring hooks vertically from the casting arm up to the armature. As shown in Photo 4, I refurbished the casting, positioned it on a centreline and fitted it. That allowed me to drill the holes for the magnet assembly siliconchip.com.au Photo 6: metal screws are used to make connections between the two sides of the Bakelite baseboard, making it an early type of doublesided circuit board. and do a trial fit, shown in Photo 5. Note the vertical position of the spring, determining where all else fits. Transformer and magnet coil To produce a transformer, I found a junk unit of the same core size as the original with a good 240V primary. I rewound the secondary to 10V AC and fitted the frame with angle-mounting foot brackets. The magnet coil needed a bobbin former to slide onto the steel core. I used a piece of 25.4mm inner diameter plastic conduit tube and for the end flanges cut the ends off a Jaycar hookup wire spool. I used plastic glue to fix the pieces into a functional bobbin, then wound on an estimated 800 turns of 0.7mm (0.028-inch) diameter Lewmex wire for the shunt winding plus a series winding tapped at 40, 80, 160 and 400 turns. The ballast resistor and modern rectifier came from stock. Next, I drilled all the holes needed in the baseboard for the parts, terminals and lead-through screws to make connections from one side to the other – see Photo 6. With the parts on hand and the board made, I could then assemble the unit and prepare it for testing. Photos 7 & 8 show the assembly in progress. Note the simplicity of the device. The casting turning the axial drive of the voltage control knob into a vertical direction to adjust the spring tension on the armature is a clever piece of design. Once all the parts were assembled, I applied a static test by driving the AVR AC input from a variac and terminating the REG terminals into a 3A 32V DC supply with a resistance load. That allowed me to adjust the vibrator running of the armature to a smooth action and to graph the output current change of the points with varying AC input voltage. Fig.4 shows the AVR static test circuit and the response curve. The DC load current varied between 0.7A and 1.7A from 255V down to 220V. As the AC voltage falls, the AVR increases the current through DC load XF. That is exactly the logic required for an exciter field load correction. When 60 turns was selected on the series winding, the slope of the response became shallower, indicating negative feedback lowering the gain of the AVR. siliconchip.com.au Photo 7: most of the parts are now mounted on the top of the board. Photo 8: the wiring on the underside of the Bakelite board. Australia's electronics magazine September 2026  93 Scope 1 shows the voltage to the actuating coil M being a 100Hz pulsing wave. The armature balances between the spring and magnetic pull sources like a see-saw and also vibrates at 100Hz, providing a ‘chopping’ action. If the AC voltage is low, the spring pull is dominant and the point duty cycle is high, as in Scope 2, and near maximum field current flows. If the AC voltage is high, the magnetic pull is dominant, the duty cycle is low (Scope 3) and the AVR has little effect on the field current, so the AC voltage drops. Some observations With the AVR set to hover at 240V, I measured the spring tension and found it close to 1200g. The spring wire is about 0.035-inches in diameter (~0.9mm) and operates with about a 10 thou (0.25mm) gap between turns. That suggests the spring is working in a linear part of its range. The magnetic path is about 20 × 20mm through the core but only 9.5 × 9.5mm through the frame parts. With a measured 0.6A DC coil current, the amp-turns of the coil is 480At (0.6A × 800 turns). The flux density in the frame is low enough that the iron path is not saturated, so it is linear except for the air gap. The pole air gap is about 3mm and the flux is concentrated in this gap. The pole face attracts the back end of the armature against the spring tension. Since the applied current is not a square wave but a sinusoidal halfwave, the effect of the vibration is to chop the point current into square waves as in Scopes 2 & 3. Back at Braybon, when I ran an AVR on a genset, the overall set noise tended to mask the buzzing. Now, when run on the bench, the noise is moderate, similar to a soft-spoken voice. You can tell how the AC voltage is going by the pitch and noise level of the points. This AVR speaks to you! Construction methods I tried to stick to old-school methods as much as possible. There are no crimped wire connections; all the lug shank connections are soldered. All joints on a 1940s production AVR were soldered with a wall-gas heated copper iron, solder stick and flux paste. AVRs used either lacquer or cotton-covered wire for the coils. Today, I used 1960s Lewmex high-temperature motor winding wire for the coils. The coil former in mass production was moulded Bakelite; mine used glued PVC sections. I had to use modern 1960s push-on quickchange terminals to make changes easy around the rectifier and series resistor. The rest is just how it was: drilled holes in the Bakelite plate, brass screws everywhere with spring or fan lock washers. I did use the flexible PVC-covered wire of the 1960s. Production AVRs had a lot of bare 16-gauge tinned copper wire connections. When I made them with the point changeover switch, you insulated the bare wires with spaghetti sleeving or used fabric-insulated wire. I painted my bare steel parts with etch primer and a light coat of hammer Fig.4: some experimental results obtained while tuning my replica AVR. 94 Silicon Chip Australia's electronics magazine silver. During 1960s production, we plated all the parts with cadmium. The shop had a plating bath with a cyanide acid solution and cadmium plates. OH&S, eat your heart out! That would not be allowed now. I did wash my hands after each use of the plating bath. Photos 9 & 10 were taken near completion of the project. Dynamic AC testing Once assembled to the point shown in those photos, I could test the AVR in closed-loop mode connected to an alternator. Getting a suitable alternator was a bit of a problem! I have a 10kVA generating set with a 1960s alternator in mothballs. Instead of the trouble of bringing that back into service, I decided to assemble a bench simulator that would provide 240V AC in response to a DC control voltage. That was accomplished using a Lucas (the prince of darkness) 12V DC car generator belt driven by an AC motor to provide a real-world ‘exciter shunt field’. I coupled that in series to a magnetic amplifier that had an AC output in proportion to the DC armature voltage input. That mimicked what a small exciter-alternator pair would look like to an AVR. The Lucas ‘exciter’ has inductance and a