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DECEMBER 2025 ISSN 1030-2662 12 9 771030 266001 $ 00* NZ $14 90 The VERY BEST DIY Projects! 14 INC GST INC GST RGB LED Star Pre-assembled and ready to decorate in time for Christmas How to design your own PCBs All the steps needed to make and order your own printed circuit boards DCC Decoder for model locomotives HUMANOID ROBOTS Portable Fridge/Freezer SALE ON SALE Thursday 4 Dec to Wednesday 24 Dec, 2025 40 * ENDS DEC 24 Whilst Stock Lasts *Selected Models Contents Vol.38, No.12 December 2025 16 Humanoid Robots, Part 2 Humanoid and android robots are now a reality; while they’re not yet perfect, they are becoming more widespread. So let’s look at some of the most interesting robots of the past and present. By Dr David Maddison, VK3DSM Robotics DCC Decoder for model locomotives 30 Power Electronics, Part 2 In this series of articles, we explore the broad term of power electronics. Part two continues the series by covering the control systems required to make DC-DC converters work. By Andrew Levido Electronic design Page 70 Earth Radio 52 How to Design PCBs, Part 1 PCB fabrication is inexpensive nowadays, with plenty of scope for customisation. To help you take advantage of this, we show you the entire process of designing and ordering your own PCBs from scratch. By Tim Blythman Making your own PCBs Page 60 96 BC-211 Frequency Meter The BC-211 is one of a family of frequency meters that were designed for military use by the USA around 1940. It needed to remain accurate even for use in the field, so how did they do it? By Ian Batty Vintage Electronics 41 RGB LED Star Ornament Jazz up your Christmas tree or anywhere around the house using this Star with its 80 RGB LEDs and pre-programmed patterns. It is also sold partially pre-assembled, with only a small amount of soldering required. By Nicholas Vinen Christmas decoration project 60 Earth Radio, Part 1 Solar and atmospheric disturbances, like storms or auroras, can be heard using this ‘natural’ radio receiver. It is battery-powered and utilises a portable loop antenna, so you can use it from nearly anywhere. By John Clarke Scientific / radio receiver project 70 DCC Decoder A DCC Decoder is essential for controlling every model train. This project is the first in a series covering all parts of a complete DCC (digital command control) system. The later parts explain you how to use DCC, build a Base Station, Remote Controller, Booster and Reverse Loop Controller. Part 1 by Tim Blythman Model train project 82 Digital Preamplifier, Part 3 This advanced preamplifier uses digital processing and can also act as a crossover. In the final part of this series, we explain how to prepare the case, mount the modules, wire it up and play some music. By Phil Prosser Audio/hifi project Part 3: Page 82 Digital Preamplifier and Crossover 2 Editorial Viewpoint 4 Mailbag 10 Product Showcase 15 Online Shop 79 Circuit Notebook 90 Serviceman’s Log 102 Ask Silicon Chip 103 Market Centre 104 Advertising Index 104 Notes & Errata 1. Pulse-counting logic probe 2. 10A ammeter using a digital voltmeter 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): $72.50 12 issues (1 year): $135 24 issues (2 years): $255 Online subscription (Worldwide) 6 issues (6 months): $52.50 12 issues (1 year): $100 24 issues (2 years): $190 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: 14 Hardner Rd, Mount Waverley VIC 3149 54 Park St, Sydney NSW 2000 2 Silicon Chip Editorial Viewpoint Dutch government fumbles with Nexperia In case you’re not aware, NXP Semiconductors, a major manufacturer of microcontrollers and other ICs, was spun off from Dutch electronics giant Philips in 2006. In 2015, NXP acquired Freescale Semiconductor, becoming one of the world’s largest automotive chipmakers. In 2017, NXP spun off its Standard Products division as a new company, Nexperia, which focuses on discrete semiconductors, logic devices and Mosfets. Nexperia was soon acquired by Chinese semiconductor firm Wingtech. Recently, the US Department of Commerce pressured the Dutch government to intervene in Nexperia due to concerns over its Chinese ownership. While Nexperia’s product line isn’t particularly sensitive, its enormous production volume and deep integration into global supply chains make it strategically important. The US demanded the removal of Nexperia’s Chinese CEO, Zhang Xuezheng, and threatened to restrict imports of Nexperia products if this didn’t happen. The Dutch government capitulated under this pressure and, on the 30th of September 2025, invoked their Goods Availability Act for the first time to intervene (see siliconchip.au/link/ac93), citing “serious governance shortcomings” and risks to the continuity of European semiconductor manufacturing. A Dutch court suspended the CEO and appointed a local director with decisive voting rights. The government claimed this would not affect day-to-day operations, but major decisions, like asset sales or leadership changes, could now be blocked or reversed by the Minister for Economic Affairs. Predictably, this has sparked internal disruption, and it seems the move may be backfiring. Nexperia’s Chinese operations have reportedly instructed staff to ignore directives from Dutch headquarters, creating a serious risk of fragmentation. Should China retaliate – for instance, by seizing control of Nexperia’s assets within China – the European side of the business would be left severely weakened. About 80% of Nexperia’s chips are assembled and tested in China, especially at its massive Guangdong facility, which handles over 50 billion units annually. While the situation continues to evolve, it highlights the fragility of international supply chains in a geopolitically tense world. I’m hoping, for everyone’s sake, that this volatile situation can be resolved and Nexperia can continue their production as normal. Ironically, in trying to safeguard European technological sovereignty, the Dutch government may have made its own semiconductor sector more vulnerable. Even if the US had cut Nexperia out of its domestic market, the fallout would likely have been more manageable. That scenario might have even shifted more production toward Europe – not less, as now seems likely. Nexperia’s products are humble yet critical: logic gates, diodes, Mosfets; the basic building blocks found in almost every piece of electronic hardware. Their strategic value comes not from cutting-edge tech, but from sheer scale and ubiquity. The current situation is a reminder that just about any large business can be at geopolitical risk. For engineers and supply managers alike, it reinforces the need to diversify sourcing and keep an eye not just on the price per unit, but on whether all the eggs are in one basket. 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”. How to avoid losing SMD parts In a moment of dubious reasoning, I ordered two kits from Silicon Chip, both with numerous, almost invisible SMD components. Previous experience with these tiny components showed how easily they can fly off my workbench and vanish forever on my relatively clean floor. Thinking of how to prevent this inevitability, I remembered what an old jeweller had used to prevent watch parts from being lost. He donned a special apron that hung from his neck as normal but then curled up and clipped under his bench. Anything that might try to escape onto the floor was caught in the apron with a good chance of being found. So I made one, as shown in the attached photo. Imagine yourself between the chair and the apron. Happy making! Stephen Somogyi, Gloucester, NSW. Comment: if you need a name for it, we propose the “SMD bib”. And if you lose anything after wearing the bib, you can then spit the dummy. Feedback on USB-C Power Monitor & SMDs Following up from an October letter you published titled “Overcoming challenges in assembling several modules”, I found several similarities to my efforts with the USB tester, identifying tiny components being the major one. I too found solder bridges, which I cleared with some solder wick. My project was all hand soldered, no re-flow equipment being available. Maybe it is a reflection on your readership but I am also in the 75+ age group. My tester is working perfectly and it calibrated easily. Ian Malcolm, Scoresby, Vic. Capacitors may act as grid leak resistors I am writing regarding the Reinartz TRF Receiver article (Reinartz 2 TRF Receiver by Philip Fitzherbert & Ian Batty, October 2025; siliconchip.au/Article/18996) and the question on page 95 to do with the absence of a grid leak resistor. I have only ever made one Reinartz-style receiver, and 4 Silicon Chip it was featured in the October 2021 issue of Silicon Chip. In that radio, I eventually wound up with a pentode as the reaction valve, having trialled triode and tetrode ideas extensively. While I was experimenting with valve types, I basically carried over the same grid leak parts. These were 1930s style: a big 250pF moulded capacitor and a 2.5MW ceramic lead end-cap resistor. At some point while experimenting, I unknowingly disconnected the resistor but did not notice as the valves were still biased OK. Then I had to change the capacitor because one brittle lead broke. After that, whatever valve I was trying did not work ‘properly’, seemingly choking up bias-wise. After much confusion, I fixed the broken wire on the old cap, swapped that back in, and all was well again. The reason was that the old capacitor had leakage in the megohms range. It was thus acting as a grid-leak bias resistor. In the final build, I used a very low-leakage mica moulded cap and mounted the 2.5MW resistor across the cap instead of being grid to ground. That proved the point; it biases just as well. I wonder if the capacitors of the day simply had enough leakage to behave this way, and Reinartz knew exactly what he was doing without saying so. On a general note, I was very impressed with how well the Reinartz ‘reaction’ theory worked on my set. Without the feedback loops, the sensitivity was poor and the selectivity bad, with stations overlapping each other. By introducing the tuning coil feedback, as well as the screen grid gain adjustment, the set then had sensitivity and selectivity almost like a superhet receiver. It was very impressive and spurred me on to produce a whole receiver, as you see in the 2021 article. Fred Lever, Toongabbie, NSW. Adding a weather station to HomeAssistant I would like to congratulate you on the articles on Home Assistant (September & October 2025; siliconchip.au/ Series/448). I’ve been running a very basic instance of HA on an Intel NUC for a couple of years now, so it was interesting to read your take on it. I currently have a Z-Wave integration and use it to drive an IR Blaster for my air conditioning system, as well as get feedback from a couple of temperature probes. One thing I’d like to see is an integration for a weather station; temperature and rain sensing is all I really want. Wind speed and direction are very unreliable unless the unit is mounted in the right place and the hardware is more complicated. I have seen some designs using a tipping-bucket rain gauge with an I2C or UART interface to Arduino and Australia's electronics magazine siliconchip.com.au Introducing 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 for live streaming to YouTube! 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 ATEM Mini is a full broadcast television switcher, so it has hidden power that’s any of the input buttons on the front panel to cut between video sources. You can unlocked using the free ATEM Software Control app. This means if you want to select from exciting transitions such as dissolve, or more dramatic effects such go further, you can start using features such as chroma keying for green screens, as dip to color, DVE squeeze and DVE push. You can even add a DVE for picture media players for graphics and the multiview for monitoring all cameras on a in picture effects with customized graphics. 