time constant. The ‘alternator’ has a time constant inherent in the magnetic cores plus added lag capacitance across the DC coil circuits if needed. In practice, the time constant obtained was about one second or slightly more. A drawback of this simulator was that the wave shape had a high harmonic content (see Scopes 4 & 5); however, this was not too dissimilar to some of the dreadful alternators I fitted AVRs to in real life! I could have added a harmonic filter to remove some of the bumps, but decided to leave that complication unless the wave shape created caused insurmountable testing problems. It turns out that this type of AVR does not like the distorted wave shapes one bit! As the smooth operation of the armature is rather dependent on the wave shape, the narrow high harmonic wave with bumps tends to make the points ‘chatter’ erratically. However, the wave shape was good enough to obtain meaningful tests. Scope 6 shows the exciter shunt field voltage delivered by the AVR, responding to conditions of no load on the left, siliconchip.com.au Scope 1: the voltage applied to the coil during operation. Scope 2: the field voltage waveform when the alternator output is 220V AC. Scope 3: the field voltage waveform when the alternator output is 260V AC. Scope 4: my test setup AC output voltage with no load. Scope 5: the AC output voltage onload. Scope 6: the exciter field with a load applied after two seconds and then removed after eight seconds. Scope 7: the alternator field voltage during the test shown in Scope 6. Scope 8: the voltage across the points with no load. Scope 9: the voltage across the points with the maximum load. load on in the centre and load off at the right, with about 12 seconds across the screen, or one second per division. Starting from the left, after approximately three seconds, the load is applied. Note how the AVR points react rapidly and overshoot the field voltage. The armature bounces a few times until stable, balanced vibrating is reached. On the load release, at around the eight-second mark, the armature tips toward the core, opening the points with the voltage diving low. It then bounces a few times before settling in balance again in the no-load condition. Scope 7 shows the exciter armature output for the same conditions of load. Here, the rotor mass absorbs most of the spiky switching of the field but follows the outline of the response, with a couple of bounces either way on load application and shed. Given the one-second-per-division scans, the time constant of the AVR and alternator from disturbance to settling is again about one second. Scope 8 shows the switching voltage across the points with no load. Here, both the duty cycle and amplitude are low. In Scope 9, on full load, the duty cycle and amplitude have risen. The point chopping contains the underlying 100Hz vibration, but this is modified by the armature rocking about trying to follow the coil’s pull. Neither condition shows a regular rhythm, as the AC voltage is always moving about and the AVR hunts slightly trying to maintain balance. Compare these with Scopes 2 and 3, which were taken while running openloop with no AVR action. would be 216V AC. Full load on this simulator was decided to be when the excitation had increased to three times that of no load. In real life, one would set the voltage high on no load, say to 250V, and live with 226V at full load. If that range sounds a bit wide, back in the day, the mains could vary by 20%, usually downwards, so a 10% drop was quite acceptable. Note also the AVR’s response may not be ‘RMS responding’. In particular, a change in wave shape could skew the magnetic pull one way or the other from RMS. Figuring out if such an AVR is responding to the RMS, peak, average of the voltage or something else would be an interesting mathematical exercise, but far beyond my capabilities! In practice, the AVR would be set with damping turns selected so it was not too unstable in response to load changes, and the resulting voltage regulation would be accepted. siliconchip.com.au Voltage regulation With this setup, the AC voltage regulation was about 10%; that is, if set to 240V at no load, the on-load voltage Australia's electronics magazine September 2026  95 Photos 9 & 10: the completed unit, ready for testing, and a custom made cover for it – like the original units had. For a three-phase alternator, Braybon always offered just single-phase sensing; this was found to be ‘good enough’. It was only when alternators were required to give closer regulation, approaching ±1% in later years, that we developed solid-state AVRs with more gain and more powerful stability circuits. Still, that is a story for another time. An early form of op amp The operation of the magnetic path and vibrating armature is more complex than at first sight. What struck me when contemplating this is how close the magnetic circuit is to being an operational amplifier. The armature pivots like a see-saw depending on the difference in the pull of the inputs, just like an op amp. The output of this ‘op amp’ is the points mounted on the end of the armature. I can liken the design to an op amp run from a chopper power supply driving a power FET to control an external DC current. If this sounds farfetched, consider Fig.5. The AC input supplies both the rail and the level-sensing applied to the ‘op amp’. The sensing level through SC drives the op amp’s negative input, and the reference spring S drives the op amp’s positive input. You can consider the spring as a zener diode. The output of the op amp chops in response to the unfiltered supply rail, and its DC level depends on the difference between the +S and -SC inputs. 96 Silicon Chip The chopped DC level then drives the output, in actuality the points. These bridge the REG terminals that are connected to the field rheostat of the exciter. The exciter current is fed back to the negative input of the ‘op amp’ by CC, tending to damp the AVR action and lower the loop gain. A copy of the Tirrell AVR? At university, the Braybon AVR came up as a topic and was dismissed as a local copy of the excellent Tirrell AVR design. Well, not really. The Tirrell is a much more complex device intended for use with large alternators of megawatt capacity with long time constants. It is also a precision device with a very high price tag, befitting its quality of build and performance. A simplified circuit diagram of the Tirrell AVR is shown in Fig.6. Comparing Fig.6 and Fig.3, the only common thing about the Braybon and the Tirrell is the use of points to control a current! The Braybon’s designer, Stan Braybon, described the AVR series in his 1940s hand-written notes I possess. Knowing the man as my employer in the 1960s, I know the inspiration for the Type 3 