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! That guarantees full compatibility with any video software and in full resolution 1080HD quality. Imagine giving a presentation on your latest research from a laboratory to software such as Zoom, Microsoft Teams, ATEM Mini Pro has a built in hardware streaming engine for live streaming to a global audience! That means you can live stream lectures or educational workshops direct to scientists 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 $505 Skype or WebEx! www.blackmagicdesign.com/au Learn More! Raspberry Pi, but the coding is beyond me. Would you look at incorporating the rain sensor into your ESPHome hub? You can buy the tipping bucket sensor with an interface board from Core Electronics SEN0575. They also sell an interesting ESP32 board that has built-in solar charging capabilities for powering this somewhere in your garden. I’d like to have your thoughts on possibly using this board to terminate the rain sensor and the interface to HA via WiFi or Zigbee. See Core Electronics DFR1075. Duncan Wilkie, Hobart, Tas. Richard Palmer responds: Good to hear that you enjoyed the Home Assistant series. A weather station would make an excellent addition to a home installation. Temperature and humidity sensing are straightforward as there are a substantial number of I2C devices that fit the bill, including the SHT40 used in the project. A few metres of cable shouldn’t upset the I2C connection. If it does, reducing the I2C frequency in the configuration file should do the trick. ESP Home supports frequencies down to 10kHz. The rain gauge is a little more complex. The Core Electronics DFROBOT Tipping Bucket Rainfall Sensor is an excellent solution in theory. However, to integrate it with ESPHome, the software library (in Python) would need to be converted into an ESPHome-compatible format, which would be a mid-level task for someone with solid programming skills. The ESPHome Developer Site (https://developers. esphome.io) provides further information. As for the Firebeetle, any platform that has I2C support is theoretically a candidate. Another approach would be to use the ESPHome UART Bus (https://esphome.io/components/uart) for the interface. The documentation is clear for writing to the serial interface, but somewhat lacking on how to process incoming data. I found a discussion on the HA forum (https://community. home-assistant.io/t/get-data-from-serial-port/799877/7) that offers an approach to processing incoming serial data. I hope this helps in your quest for a HA-integrated weather station. Testing infrared remote controls Regarding the query by M. S. of Melbourne, Victoria, in the October 2025 edition of Silicon Chip magazine on page 104, there is a simple, convenient method of testing infrared remote controls. All that is required is a mobile phone with a camera facility. Open up the camera app on the phone and point the remote at the camera lens. Pressing any button on the remote will cause it to emit a beam of IR pulses that can be seen on the phone screen, since phone cameras are sensitive to IR as well as normal visible light. The response from the remote will be a white flickering glow from the IR LED, easily visible on the camera. Keep up the great work with this magazine. Keith Gooley, VK5OQ, One Tree Hill, SA. File synchronisation problems with OneDrive I’ve noticed that the editors of Silicon Chip sometimes make unkind remarks about Windows 11. I’ve also noticed recently that this magazine publishes amazingly thorough and informative articles about modern technologies only indirectly related to silicon chips. The latest example has been drones. Well done, and thanks. 6 Silicon Chip Australia's electronics magazine siliconchip.com.au My grizzle about Windows 11 is that one of my computers has a weird fault that experts seem unable to fix, other than to charge a diagnostic fee to provide a report that reads, “Unable to replicate problem”. Friends advise me for free that “it seems like a hardware problem” and then bombard me with suggestions, none of which I understand, other than that my computer probably uses special screws that discourage DIY repair. The weird problem is that although my computer works most of the time, if I’ve just done a lot of work since I last opened an important file, my computer stops spontaneously, just before autosave tries to save what I’ve done. The second part of my grizzle is that when my computer is working, it seems impossible to persuade it to cease pestering me about some cloud thingy. In desperation, I bought a second computer, and it seems not to exhibit the weird problem, confirming the “It seems like hardware” diagnosis, but having two computers mostly just doubles my “where are my files hiding now?” problems. I’m experiencing a Catch-22 trying to get my files from the broken computer to the unbroken computer. Yes, I know Microsoft claims that their cloud thingy makes this easy. I’ve read way more Windows Help files than my sanity can bear, but they seem to be written in a modern version of Klingon that Google is unable to translate, other than into an older version of Klingon. Maybe this is an opportunity for Silicon Chip. Magazines dedicated to Windows want to tell me about advanced features and the latest technology that I just find mysterious. Maybe Silicon Chip might be the right place to publish an article explaining the basics of how to use a modern computer efficiently. After all, when I buy a new car, I don’t expect some mysterious cloud thingy to transfer all the dents, scratches, rattles, squeaks, and clonks from my old car to my new one! Keith Anderson, Kingston, Tas. Comment: we aren’t aiming for “unkind remarks”, but Microsoft’s push for Windows 10 users to upgrade to Windows 11 seems driven more by what benefits Microsoft than by what benefits those users. Like you, we don’t find OneDrive useful. Computers should do things in the way the user wants, not tell the user how they should do things. Have you tried FreeFileSync? We find it works really well for keeping files synchronised between multiple computers via shared storage. And, importantly, it tells you what it is going to do before it does it. If your problematic computer has an Intel 13th or 14th generation CPU (“Raptor Lake”), be aware that many of those CPUs are defective. They might work fine at first but can degrade, and start crashing frequently. It’s also good practice to save your work regularly, just in case. Get in the habit of pressing CTRL+S every few minutes! Mains Timer from Circuit Notebook works well Some time ago, I asked for details on a mains timer unit I built that I had lost the details of. If it may help others, I found that it was published as a Circuit Notebook entry by David Eather in the October 2011 issue, titled “Mains timer has no stand-by power” (siliconchip.au/Article/1187). SC James Newman, Mt Maunganui, New Zealand. ourPCB LOCAL SERVICE <at> OVERSEAS PRICES AUSTRALIA PCB Manufacturing Full Turnkey Assembly Wiring Harnesses Solder Paste Stencils small or large volume orders premium-grade wiring low cost PCB assembly laser-cut and electropolished Instant Online Buying of Prototype PCBs www.ourpcb.com.au 8 Silicon Chip Australia's electronics magazine 0417 264 974 siliconchip.com.au $ 89 (Q170) You’ll never need a 20mm Battery Again The new calipers are available with either IP-54 (dust and water resistant) or IP-67 (coolant proof) protection. The series is offered in 150mm, 200mm and 300mm sizes. Each model recharges in under three hours, so you will never look for a battery again! 129 (Q175) FOR MORE PRODUCTS VISIT MACHINERYHOUSE.COM.AU/ SIC2511 SYDNEY BRISBANE MELBOURNE (03) 9212 4422 (08) 9373 9999 (08) 9373 9969 1/2 Windsor Rd, Northmead 625 Boundary Rd, Coopers Plains 4 Abbotts Rd, Dandenong 11 Valentine St, Kewdale Unit 11/20 Cheltenham Pde, Woodville (02) 9890 9111 (07) 3715 2200 PERTH Specifications & prices are subject to change without notification. ADELAIDE 10_SIC_271125 $ PRODUCT SHOWCASE Electronex 2026 in Sydney: Rosehill Gardens Event Centre, 3rd & 4th June Following the outstanding success of last May’s Electronex Electronics Design and Assembly Expo held in Melbourne, Electronex 2026 Sydney has generated a strong response, with over 85% of exhibition space already booked. The 2026 Expo will take place at Rosehill Gardens Event Centre, Sydney, on the 3rd and 4th of June 2026, in conjunction with the annual Surface Mount and Circuit Board Association (SMCBA) conference. The Melbourne show, staged alongside National Manufacturing Week (NMW), attracted a record number of engineers, managers and senior decision-­makers from across the manufacturing and electronics industries. With 85 stands representing more than 100 companies, it was the largest Electronex held to date. More than 250 exhibiting staff also took part in valuable networking opportunities, including the popular Exhibitor Networking Function. Visitor numbers were around 15% higher than Electronex 2023 (the last time it was held in Melbourne), and exhibitors were delighted with the quality of enquiries. Face-to-face contact remains invaluable for discussing technical requirements and discovering new technologies in this vital high-tech manufacturing sector. 10 Silicon Chip Electronex is now in its 14th year and firmly established as Australia’s premier event for the electronics industry. Strong visitor engagement & industry support The Melbourne Expo drew around 2000 trade visitors from all Australian states and New Zealand, reflecting the show’s national reach. The concurrent Surface Mount and Circuit Board conference featured a stellar lineup of local and international speakers, offering deep insights into current and emerging technologies. A post-show visitor survey reinforced the event’s value. 75% of attendees were directly involved in purchasing, specifying, or recommending products. 93% discovered new companies, while 85% discovered new products or technologies. 97% said that a dedicated event such as Electronex is beneficial to their industry. Notably, 56% were first-time visitors, and many long-term exhibitors reported that it was one of the most successful events they had attended. Competitions and technical highlights The free on-floor seminars were also well attended, giving exhibitors a platform to present the latest innovations Australia's electronics magazine Australasian Exhibitions and Events Pty Ltd Tel: (03) 9676 2133 mail: info<at>auexhibitions.com.au Web: www.electronex.com.au and solutions. The SMCBA, Australia’s association for electronics design and assembly professionals, with support from IPC International, also conducted the Australian rounds of the IPC Hand Soldering Competition and the inaugural IPC/WHMA Wire Harness Competition. In a closely contested HSC, Tony Cimino from the Australian Centre for Robotics took first place, ahead of Rodney Tacey from Bluefish444. Tony received a JBC B-Iron Soldering Station and an invitation to compete in the World Finals in Munich, Germany. The Wire Harness Competition winner was Sai Praneeth Jasti from Design 2000, with Tony Cimino taking runnerup honours. Join us at Electronex 2026 With almost all the exhibition space already sold, companies interested in participating in Electronex 2026 Sydney who haven’t already booked are encouraged to secure their stands as soon as possible. siliconchip.com.au TECHY GIFTS WORTH DROPPING HINTS ABOUT? MARK YOUR CALENDAR: Thurs 4th of Dec to Wed 24th of Dec, 2025 2" LCD WHILE STOCKS LAST BUY 2 FOR $ 25 $ SAVE 15% NOW ONLY 3495 . Waterproof Smart Fitness Band Features step counter, smartphone push notifications and more. QC3113 RRP $14.95EA SAVE 30% 1080p Dash Camera 120° wide-angle lens, motion sensing and parking mode. QV3874 $ UP TO 500MM/S PRINT SPEED 359 SAVE $40 SEE NEXT PAGE FOR MORE 3D PRINTERS UP TO 225 X 225 X 265MM BUILD VOLUME NOW ONLY 2495 $ . SAVE 25% NOW ONLY 14 $ 95 EA . 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Simply order before 2PM *Conditions apply - see website for full details SCORE FREE DELIVERY ON US With online orders over $99 *Conditions apply - see website for full details ONLINESHOP SILICON CHIP .com.au/shop PCBs, CASE PIECES AND PANELS 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 SSB SHORTWAVE RECEIVER PCB SET ↳ FRONT PANEL (BLACK) 433MHz RECEIVER SMARTPROBE ↳ SWD PROGRAMMING ADAPTOR MAR25 MAR25 APR25 APR25 APR25 APR25 MAY25 MAY25 MAY25 MAY25 MAY25 JUN25 JUN25 JUN25 JUN25 JUL25 JUL25 Subscribers get a 10% discount on all orders for parts 04104251 $5.00 04107231 $5.00 07104251 $5.00 07104252/3 $10.00 09101251 $2.50 15103251 $2.50 11104251 $5.00 11104252 $7.50 10104251 $5.00 19101251 $15.00 18101251 $2.50 18110241 $20.00 CSE250202-3 $15.00 CSE250204 $7.50 15103252 $2.50 P9054-04 $5.00 P9045-A $2.50 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 AUG25 AUG25 AUG25 AUG25 AUG25 SEP25 SEP25 OCT25 OCT25 OCT25 OCT25 OCT25 