comes from his experience with motorcycle and road vehicle voltage regulators and ignition system magnetos. If you like, the Braybon Type 3 is a much-enlarged Bosch or Lucas 12/24V vibrating point battery charge regulator. Much enlarged means shifting sensing coil operation from 12/24V Australia's electronics magazine DC to 240V AC, points operation from 12/24V DC to 100V DC, chopping the points to give astatic operation and employing feedback to accelerate the armature response and to reduce hunting (stabilisation). His design notes show the evolution of the AVR types; the ‘bulletproof’ mechanical construction reflects the engineering experience of the man. The Braybon AVR Type 3 may have been ‘cheap and cheerful’, but it was an advanced design that was very effective in its diverse usage. Final thoughts While working for Braybon Bros as an electrical fitter, I encountered many types of voltage regulating systems fitted to many different types of generating sets. These systems ranged from open-loop compensating arrangements with shunt/compound field control, magnetic control via saturable reactors, to even having the voltage set by a manual control. Better-managed alternators used closed-loop devices that can be called AVRs, where the voltage level was sampled, compared to a reference, and the excitation level adjusted automatically to a standard. I came across many brands of AVRs when I was discarding a failed AVR and fitting a Braybon unit. Some I can remember are: Brown Boveri and Cie (Co), Metropolitan Vickers, GEC and Westinghouse. All of those were motorised rheostat types. There were others employing contact points, like the siliconchip.com.au engine-driven Tillitson and the Tirrell nodding point type. My brief at the time was to disconnect or remove the original AVR and graft in a Braybon unit. How I wished I had simply picked up and kept some of the marvels of engineering that went to the scrap heap! I never saw any other AVR brand that worked quite the same way as the Braybon, with a single balanced armature controlled by various sets of field coils. I thought I had found such a unit bolted to a competitor’s set, with the competitor’s nameplate attached. I was informed strongly that it was a Braybon unit of a very early build, with three coils and a rheostat volt control, along the lines of the NZ patent! That one I repaired with new points and a Braybon sticker attached. The point about the Braybon it that it is not what was called a ‘static regulator’ that moved from one excitation position to the next within a regulation band, like most motorised rheostats do in response to a load change. It is an ‘astatic’ type, where the control never sits still, hunts for a set value and responds in a non-linear way to load changes. Students will recognise that this is how a control system with a PID (proportional, integral and differential) feedback loop acts. The oscilloscope trace in Scope 6 shows some of this trait. On a load change, the excitation level accelerates almost instantly, then decelerates with overshoot to the next mean level. The effect on the AC level is to achieve the new level with almost the desired single over/ under shoot. From the more complex three-coil winding vibrator of early examples, the designer discovered fairly quickly that one coil winding could be dispensed with. Even with a mechanical voltage adjustment varying the reference spring, the AVR still exhibited fast response and satisfactory regulation and stability. After about 30 units were made, the design was settled, and mass production followed for defence and private purposes. In a nutshell, this archaic-looking vibrating-point magnetic field AVR embodied the classic features of a modern solid-state AVR, with switchmode power control and PID feedback, even if the designer may not have fully SC realised it. siliconchip.com.au Fig.5: you can think of the AVR a bit like an op amp since it uses negative feedback to regulate a voltage. Fig.6: the Tirrell AVR does the same job using a similar principle but with a different configuration. Australia's electronics magazine September 2026  97 SILICON CHIP .com.au/shop ONLINESHOP HOW TO ORDER INTERNET (24/7) PAYPAL (24/7) eMAIL (24/7) MAIL (24/7) PHONE – (9-5:00 AET, Mon-Fri) siliconchip.com.au/Shop silicon<at>siliconchip.com.au silicon<at>siliconchip.com.au PO Box 194, MATRAVILLE, NSW 2036 (02) 9939 3295, +612 for international You can also pay by cheque/money order (Orders by mail only) or bank transfer. Make cheques payable to Silicon Chip. 09/26 YES! You can also order or renew your Silicon Chip subscription via any of these methods as well! The best benefit, apart from the magazine? Subscribers get a 10% discount on all orders for parts. PRE-PROGRAMMED MICROS For a complete list, go to siliconchip.com.au/Shop/9 $10 MICROS $15 MICROS ATtiny85-20PU Graphing Thermometer (Mar26), Simple LC Meter (May26) Simple USB Power Monitor (Jun26), Transceiver Test Set (Aug26) ATmega328PB-AU Low-Power FM Transmitter (Sep26) PIC12F617-I/P Active Mains Soft Starter (Feb23), Model Railway Uncoupler (Jul23) Battery-Powered Model Railway Transmitter (Jan25) PIC16F1455-I/P Battery-Powered Model Railway TH Receiver (Jan25) Dual Train Controller (Transmitter / TH Receiver, Oct25) PIC16F1455-I/SL Battery-Powered Model Railway SMD Receiver (Jan25) USB Programmable Frequency Divider (Feb25) Dual Train Controller (SMD Receiver, Oct25) PIC16LF1455-I/P New GPS-Synchronised Analog Clock (Sep22) PIC16F1459-I/P Railway Points Controller Transmitter / Receiver (2 versions; Feb24) Mains Power-Up Sequencer (Feb24 | repurposed firmware Jul24) 8CH Learning IR Remote (Oct24), Heat Transfer Controller (Aug25) Vacuum Controller (Oct25), Adjustable Ultrasonic Cleaner (Jul26) PIC16F15214-I/SN Silicon Chirp Cricket (Apr23), Mic The Mouse (Aug25) PIC16F15214-I/P Filament Dryer (Oct24), Tool Safety Timer (May25) PIC16F15224-I/SL Multi-Channel Volume Control (OLED Module; Dec23) NFC IR Keyfob Transmitter (Feb25), Rotating Light (Apr25) PIC16F18115-I/SN Model Railway Destination Display (Aug26) PIC16F18126-I/SL RGB LED Star (Dec25), DCC/DC Stepper Motor Driver (Apr26) μDCC Decoder (May26; bell [G] or whistle [W]) PIC16F18146-I/SO Versatile Battery Checker (May25), RGB LED ‘Analog’ Clock (May25) USB-C Power Monitor (Aug25), DCC Remote Controller (Feb26) DCC Booster & Reverse Loop Controller (Mar26) DCC Accessory Decoder (Snap / Servo-type, Jul26) STM32G030K6T6 Variable Speed Drive Mk2 (Nov24) PIC16F1847-I/P PIC16F18877-I/PT Digital Capacitance Meter (Jan25) Dual-Channel Breadboard PSU Display Adaptor (Dec22) Wideband Fuel Mixture Display (WFMD; Apr23) PIC16F88-I/P Battery Charge Controller (Jun22), Railway Semaphore (Apr22) PIC24FJ256GA702-I/SS Ohmmeter (Aug22), Advanced SMD Test Tweezers (Feb23) ESR Test Tweezers (Jun24), Human Comfort Indicator (Jun26) PIC32MX170F256D-501P/T 44-pin Micromite Mk2 (Aug14), 4DoF Simulation Seat (Sep19) PIC32MX170F256B-50I/SP Micromite LCD BackPack V1-V3 (Feb16 / May17 / Aug19) Advanced GPS Computer (Jun21), Touchscreen Digital Preamp (Sep21) PIC32MX170F256B-I/SO Battery Multi Logger (Feb21), Battery Manager BackPack (Aug21) PIC32MX270F256B-50I/SP ASCII Video Terminal (Jul14), USB M&K Adaptor (Feb19) STM32L031F6P6 SmartProbe (Jul25) $20 MICROS ATmega32U4 ATmega644PA-AU PIC32MK0128MCA048 PIC32MX270F256D-50I/PT Wii Nunchuk RGB Light Driver (Mar24) AM-FM DDS Signal Generator (May22) Power LCR Meter (Mar25) Digital Preamplifier (Oct25) $25 MICROS PIC32MX170F256B-50I/SO + PIC16F1455-I/SL Micromite Explore-40 (SC5157, Oct24) PIC32MX470F512H-120/PT Micromite Explore 64 (Aug 16), Micromite Plus (Nov16) PIC32MX470F512L-120/PT Micromite Explore 100 (Sep16) $30 MICROS PIC32MX695F512H-80I/PT Touchscreen Audio Recorder (Jun14) PIC32MZ2048EFH064-I/PT DSP Crossover/Equaliser (May19), Low-Distortion DDS (Feb20) DIY Reflow Oven Controller (Apr20), Dual Hybrid Supply (Feb22) KITS, SPECIALISED COMPONENTS ETC SEMICONDUCTOR ANALYSER (SC7725) (SEP 26) Kit: includes an assembled PCB with the top-side components already fitted, plus all other non-optional parts except for the case, battery & label (see p32, Sep26) $95.00 - Hammond 1593XBK plastic case (SC7732) $17.50 LOW-POWER FM TRANSMITTER - Elechouse FM transmitter module (SC7712) - 0.96in OLED display module, white (SC6936) or cyan (SC6176) - ND0205MA 3V-to-5V DC step-up converter module (SC7713) (SEP 26) BATTERY BACKPACK KIT (SC7707) (SEP 26) MODEL RAILWAY DESTINATION DISPLAY (SC7697) (AUG 26) Kit: includes all the parts except for a Li-ion cell (see p82, Sep26) Kit: includes all parts, except for the OLED screen (see p54, Aug26) - 0.32in white OLED screen (SC7698) - 0.50in white OLED screen (SC7699) DCC ACCESSORY DECODERS (JUL 26) I2C CONTROLLER COMPLETE KIT (SC7690) (JUL 26) Snap-type (SC7685): includes the PCB and all non-optional onboard parts Servo-type (SC7686): includes the PCB and all non-optional onboard parts Includes the PCB and all onboard parts (see p83, Jul26) HUMAN COMFORT INDICATOR (SC7646) (JUN 26) Kit: includes all parts, except the case and battery (see p49, Jun26) - white 3D-printed case: portrait (SC7453) or landscape (SC7684) version - 3.3V GY-BME280 module (SC5482) $10.00 $10.00 $5.00 $25.00 $22.50 $5.00 $6.50 $40.00 $40.00 $30.00 $60.00 $12.50 $10.00 siliconchip.com.au/Shop/ SIMPLE LC METER COMPLETE KIT (SC7657) (MAY 26) μDCC DECODER KIT (SC7617) (MAY 26) POWER AMPLIFIER CLIPPING INDICATOR (SC7649) (MAY 26) STEPPER MOTOR DRIVER KIT (SC7601) (APR 26) CALLIOPE AMPLIFIER PARTS (SC6021) (APR 26) DCC BOOSTER / REVERSE LOOP CONTROLLER KIT (SC7579) (MAR 26) Includes all the parts and the 3D-printed enclosure (see p67, May26) Includes all the parts and the optional piezo (wire not included). Specify if you want a bell or whistle sound for the microcontroller (see p88, May26) $45.00 $25.00 Short-form kit: includes the PCB and all onboard parts, the case and power supply are not included (see p35, May26) $95.00 - pair of red & white PCB-mounting RCA sockets (SC2615) $4.00 Includes all required parts for DCC or DC mode (see p55, Apr26) Includes some of the harder-to-get transistors, resistors and a capacitor Includes all required parts, except for the Jiffy box, OLED screen (see below), power supply and front panel (see p58, Mar26) - 0.91-inch OLED screen (SC7484) DCC REMOTE CONTROLLER KIT (SC7552) (FEB 26) MAINS HUM NOTCH FILTER (SC7598) (FEB 26) DCC BASE STATION KIT (SC7539) (JAN 26) DCC DECODER KIT (SC7524) (DEC 25) $35.00 $15.00 $45.00 $7.50 Includes all required parts, except for the case and wire/cable (see p63, Feb26) $35.00 Includes everything except for the case and power supply (see p53, Feb26) $50.00 Includes everything but the plastic case, power supply and some optional parts. (JUN 26) $90.00 Control Board (SC7659): includes the PCB and all non-optional onboard parts $150.00 The Pico 2 is supplied but not programmed (see p39, Jan26) Power Supply (SC7680): includes the PCB and all onboard parts $50.00 RGB LED STAR KIT (SC7535) (DEC 25) Cable & Connector Set (SC7681): includes 17 10-pin box headers, 34 10-pin IDC Includes the mostly-assembled board and all non-optional components connectors, 10m of 10-way ribbon cable, 30 2-way pluggable terminal blocks except the power supply (see p43, Dec25) $80.00 and 20 2-way polarised headers $65.00 PINBALL MACHINE KITS SIMPLE USB POWER MONITOR (SC7683) Includes the PCB and all onboard parts (see p63, Jun26) - 0.96in OLED display module, white (SC6936) or cyan (SC6176) (JUN 26) $50.00 $10.00 Includes everything in the parts list (see p73, Dec25) *Prices valid for month of magazine issue only. All prices in Australian dollars and include GST where applicable. # Overseas? Place an order on our website for a quote. $25.00 PRINTED CIRCUIT BOARDS PRINTED CIRCUIT BOARD TO SUIT PROJECT MICROPHONE PREAMPLIFIER ↳ EMBEDDED VERSION RAILWAY POINTS CONTROLLER TRANSMITTER ↳ RECEIVER LASER COMMUNICATOR TRANSMITTER ↳ RECEIVER PICO DIGITAL VIDEO TERMINAL ↳ FRONT PANEL FOR ALTRONICS H0190 (BLACK) ↳ FRONT PANEL FOR ALTRONICS H0191 (BLACK) ARDUINO FOR ARDUINIANS (PACK OF SIX PCBS) ↳ PROJECT 27 PCB WII NUNCHUK RGB LIGHT DRIVER (BLACK) SKILL TESTER 9000 PICO GAMER ESP32-CAM BACKPACK WIFI DDS FUNCTION GENERATOR 10MHz to 1MHz / 1Hz FREQUENCY DIVIDER (BLUE) FAN SPEED CONTROLLER MK2 ESR TEST TWEEZERS (SET OF FOUR, WHITE) DC SUPPLY PROTECTOR (ADJUSTABLE SMD) ↳ ADJUSTABLE THROUGH-HOLE ↳ FIXED THROUGH-HOLE USB-C SERIAL ADAPTOR (BLACK) AUTOMATIC LQ METER MAIN AUTOMATIC LQ METER FRONT PANEL (BLACK) 180-230V DC MOTOR SPEED CONTROLLER STYLOCLONE (CASE VERSION) ↳ STANDALONE VERSION DUAL MINI LED DICE (THROUGH-HOLE LEDs) ↳ SMD LEDs GUITAR PICKGUARD (FENDER JAZZ BASS) ↳ J&D T-STYLE BASS ↳ MUSIC MAN STINGRAY BASS ↳ FENDER TELECASTER COMPACT OLED CLOCK & TIMER USB MIXED-SIGNAL LOGIC ANALYSER (PicoMSA) DISCRETE IDEAL BRIDGE RECTIFIER (TH) ↳ SMD VERSION MICROMITE EXPLORE-40 (BLUE) PICO BACKPACK AUDIO BREAKOUT (with conns.) 