OCT25 NOV25 DEC25 DEC25 DEC25 17101251 17101252 17101253 SC7528 SC7527 15104251 18106251 09110245 01107251 01107252 01107253 10109251 10109252 P9058-1-C 16112251 06110251 09111241 $10.00 $2.50 $2.50 $7.50 $7.50 $3.50 $2.00 $3.00 $30.00 $2.50 $7.50 $10.00 $2.50 $5.00 $12.50 $5.00 $2.50 PRE-PROGRAMMED MICROS As a service to readers, Silicon Chip Online Shop stocks microcontrollers and microprocessors used in new projects (from 2012 on) and some selected older projects – pre-programmed and ready to fly! Some micros from copyrighted and/or contributed projects may not be available. $10 MICROS $15 MICROS PIC12F617-I/P PIC16F1455-I/P Battery-Powered Model Railway Transmitter (Jan25) 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 8CH Learning IR Remote (Oct24), Heat Transfer Controller (Aug25) Vacuum Controller (Oct25) PIC16F15214-I/SN Silicon Chirp Cricket (Apr23), Mic The Mouse (Aug25) PIC16F15214-I/P Filament Dryer (Oct24), Tool Safety Timer (May25) PIC16F15224-I/SL NFC IR Keyfob Transmitter (Feb25), Rotating Light (Apr25) PIC16F18126-I/SL DCC Decoder (Dec25), RGB LED Star (Dec25) PIC16F18146-I/SO Versatile Battery Checker (May25), RGB LED ‘Analog’ Clock (May25) USB-C Power Monitor (Aug25) PIC16LF15323-I/SL Remote Mains Switch (TX, Jul22), Secure Remote Switch (TX, Dec23) STM32G030K6T6 Variable Speed Drive Mk2 (Nov24) PIC16F1847-I/P PIC16F88-I/P PIC24FJ256GA702-I/SS PIC32MX170F256B-50I/SP STM32L031F6P6 Digital Capacitance Meter (Jan25) Battery Charge Controller (Jun22), Railway Semaphore (Apr22) ESR Test Tweezers (Jun24) Micromite LCD BackPack V1-V3 (Feb16 / May17 / Aug19) SmartProbe (Jul25) $20 MICROS PIC32MK0128MCA048 Power LCR Meter (Mar25) PIC32MX270F256D-50I/PT 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 PIC32MZ2048EFH064-I/PT DSP Crossover/Equaliser (May19), Low-Distortion DDS (Feb20) DIY Reflow Oven Controller (Apr20), Dual Hybrid Supply (Feb22) KITS & SPECIALISED COMPONENTS RGB LED STAR KIT (SC7535) Includes the mostly-assembled board and all non-optional components except the power supply (see p43, Dec25) EARTH RADIO KIT (SC7582) Includes everything to build the radio itself except the case and battery, plus the plug for the antenna (see p65, Dec25) RP2350B COMPUTER (NOV 25) Assembled Board: a fully-assembled PCB with all non-optional components, front and rear panels are sold separately below (SC7531; see p28, Nov25) - front & rear panels (SC7532) (OCT 25) - PIC16F1455-I/P programmed with 0911024D.HEX (Transmitter) - PIC16F1455-I/P programmed with 0911024S(or T).HEX (Receiver, TH) - PIC16F1455-I/SL programmed with 0911024S(or T).HEX (Receiver, SMD) firmware ending with “S.HEX” is for train 1, while “T.HEX” is for train 2 PICKIT BASIC POWER BREAKOUT KIT (SC7512) Includes all parts except the jumper wire and glue (see p39, Sep25) USB-C POWER MONITOR KIT (SC7489) (AUG 25) $55.00 433MHz RECEIVER KIT (SC7447) (JUN 25) $25.00 VERSATILE BATTERY CHECKER KIT (SC7465) (MAY 25) RGB LED ‘ANALOG’ CLOCK KIT (SC7416) (MAY 25) USB POWER ADAPTOR COMPLETE KIT (SC7433) (MAY 25) (SEP 25) 433MHz TRANSMITTER KIT (SC7430) (APR 25) Includes all parts except a CR2032 cell (see p64, Aug25) Includes all non-optional parts except the case, cell & glue (see p39, Aug25) (DEC 25) (DEC 25) DUAL TRAIN CONTROLLER MICROCONTROLLERS (AUG 25) $80.00 DCC DECODER KIT (SC7524) Includes everything in the parts list (see p73, Dec25) MIC THE MOUSE KIT (SC7508) (DEC 25) $90.00 $7.50 $10.00 $10.00 $10.00 Includes the PCB and all onboard parts (see p66, Jun25) Includes everything in the parts list (including the case), except the optional components, batteries and glue (see p30, May25) $37.50 $60.00 $20.00 $65.00 Includes all the parts except the power supply. When buying the kit select either a BZ-121 GPS module or Pico W (unprogrammed) for the time source (see p66, May25) $65.00 Includes everything in the parts list and a choice of one USB socket: USB-C power only; USB-C power+data; Type-B mini; or Type-B micro (see p80, May25) $10.00 Includes the PCB and all onboard parts (see p75, Apr25) $20.00 $20.00 $12 flat rate for postage within Australia. Overseas? Place an order via our website for a quote. All items subect to availability. Prices valid for month of magazine issue only. All prices in Australian dollars & include GST where applicable. 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. 12/25 HUMANOID & AN Agility Digit www.agilityrobotics.com Boston Dynamics Atlas https://bostondynamics.com/atlas Unitree H1 www.unitree.com/h1 Tesla Optimus www.tesla.com/en_eu/AI Like many ideas that started as science fiction, humanoid and android robots are now a reality. They have not yet been perfected – but they are here. Last month we covered the tech; and now we showcase some of the most interesting robots. W e will now look at some historical humanoid robots, followed by those that are under active development or are available commercially. Leonardo da Vinci’s mechanical knight Around 1495, Leonardo conceived a “mechanical knight” that could perform actions such as moving its arms, neck & jaw, raising its visor, sitting and standing (Fig.18). It was operated with gears and pulleys but obviously had no electronic intelligence. The significance of this robot is that it is thought to be the first demonstration of the ability to mimic human-like actions by mechanical means. Leonardo’s sketches for this machine were rediscovered in the 1950s by Carlo 16 Silicon Chip Pedretti and, in 1993, Mark Rosheim collaborated with him to reconstruct the robot. As designed by Leonardo, it has three degrees of freedom in its legs and four in its arms. Pedretti described it as “the first articulated humanoid robot in the history of Western Civilisation”. There is a BBC audio presentation about the robot at www.bbc. co.uk/sounds/play/m0004mf2 Elecktro Elektro was an early humanoid robot built by Westinghouse in 1937 for the 1939/1940 World’s Fair in New York. It was extremely impressive and popular at the time, and it was featured in daily shows at the fair – see Fig.19. It was 2.1m tall, weighed 120kg, Australia's electronics magazine could perform 26 actions and had 700 words stored on eight 78 RPM records in its chest cavity. The 26 actions were hard-wired in a fixed sequence so that in a