8-CHANNEL LEARNING IR REMOTE (BLUE) 3D PRINTER FILAMENT DRYER DUAL-RAIL LOAD PROTECTOR VARIABLE SPEED DRIVE Mk2 (BLACK) FLEXIDICE (RED, PAIR OF PCBs) SURF SOUND SIMULATOR (BLUE) COMPACT HIFI HEADPHONE AMP (BLUE) CAPACITOR DISCHARGER PICO COMPUTER ↳ FRONT PANEL (BLACK) ↳ PWM AUDIO MODULE DIGITAL CAPACITANCE METER 5MHZ 40A CURRENT PROBE (BLACK) BATTERY MODEL RAILWAY TRANSMITTER ↳ THROUGH-HOLE (TH) RECEIVER ↳ SMD RECEIVER ↳ CHARGER USB PROGRAMMABLE FREQUENCY DIVIDER HIGH-BANDWIDTH DIFFERENTIAL PROBE NFC IR KEYFOB TRANSMITTER POWER LCR METER WAVEFORM GENERATOR PICO 2 AUDIO ANALYSER (BLACK) PICO/2/COMPUTER ↳ FRONT & REAR PANELS (BLACK) ROTATING LIGHT (BLACK) 433MHZ TRANSMITTER VERSATILE BATTERY CHECKER ↳ FRONT PANEL (BLACK, 0.8mm) TOOL SAFETY TIMER RGB LED ANALOG CLOCK (BLACK) USB POWER ADAPTOR (BLACK, 1mm) HWS SOLAR DIVERTER PCB & INSULATING PANELS DATE FEB24 FEB24 FEB24 FEB24 MAR24 MAR24 MAR24 MAR24 MAR24 MAR24 MAR24 MAR24 APR24 APR24 APR24 MAY24 MAY24 MAY24 JUN24 JUN24 JUN24 JUN24 JUN24 JUL24 JUL24 JUL24 AUG24 AUG24 AUG24 AUG24 SEP24 SEP24 SEP24 SEP24 SEP24 SEP24 SEP24 SEP24 OCT24 OCT24 OCT24 OCT24 OCT24 NOV24 NOV24 NOV24 DEC24 DEC24 DEC24 DEC24 DEC24 JAN25 JAN25 JAN25 JAN25 JAN25 JAN25 FEB25 FEB25 FEB25 MAR25 MAR25 MAR25 APR25 APR25 APR25 APR25 MAY25 MAY25 MAY25 MAY25 MAY25 JUN25 For a complete list, go to siliconchip.com.au/Shop/8 PCB CODE 01110231 01110232 09101241 09101242 16102241 16102242 07112231 07112232 07112233 SC6903 SC6904 16103241 08101241 08104241 07102241 04104241 04112231 10104241 SC6963 08106241 08106242 08106243 24106241 CSE240203A CSE240204A 11104241 23106241 23106242 08103241 08103242 23109241 23109242 23109243 23109244 19101231 04109241 18108241 18108242 07106241 07101222 15108241 28110241 18109241 11111241 08107241/2 01111241 01103241 9047-01 07112234 07112235 07112238 04111241 9049-01 09110241 09110242 09110243 09110244 04108241 9015-D 15109231 04103251 04104251 04107231 07104251 07104252/3 09101251 15103251 11104251 11104252 10104251 19101251 18101251 18110241 Price $7.50 $7.50 $5.00 $2.50 $5.00 $2.50 $5.00 $2.50 $2.50 $20.00 $7.50 $20.00 $15.00 $10.00 $5.00 $10.00 $2.50 $5.00 $10.00 $2.50 $2.50 $2.50 $2.50 $5.00 $5.00 $15.00 $10.00 $12.50 $2.50 $2.50 $10.00 $10.00 $10.00 $5.00 $5.00 $7.50 $5.00 $2.50 $2.50 $2.50 $7.50 $7.50 $5.00 $15.00 $5.00 $10.00 $7.50 $5.00 $5.00 $2.50 $2.50 $5.00 $5.00 $2.50 $2.50 $2.50 $2.50 $5.00 $5.00 $2.50 $10.00 $5.00 $5.00 $5.00 $10.00 $2.50 $2.50 $5.00 $7.50 $5.00 $15.00 $2.50 $20.00 PRINTED CIRCUIT BOARD TO SUIT PROJECT SSB SHORTWAVE RECEIVER PCB SET ↳ FRONT PANEL (BLACK) 433MHz RECEIVER SMARTPROBE ↳ SWD PROGRAMMING ADAPTOR DUCTED HEAT TRANSFER CONTROLLER ↳ TEMPERATURE SENSOR ADAPTOR ↳ CONTROL PANEL MIC THE MOUSE (PCB SET, WHITE) USB-C POWER MONITOR (PCB SET, INCLUDES FFC) HOME AUTOMATION SATELLITE PICKIT BASIC POWER BREAKOUT DUAL TRAIN CONTROLLER TRANSMITTER DIGITAL PREAMPLIFIER MAIN PCB (4 LAYERS) ↳ FRONT PANEL CONTROL ↳ POWER SUPPLY VACUUM CONTROLLER MAIN PCB ↳ BLAST GATE ADAPTOR POWER RAIL PROBE RGB LED STAR EARTH RADIO DCC DECODER DCC BASE STATION MAIN PCB ↳ FRONT PANEL REMOTE SPEAKER SWITCH ↳ CONTROL PANEL DCC REMOTE CONTROLLER MAINS HUM NOTCH FILTER MAINS LED INDICATOR DCC BOOSTER / REVERSE LOOP CONTROLLER ↳ FRONT PANEL SOLAR PANEL PROTECTOR (WHITE) GRAPHING THERMOMETER PICOSDR CONTROL PCB ↳ RF PCB ↳ FRONT PANEL (BLACK) DCC/DC STEPPER MOTOR DRIVER CALLIOPE AMPLIFIER MICROMITE AUDIO PLAYER ADD-ON ↳ ALL-IN-ONE μDCC DECODER SIMPLE LC METER WIFI ALARM MONITOR POWER AMPLIFIER CLIPPING INDICATOR PINBALL MACHINE CONTROL BOARD ↳ POWER SUPPLY ↳ PLAYER LED BOARD ↳ SCORE LED BOARD ↳ LED OUTPUT BOARD ↳ BUMPER LED BOARD ↳ CASCADE LED BOARD ↳ SWITCH INPUT BOARD ↳ GENERAL INPUT BOARD ↳ HIGH-CURRENT INTERFACE ↳ ROLLOVER INTERFACE ↳ BUMPER DRIVER SSB TRANSMITTER (MikeOne/Two/Three) SIMPLE USB POWER MONITOR HUMAN COMFORT INDICATOR ADJUSTABLE ULTRASONIC CLEANER MAIN PCB ↳ FRONT PANEL CONTROL PCB SNAP-TYPE DCC ACCESSORY DECODER ↳ SERVO-TYPE I2C CONTROLLER TRANSCEIVER TEST SET RF/AUDIO PCB ↳ CONTROL PCB MODEL RAILWAY DESTINATION DISPLAY ↳ FLEX ANTENNA PCB DATE JUN25 JUN25 JUN25 JUL25 JUL25 AUG25 AUG25 AUG25 AUG25 AUG25 SEP25 SEP25 OCT25 OCT25 OCT25 OCT25 OCT25 OCT25 NOV25 DEC25 DEC25 DEC25 JAN26 JAN26 JAN26 JAN26 FEB26 FEB26 FEB26 MAR26 MAR26 MAR26 MAR26 APR26 APR26 APR26 APR26 APR26 APR26 APR26 MAY26 MAY26 MAY26 MAY26 JUN26 JUN26 JUN26 JUN26 JUN26 JUN26 JUN26 JUN26 JUN26 JUN26 JUN26 JUN26 JUN26 JUN26 JUN26 JUL26 JUL26 JUL26 JUL26 JUL26 AUG26 AUG26 AUG26 AUG26 PCB CODE Price CSE250202-3 $15.00 CSE250204 $7.50 15103252 $2.50 P9054-04 $5.00 P9045-A $2.50 17101251 $10.00 17101252 $2.50 17101253 $2.50 SC7528 $7.50 SC7527 $7.50 15104251 $3.50 18106251 $2.00 09110245 $3.00 01107251 $30.00 01107252 $2.50 01107253 $7.50 10109251 $10.00 10109252 $2.50 P9058-1-C $5.00 16112251 $12.50 06110251 $5.00 09111241 $2.50 09111243 $5.00 09111244 $5.00 01106251 $5.00 01106252 $2.50 09111245 $5.00 01003261 $7.50 10111251 $2.50 09111248 $5.00 09111249 $5.00 17112251 $7.50 04102261 $3.00 CSE251101 $5.00 CSE251102 $5.00 CSE251103 $7.50 09111242 $2.00 01111212 $5.00 01110251 $2.50 01110252 $5.00 09111247 $1.50 04103261 $2.50 01304261 $2.50 01104261 $15.00 08107261 $25.00 08107262 $7.50 08107263 $2.50 08107264 $5.00 08107265 $2.50 08107266 $5.00 08107267 $5.00 08107268 $2.50 08107269 $2.50 08107260 $2.50 08117261 $2.50 08117262 $5.00 06103261 $2.50 04104261 $5.00 21105261 $5.00 04105261 $7.50 04105262 $5.00 09111254 $3.00 09111255 $3.00 09111256 $3.00 06104261 $5.00 06104262 $5.00 09111252 $2.00 06101233 $2.00 SEMICONDUCTOR ANALYSER LOW-POWER FM TRANSMITTER ↳ LID (BLACK, 0.8mm) BATTERY BACKPACK SEP26 SEP26 SEP26 SEP26 P9062-1-C $7.50 CSE260501C $5.00 CSE260502 $5.00 11105261 $3.00 NEW PCBs We also sell the Silicon Chip PDFs on USB, RTV&H USB, Vintage Radio USB and more at siliconchip.com.au/Shop/3 ASK SILICON CHIP Got a technical problem? Can’t understand a piece of jargon or some technical principle? Drop us a line and we’ll answer your question. Send your email to silicon<at>siliconchip.com.au Simple USB Power Monitor query I’ve just completed the Simple USB Power Monitor (June 2026 issue; siliconchip.au/Article/20365). It appears to work well, but there is a niggling problem I thought you may be able to shed light on. When I connect it in the charging path of an Android tablet or phone, the device gives me a warning saying “Check your charger connection. Make sure the charger and cable are correctly connected. If this warning repeatedly occurs, consider replacing the charging cable.” In spite of this warning, the device still charges. Why do you think I’m getting this warning? Do you think some of the USB connections on the charger have not been properly soldered? (J. H., Nathan, Qld) ● The designer, Richard Palmer, responds: Sorry to hear that you are having a problem. It would help to have some more information. Firstly, does the monitor show reasonable readings while the device is charging? As indicated in the article, USB comes in many formats, and some combinations cause the connection to fall back to the most basic 5V/500mA settings if the Power Delivery negotiation fails. This is common with USB-C chargers. Does the charger have a USB-A or USB-C socket? Is there a USB-C plug/socket anywhere in the connection, other than on the phone/ tablet itself? What are the ratings (the range of voltages and currents available) on the charger you have it connected to? Does the monitor work properly with other USB-A