show, the same script from the handler resulting in the same sequence of actions every time. The robot gave the illusion of being voice-controlled, but electronics of the era were not advanced enough to perform voice recognition. The robot responded to the controller’s voice, but only to the rhythm. For example, speaking one word might correspond to one impulse to close a relay. Two words would correspond to two impulses which might perform another electro-mechanical action. It could also recognise red or green colours with a photocell, and state siliconchip.com.au NDROID ROBOTS Part 2: by Dr David Maddison, VK3DSM Figure 02 www.figure.ai 1X NEO Gamma www.1x.tech/neo Apptronik Apollo https://apptronik.com/apollo Fig.18: a reconstruction of Leonardo’s “mechanical knight” robot. Source: https://w. wiki/EotZ Fig.19: the Elektro robot. Source: www.computertimeline. com/timeline/ the-robots-ofwestinghouse the colour. See the video titled “Elektro the Smoking Robot (Odd History)” at https://youtu.be/sxGDdwbcfJg and “The World’s First Celebrity Robot” at https://youtu.be/dwBLOluOiUY The system of voice control had its origins in Westinghouse engineer Roy James Wensley, who was issued a patent in 1929 for a “supervisory control system” that enabled the control siliconchip.com.au of machinery over telephone or radio connection by “voice”. It wasn’t truly voice recognition, but a series of tones generated with pitch pipes or tuning forks at 600, 900 and 1400Hz. These activated relays via tuned circuits, enabling equipment such as at a power plant or telephone exchange to be remotely controlled. Australia's electronics magazine Booster Robotics T1 www.boosterobotics.com/robots/ The system was demonstrated with a robot prior to Elektro called Herbert Televox. Presumably, this system was modified for Elektro to use impulses instead of tones. This system of tone control became the basis of the DTMF (dual-tone multi-frequency) signalling used in traditional telephone exchanges. Current humanoid and android robots We do not have space to include a description of every available humanoid robot, so have selected the most interesting ones. 1X Technologies www.1x.tech The Neo Gamma is a humanoid robot under development by a Norwegian AI and robotics company, intended for domestic service (Fig.20). Neo Gamma uses artificial intelligence and is trained on household tasks using human motion-capture data. It December 2025  17 Fig.20: the Neo Gamma from 1X Technologies performs domestic duties. Source: www. therobotreport.com/1xbuilt-humanoid-neogamma-better-fit-home Fig.21: Australian Abi companion robots. Source: www.dromeda. com.au/product is covered in a knitted fabric to give it a friendly appearance. EVE is another humanoid robot from 1X. Abi www.dromeda.com.au Abi is an Australian humanoid robot, made in Melbourne – see Fig.21. It is intended as a companion robot with “playful features and infinite empathy”, for use in nursing homes and similar care facilities. It uses advanced AI and machine learning to recognise faces, understand and express emotions and remember conversations from days or months ago. It interacts with each resident based on their personal cues, including speaking their preferred language. Abi is fluent in 90 languages and can participate in small group activities such as singing, dancing, games and conversation. Actroids www.kokoro-dreams.co.jp The Actroid-DER series of android robots is designed with strong human likeness by Osaka University and manufactured by the Kokoro Company Ltd. Actroids were first displayed in 2003, so they are relatively old, and various models are available to rent. A model from the Actroid-DER series is shown here. It performs simple functions like blinking and speaking using AI. Later models have 47 actuators, which are pneumatically driven. For more information, see the video at https://youtu.be/l8qHXdKF300 AgiBot www.agibot.com This Chinese company has developed the AI-powered Genie Operator-1 (GO-1) model for quickly training humanoid robots in a variety of tasks – see Fig.23. According to Agibot, the model enables robots to “understand instructions in natural language and perform reasoning, rather than being limited to preprogrammed routines”. AgiBot also produces a variety of robots, such as the AgiBot A2 interactive service robot. The open-source AgiBot X1 robot is documented at www.agibot.com/DOCS/OS Apollo A1 https://apptronik.com Apptronik from Austin, Texas, has developed the Apollo A1 general-­ purpose humanoid robot, intended for jobs in warehouses, manufacturing plants and so on – see Fig.24. As the robot is further developed, its use will be extended to areas like construction, oil and gas extraction, electronics production, retail, home delivery, aged care and others. Apptronik has origins in the development of robots for NASA, such as Valkyrie (described below). Apollo is 172cm tall, has a run time of four hours with a swappable battery pack, weighs 72kg, can carry 25kg, is modular and the torso can even be mounted on a wheeled or stationary platform. It has a chest-mounted display that shows its status. Apollo can be tethered to a power supply to allow continuous operation. It has ‘force control’ systems to limit the amount of mechanical force it can impose to enhance safety around humans. It walks within a defined perimeter, so it does not get too close to people or objects. It immediately pauses when moving objects are detected in its vicinity. Mercedes Benz has an agreement to test Apollo in its manufacturing operations. See the video at https://youtu. be/TfUOg38iXxo ASIMO https://global.honda/en/robotics/ Apart from cars, Honda is famous for one of the earliest humanoid robots, ASIMO (Advanced Step in Innovative Mobility) – see Fig.25. It was retired in 2018. Honda researchers researched the following things with ASIMO: O Moving around while sharing the same space with people. O Performing tasks using its hands. O Interacting with people, including Fig.22: an Actroid-DER series android. Source: www.kokoro-dreams.co.jp/ english/rt_tokutyu/actroid/ Fig.23: AgiBot robots running GO-1 perform some kitchen tasks. Source: https://youtu.be/9dvygD4G93c 18 Silicon Chip Australia's electronics magazine siliconchip.com.au