devices? Charging a power bank is a good test. You could try something that sends data through the monitor: a headset, portable hard drive etc. That will reveal if there are any problems with the two main data lines. The plug and socket on the monitor are nine-pin types, so you can get some interesting issues if one of the ‘extra’ pins is not 100 Silicon Chip soldered properly or you have a ninewire cable on one side and a five-wire cable on the other. MP3 player design wanted Have you published an MP3/WMA player using a USB stick or similar for storage? I would ideally like one that’s powered by an internal Li-ion battery, providing both line and headphone outputs. A graphic equaliser would be a bonus. (J. K., Freshwater, NSW) ● We have published a couple of basic MP3 players, most recently the Micromite-based Music Player in the April 2026 issue, which can play MP3s (siliconchip.au/Article/20086). You might also like to refer to the article “El cheapo modules, part 21: stamp-sized audio player” in the December 2018 issue (siliconchip.au/Article/11341). We can supply the MP3 player module; see siliconchip.com.au/ Shop/7/4789 You could add a Li-ion battery-based supply or just use a USB power bank. The output can drive headphones or a line input. Difficulty in calibrating the Power LCR Meter I built this project from the March & April 2025 issues (siliconchip. au/Series/436) but am experiencing problems with the calibration process. Step one of the calibration is to measure the 10mA current controlled by the current sink (Q5) by inserting a meter between the DUT terminals and setting the value using the up/ down buttons. The problem is that the value I read on my digital ammeter is 5.4mA. I have verified this with another meter. This value is outside the calibration range in the software, which only goes down to 7.0mA. As a result, any resistance measurements I make using the Meter are incorrect. I have verified all the component values in the TIP121/IC7/IC2 areas of the circuit, and they are all correct. I have changed Q5 (TIP121), IC7 (TLC072) and R42 (1W) but the problem is still present. Before changing IC7, I was measuring 6.3mA, but I assume that the change is due to the component. If I proceed with the calibration process for the 100mA and 1A calibration, then I get acceptable readings on the digital ammeter with values close to nominal. I am assuming that the software initially sets the voltage at DAC IC2 pin 6 to a value that causes Q5 (TIP121) via IC7b (TLC072) to allow 10mA to flow between DUT+ and DUT− via the ammeter. The +10V, +10V filtered, +3.3V and -3.3V supplies are all OK. I have looked at the voltage rails with an oscilloscope and they are stable. The 1W resistor (R42) measured 1.03W on three different instruments, so I don’t think that is a problem. Any help would be appreciated. (K. F., Pullenvale, Qld) ● Phil Prosser responds: I ran one of my prototypes through the calibration procedure and made some measurements. You should see something like: 10mA test: • measured current = 10.5mA • voltage across the 1W resistor = 0.0105V Heatsink-mounted thermistor for Temperature Switch I want to build the Temperature Switch Mk2 from the June 2018 issue (siliconchip. au/Article/11101) to monitor a heatsink’s temperature for a power supply. How do I attach the thermistor to the heatsink? Can I purchase a thermistor already suited to this purpose, or is there a better way of doing this? (R. M., Melville, WA) ● Altronics sells a suitable 10kW thermistor, Cat R4112. It is attached to an eyelet, making it easy to mount on a heatsink. Australia's electronics magazine siliconchip.com.au SOnline ilicon Chip Shop Kits, parts and much more www.siliconchip.com.au/Shop/ Simple USB Power Monitor June 2026 Complete Kit SC7683: $50 siliconchip.au/Article/20365 Includes the PCB, all onboard parts and some clear heatshrink tubing to encase it. This unit is not recommended for USB 3.1 PD above 36V. Dual Mini LED Dice August 2024 SMD LED Complete Kit SC6961: $17.50 TH LED Complete Kit SC6849: $17.50 USB Power Adaptors May 2025 Complete Kit with choice of USB socket SC7433: $10 siliconchip.au/Article/16418 siliconchip.au/Article/18112 Includes either 3mm through-hole or 1206sized SMD LEDs. Choice of either white or black PCB. CR2032 coin cell not included. You can choose from one of four USB sockets (USB-C power only, USB-C power+data, mini-B or micro-B). The kit includes all other parts. DCC Base Station Short-form Kit SC7539: $90 Human Comfort Indicator June 2026 January 2026 siliconchip.au/Article/19558 This kit includes all non-optional components in the parts list (and the RJ45 socket, CON6). It does not include the case, DC power supply, glue, CON4 screw terminal and CON5 locking header. Mic the Mouse Complete Kit SC7508: $37.50 August 2025 siliconchip.au/Article/18637 It includes everything needed to build one Mic the Mouse, except for solder, glue and a CR2032 cell. Complete Kit SC7646: $60 siliconchip.au/Article/20362 Includes everything, except for the case and Li-ion cell. You can either use a 3D printed case (available separately) or a UB3 Jiffy box. → Subscribers receive a 10% discount on all purchases, except for subscriptions (postage is not discounted). → Prices listed do not include postage. Postage rates within Australia start at $12, rates are calculated at the checkout. siliconchip.com.au Australia's electronics magazine September 2026  101 • TL072 pin 5 = 0.0104V • TL072 pin 7 = 1.18V (the Vbe of a Darlington transistor) 100mA test: • measured current = 98.3mA • voltage across the 1W resistor = 0.0983V • TL072 pin 5 = 0.0998V • TL072 pin 7 = 1.308V It seems like your DAC output is much lower than it ought to be. That part is clearly at the extreme limit of its specifications. That doesn’t mean the instrument won’t work, but it does affect the calibration procedure. I am thinking of modifying the software; there are various options: • changing the calibration current from 10mA to something higher; • allowing a wider range of calibration settings; • changing the calibration approach to actually modify the DAC drive instead of measuring the current. I will perform some further testing, but my inclination is to modify the software to widen the calibration window. We can then see if that fixes your problem. Large clock needs higher current drive I have been reading Silicon Chip since the days when it took over from Electronics Australia, and I had read that from when I was about 15 years old. I am now 75! I have built many of your kits, and I am very grateful for all the work you put in developing them. I recently purchased your kit for the GPS Synchronised Analog Clock (September 2022; siliconchip.au/ Series/391) with the Clayton’s GPS WiFi module (November 2022). I have built