understanding spoken words and controlling movement/behaviour by estimating the intention of nearby people. Over its 20 years of demonstrations, it walked a total of 7907km. Honda continues to develop robotics, but with a focus on developing a variety of robots with specific functions rather than just one general-­ purpose humanoid robot. Fig.24: the Apptronik A1 generalpurpose robot. Source: https:// apptronik.com Digit www.agilityrobotics.com Agility Robotics from Albany, Oregon, USA has its origins in Oregon State University. In 2023, they released the current version of their bipedal robot, digit – see Fig.26. Digit is said to be the world’s first commercially deployed humanoid robot. They are currently in operation at Amazon and GXO Logistics. Amazon has also stated they intend to replace their entire human workforce by 2030. The robots cost around US$250,000 ($380,000) each. See https://youtu.be/NvYsGcQvMw8 and https://youtu.be/MYzRPJ7TaLc AheadForm www.aheadform.com Heads capable of expression are among the most complicated things to build for such robots, apart from the software. Just as motor vehicle manufacturers might contract out specialists to make certain components, the same concept can apply to robots. AheadForm specialises in making robot heads capable of a wide variety of human-like facial expressions, including subtle but important ones such as smirks – see Fig.27. These highly realistic heads are said to avoid the “uncanny valley” in which humanlike heads are seen as not quite realistic enough, and thus disturbing to the viewer. AiMOGA Robot Artificial Intelligence with Multi-Objective Genetic Algorithm was the robot’s original name, but it is now called Mornine. Its job is as an intelligent sales consultant for some Chery car dealerships, with about 220 to be delivered – see Fig.28. The robot is manufactured by Chery. It uses a ‘multimodal sensory model’ to perceive a customer’s gestures and questions by combining sensory data from speech, visual and other data. It then uses an “advanced emotion and personality engine” for personalised interactions. siliconchip.com.au Fig.25: Honda’s ASIMO (Advanced Step in Innovative Mobility) showcased at the Tokyo Motor Show in 2011. Source: https://w.wiki/EpwP Fig.26: a demonstration of Digit robots from Agility Robotics at work in a warehouse. Fig.27: the AheadForm robot head (left) and its creator (right). Source: https://youtu. be/gnyFtUU-TJ8 Fig.28: a Mornine robot at a Chery dealership. Source: https:// motorillustrated.com/ chery-debuts-humanoidrobot-mornine-to-dealerstheir-future-salesreps/153657/ Australia's electronics magazine December 2025  19 It takes advantage of DeepSeek’s AI and CheryGPT’s large language models to understand natural language, give appropriate responses and chat in any of ten languages. The robot can be used for other purposes apart from car sales, such as a bookshop assistant, other sales roles, companionship, shopping guide or caregiver. The robot is 166cm tall and weighs 55kg. Chery intends to launch the robot commercially in 2027. One estimate is that the robots will cost around $89,000 each. Fig.29: Alter3 after being instructed to “take a selfie”. Source: https://arxiv. org/pdf/2312.06571 Fig.30: the Ameca robot. Source: https://engineeredarts.com/gallery/ Fig.31: the Berkeley Humanoid Lite. Fig.32: the Booster T1 robot. Source: www. boosterobotics.com/ robots 20 Alter3 https://tnoinkwms.github.io/ALTER-LLM Alter3 is an experimental humanoid robot from the University of Tokyo that uses the GPT-4 large language model to interpret instructions, then produces separate code to generate the required motions – see Fig.29. There is a video showing the implementation of this code at https://youtu. be/l4d6N_Rf8mk Ameca https://engineeredarts.com A humanoid robot by UK company Engineered Arts (Fig.30), Ameca is designed for interaction with humans; its legs are still under development. Despite that, Ameca has advanced facial expression capabilities, extremely advanced emotional intelligence and conversational capabilities in multiple languages. It runs an in-house-developed operating framework called Tritium; for its language model, it uses OpenAI’s GPT 4.0 or can be controlled by teleoperation. It can watch, listen, learn, track faces in real time, analyse the emotional state of someone speaking to it and respond appropriately. Ameca is available for purchase or rental. See https://youtu.be/b9xFM61KKyc Fig.33: the latest version of Boston Dynamics’ Atlas robot. Source: https://bostondynamics.com/atlas/ Berkeley Humanoid Lite Berkeley Humanoid Lite (https:// lite.berkeley-humanoid.org) is an open-source, customisable 3D-printed humanoid robot – see Fig.31. It is 88cm tall and weighs 16kg. Its developers state that it can be made for around US$5000 ($7650) with readily available components. It can perform several useful tasks, including manipulating a Rubik’s Cube, and it can also be teleoperated. To make it, you need a 3D printer that can produce parts within a 20 × 20 × 20cm workspace. For more details, see https://youtu.be/dIdJGkMDFl4 Australia's electronics magazine siliconchip.com.au Fig.34: Atlas picks and places parts in an experimental situation. Source: https://bostondynamics.com/video/ pick-carry-place-repeat Fig.35: the torso of the Clone android showing anatomical similarity to a human. Source: https://x.com/ clonerobotics Fig.36: the Pudu D9 robot. Source: www.pudurobotics.com/en/products/ d9 Booster T1 www.boosterobotics.com Booster Robotics is a Chinese company that has developed a humanoid robot platform intended for developers to write their own software for – see Fig.32. There is an online manual describing the robot with details for developers at siliconchip.au/link/ ac7h picking and placing a part is shown in Fig.34. D9 www.pudurobotics.com Pudu Robotics is developing the general-purpose D9 robot – see Fig.36 and the video at https://youtu.be/ gd5DdfJX_RM Boston Dynamics Atlas Boston Dynamics (website: https:// bostondynamics.com) is a subsidiary of Hyundai. It was one of the first companies with a functional humanoid robot called Atlas, and there are many impressive videos of its products on YouTube. The latest version of Atlas is shown in