it and it works as designed. However, the clock I am using has a ‘high torque’ stepping motor, which is needed to drive its more than usually substantial metal hands. The driver sends impulses to the clock’s motor, and I can hear it ticking, but they are of insufficient power to actually drive the clock. Still, the clock works normally on a single AA cell when I reconnect the coil to its own circuit board. The resistance of the coil is about 130W. Is there any way I can boost the power output of the clock driver to drive this clock motor? Many thanks from a long-time reader. (C. H., Camperdown, NSW) 102 Silicon Chip ● The designer, Geoff Graham, responds: the MCP6041 can just drive a 200W load but 130W is too low and the output voltage will sag to a level that will not drive the motor. You could try buffering the output of the MCP6041 with a couple of Mosfets but I have never tried it. A better solution might be to solder a second MCP6041 in piggyback fashion on top of the first MCP6041 with like pins connecting to like pins. Theoretically, this would double the drive to the clock’s motor, but again, I have never tried it. It could also be a difficult soldering job. The best solution would be to simply replace the movement with one that is less demanding, but then it might not be able to move your extralarge clock hands. Sorry that I cannot be of more help. Note: another option would be to substitute an OP391 on an SMD adaptor for the MCP6041. It’s more expensive and has a higher quiescent current, but it seems that it would be able to drive a 100W load or even lower. SC200 amplifier questions I have the following questions on the SC200 amplifier published in the January 2017 to March 2017 issues (siliconchip.au/Series/308): 1. In the circuit description, you recommend mounting the inductor in a vertical position. In the photos (for example, in the February 2017 issue, on page 78) of the assembled board, the inductor is in a horizontal position. Which way is correct? 2. In the main photos (February 2017 issue, page 78), transistors Q10, Q11 and Q12 are in a row. However, in the drawing (February 2017 issue, page 83) they are placed in a different way. Which way is correct? 3. Where do you recommend placing the temperature sensor on the heatsink for the Altronics K5167 Loudspeaker Protector Kit? 4. I purchased your set of transistors. Instead of KSC2690 and KSA1220, you sent me TTA004B and TTC004B as replacements. The KCS2690 and KSA1220 have a metal tab at the back, but the replacement transistors do not. They are completely plastic moulded, so the collector has no electrical connection to a heatsink. Can I mount these transistors without insulators? Australia's electronics magazine 5. Regarding the clipping detector, I am going to use a lower voltage power supply of around ±45V, maybe even less. Do I need to change resistor values in the circuit which is designed for a ±57V DC power supply? (Y. A., Kellyville, NSW) ● Our reply to each point is listed below: 1. In the text on page 78 of the February 2017 issue (left-hand column) and the caption for the photo at the top of p79, it explains that vertical mounting gives better performance. The vertical orientation was found to work better after most of the photos had been taken. It will still function with a horizontal inductor, but the performance won’t be as good. 2. The drilling diagram is designed for compatibility with other amplifiers in the Ultra-LD series where the transistors are different sizes, so the holes aren’t at the same height. It won’t make any difference whether you choose to put them in a line or follow our recommended hole positions. 3. We recommend the sensor be mounted near one of the larger transistors since that will be the hottest point. 4. Since they don’t have an exposed metal tab, you don’t need to add insulators to those transistors. 5. We don’t think it’s necessary to change the resistors as ±45V is only about 20% lower than ±57V and the circuit is designed to track the supply rails. However, if you want to change the values to be optimal, we suggest changing the 68kW resistor to 51kW (or 56kW) and the 33kW resistor to 24kW (or 27kW). Getting rid of hum in a stereo amplifier I have recently built a pair of Ultra-LD Mk.4 amplifier modules (August-October 2015; siliconchip. au/Series/289) to make a stereo power amplifier in one aluminium chassis. Each module was powered using the power supply board recommended (and the Universal Loudspeaker Protector) and it seems to be working just fine. Each module is dead quiet when separately connected to a Yamaha preamplifier. However, when both modules are connected via RCA cable, I hear an audible hum in both speakers. Unplugging either the left or right continued on page 104 siliconchip.com.au MARKET CENTRE Advertise your product or services here in Silicon Chip KIT ASSEMBLY & REPAIR FOR SALE DAVE THOMPSON (the Serviceman from Silicon Chip) is available to help you with kit assembly, project troubleshooting, general electronics and custom design work. No job too small. Based in Christchurch, New Zealand, but service available Australia/NZ wide. Email dave<at>davethompson.co.nz LEDsales KEITH RIPPON KIT ASSEMBLY & REPAIR: * Australia & New Zealand; * Small production runs. Phone Keith: 0409 662 794 keith.rippon<at>gmail.com LEDS, BRAND NAME AND GENERIC LEDs, filament LEDs, LED drivers, heatsinks, power supplies, kits and modules, components, breadboards, hardware, magnets. Please visit www. ledsales.com.au Micromite Explore-40 October 2024 Complete Kit SC6991: $35 siliconchip.au/Article/16677 PMD WAY offers (almost) everything for the electronics enthusiast – with full warranty, technical support and free delivery worldwide. Visit pmdway.com to get started. Includes the PCB and all onboard parts. Audio Breakout board and Pico BackPack are sold separately. WANTED PCB PRODUCTION TELEX MACHINE OR TELEPRINTER: I am looking to purchase a Siemens Telex Machine or Creed teleprinter (any models). Also, any teleprinter parts. Phone Pete: 0419 036 213 PCB MANUFACTURE: single to multilayer. Bare board tested. One-offs to any quantity. 48 hour service. Artwork design. Excellent prices. Check out our specials: www.ldelectronics.com.au ADVERTISING IN MARKET CENTRE Classified Ad Rates: $32.00 for up to 20 words (punctuation not charged) plus $1.20 for each additional word. Display ads in Market Centre (minimum 2cm deep, maximum 10cm deep): $82.50 per column centimetre per insertion. All prices include GST. Closing date: 5 weeks prior to month of sale. To book, email the text to silicon<at>siliconchip.com.au and include your name, address & credit card details, or phone (02) 9939 3295. WARNING! Silicon Chip magazine regularly describes projects which employ a mains power supply or produce high voltage. All such projects should be considered dangerous or even lethal if not used safely. Readers are warned that high voltage wiring should be carried out according to the instructions in the articles. When working on these projects use extreme care to ensure that you do not accidentally come into contact with mains AC voltages or high voltage DC. If you are not confident about working with projects employing mains voltages or other high voltages, you are advised not to attempt work on them. Silicon Chip Publications Pty Ltd disclaims any liability for damages should anyone be killed or injured while working on a project or circuit described in any issue of Silicon Chip magazine. Devices or circuits described in Silicon Chip may be covered by patents. Silicon Chip disclaims any liability for the infringement of such patents by the manufacturing or selling of any such equipment. Silicon Chip also disclaims any liability for projects which are used in such a way as to infringe relevant government regulations and by-laws. Advertisers are warned that they are responsible for the content of all advertisements and that they must conform to the Competition & Consumer Act 2010 or as subsequently amended and to any governmental regulations which are applicable. siliconchip.com.au Australia's electronics magazine September 2026  103 Advertising Index Altronics.................................41-44 Blackmagic Design....................... 7 Dave Thompson........................ 103 DigiKey Electronics..................OBC Emona Instruments.................. IBC Hare & Forbes............................... 9 Jaycar............................. IFC, 24-27 Keith Rippon Kit Assembly....... 103 LD Electronics........................... 103 LEDsales................................... 103 Microchip Technology.................. 5 Mouser Electronics....................... 3 PCBWay....................................... 11 PE Back Issues............................. 8 PMD Way................................... 103 SC Micromite Explore-40......... 103 Silicon Chip PDFs on USB......... 82 Silicon Chip Kits...................... 101 Silicon Chip Shop.................98-99 Silicon Chip Subscriptions........ 83 The Loudspeaker Kit.com.......... 89 Wagner Electronics..................... 10 cable and replacing it with a shorting plug (or not!) results in both speakers being dead quiet! Playing music loud to consign the hum well into the background and the stereo pair sounds just fantastic, better than my late-model Yamaha power amplifier. Has anyone else had the same problem? Have I missed an erratum? Any help would be appreciated as this is a superb amplifier, but I can’t stand the hum late at night when the volume is down. (J. D. S., Endeavour Hills, Vic) ● It sounds like there is an Earth hum loop. We suggest you try disconnecting one of the 10W resistors at the input of one of the circuit boards. The 10W resistor is intended to reduce any hum loop current, but it appears not to be sufficient in your case. You could use a 100W resistor instead, or just remove one. You should also check the power supply Earthing arrangement. Ensure you use a common point for all the power Earths and it should not be at the capacitor bank. Note: J.D.S. responded that disconnecting one of the 10W resistors from input to Earth completely eliminated the hum. Controlling many relays using two wires I am a long-term subscriber and have bought every issue. I think you do an amazing job. I am wondering if you can suggest or point me to a project or circuit for me. I’d like to control 32 LEDs or relays from 32 switches, on a oneto-one basis, but with only two wires between the loads and the switches. Visualise one box with 32 relays, another box with 32 switches about Errata and on-sale date for the next issue Simple USB Power Monitor, June 2026: the alternative regulator was specified as the MIC1973-330OT but it should have been MCP1793T-3302H/ OT. Many of the MIC5233-3.3YM5 regulators available from both DigiKey and Mouser around the time of publication appear to be faulty, producing no output. If you purchased a kit and the regulator doesn’t work, contact us and we’ll send you a replacement MCP1793T-3302H/OT regulator (no such problems have been reported with those). USB-C Power Monitor, September 2025 (part two): the caption at the bottom of p79 states “A row of header pins can be fitted to CON5…”. It should refer to CON3 instead. Next Issue: the October 2026 issue is due on sale in newsagents by Monday, September 28th. Expect postal delivery of subscription copies in Australia between September 25th and October 14th. 104 Silicon Chip Australia's electronics magazine say 10m away, but only two wires between them. I’m imagining some kind of scanning of the switches, serialising the data in the ‘switch box’ and decoding and driving at the other end. Can you point me to anything suitable that may already exist, or do you have any ideas? Thank you. (C. B., Seacombe Heights, SA) ● The simplest way to do that would be to take our 10-Channel Remote Control Receiver from the June 2013 issue (siliconchip.au/Article/3811) and remove the infrared receiver. A twinwire cable can connect to its pads 1 and 2 instead. A device like an Arduino can send RC5-encoded signals over those two wires to switch any of the ten attached loads on and off. Multiple Remote Control Receivers can be connected to the same pair of wires and set to use different RC5 encodings. While this would involve developing some Arduino software to sense switch inputs and send the required on/off codes, that should be pretty simple. Sensing switch closures is done easily by adding pull-up resistors (or enabling pull-up currents) and then checking the digital input state. There are many infrared encoding libraries available; while they are intended to drive IR LEDs, the signals are the same, so they should do the job. A board like the Arduino Mega2560 would likely be required to get enough inputs for 32 switches. Replacement transformer for EA amp A while ago I built the Electronics Australia High Quality Audio Amp Module (88ma12, January 1989). The transformer has failed and I can’t find suitable E-core transformers anymore. Can I use a toroidal transformer? I have seen a 160VA 25-0-25V 3.2A toroidal transformer. Will this be too much for the 50/80W amplifier? (W. O., Miller, NSW) ● The 160VA 25-0-25 transformer would be a good match for the amplifier. The amplifier will only draw the power it requires, so provided that the supply voltages are correct for the amplifier module (which they will be in this case), there’s nothing wrong with using a transformer with a higher power or current rating. 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