Fig.33. Unlike previous versions, which were hydraulically actuated, this one is fully electric. That makes it quieter, less complicated, more compact, with more natural movements and some say less intimidating. Computationally, Atlas uses NVIDIA’s Isaac GR00T framework and the NVIDIA Jetson Thor computing platform, which is specifically designed for humanoid robots. It uses the Blackwell GPU architecture with 2560 CUDA cores, 96 tensor cores and Arm Neoverse V3AE CPUs with 14 cores and 128GB of LPDDR5X memory, giving up to 2070 FP4 teraflops of AI computing power (FP4 is floating point 4-bit operations). Hyundai are testing Boston Dynamics Atlas robots for building electric cars in their Georgia, US manufacturing facilities and plan to roll them out globally. A demonstration of Atlas Clone https://clonerobotics.com Polish company Clone Robotics developed Clone, which they describe as an android. Clone adopts a different approach from other humanoid robots. It seeks to emulate the actual structure of the human body, with anatomically accurate bones, joints, muscles and a nervous system – see Fig.35. It is hydraulically actuated with a 500W pump. It has an unusually large 200 degrees of freedom, 206 bones, 1000 artificial muscle materials (known as myofibres) and 200 sensors. Myofibres are composed of mesh tubes that contain balloons of hydraulic fluid. These are inflated or deflated by hydraulic pressure. The aforementioned pump acts as the ‘heart’ of the system. The idea is based on the McKibben artificial muscle concept first invented in the 1950s. For cooling, it even ‘sweats’ fluid, just like a human. In essence, Clone seeks to mimic the human body. Clone runs on an NVIDIA Jetson Thor inference GPU in the skull running Cybernet, Clone’s visuomotor foundation model. See Clone Robotics’s YouTube channel (www.youtube. com/<at>CloneRobotics). siliconchip.com.au Certis www.certisgroup.com Certis is a Singapore company that is using an Agibot humanoid robot to study potential applications for humanoid robots in security and integrated facilities management. Australia's electronics magazine EveR-4 android The EveR-4 is an android that was developed at the Korea Institute of Industrial Technology and was exhibited at RoboWorld 2011 in Seoul. It is one of a series of four such androids. It can exhibit a wide range of facial expressions and has over thirty motors actuating its face. The EverR-4 has been investigated for use in jobs such as a medical receptionist, where the robot was positively received by patients. See the videos at https://youtu.be/OvsZqPcnNIE and https://youtu.be/b2GtBAPG1ho (EveR-4 as a receptionist). FALCON FALCON is an experimental robot control framework from Carnegie Mellon University, designed to train humanoid robots for a complex tasks called “force-adaptive loco-manipulation”. This is walking or standing and using their arms at the same time, while applying strong, precise force. Examples include pushing a wheelbarrow, opening a door or engaging in a tug -of-war with another robot. Two AI agents are used to accomplish such tasks; one for the legs and the other December 2025  21 for the arms – they communicate with each other. The systems is tested on humanoid platforms from Unitree and Booster Robotics. See https://youtu.be/ OfsvJ5-Fyzg Figure AI www.figure.ai An American robotics company, its investors include OpenAI, NVIDIA and Jeff Bezos. It is aiming to build robots that learn and reason like humans. Their most advanced robot is Figure 02 (shown in Fig.37), which runs the Helix generalist vision language action (VLA) model. The aim of Figure AI is to enable robots to work in an unstructured home environment with thousands of objects it has never encountered before, and to reason how to deal with them without prior programming or demonstrations. Helix can also run on two robots simultaneously, enabling them to cooperatively solve problems and share learning. This is shown by putting groceries away in the video at https://youtu.be/Z3yQHYNXPws Figure 02 with Helix is also intended for industrial use, where it will be introduced first, before being brought to the domestic market. An industrial environment is much more structured than a home environment, and should be easier to operate in. According to Figure AI: VLM (vision language model) backbones are general, but not fast, while robot visuomotor policies are fast but not general. Helix resolves this trade-off through two complementary systems, trained end-to-end to communicate: O System 2 (S2) is an onboard internet-­pretrained VLM operating at 7-9Hz for scene understanding and language comprehension, enabling broad generalisation across objects and contexts. O System 1 (S1) is a fast reactive visuomotor policy that translates the latent semantic representations produced by S2 into precise continuous robot actions at 200Hz. Fig.37: Figure AI’s Figure 02 robot uses a VLA model for control. Source: www.figure.ai 22 Silicon Chip This decoupled architecture allows each system to operate at its optimal timescale. S2 can “think slow” about high-level goals, while S1 can “think fast” to execute and adjust actions in real-time. For example, during collaborative behaviour, S1 quickly adapts to the changing motions of a partner robot while maintaining S2’s semantic objectives. Gary www.hospital-robots.com Gary is a wheeled hospital robot from Israeli company Unlimited Robotics – see Fig.40. It can perform tasks such as providing bedside assistance to staff, medication reminders, provide companionship to patients, deliver supplies, sanitise, clean, automate mundane tasks etc. It runs on an Intel Core i7 processor with 512GB of memory and has a Google tensor processing unit (TPU) for its AI for neural network machine learning. For software, it uses the Linux operating system with the in-house developed Ra-Ya platform, which is designed to make it easier for inexperienced robot programmers to build applications if they are familiar with Python or JavaScript. Geminoid HI-6 In 2006, Professor Hiroshi Ishiguro of the Intelligent Robotics Laboratory Fig.38 (below): the HUBO2 robot. Source: www.rainbowrobotics.com/en_hubo2 at The University of Osaka made an android replica of himself, the Geminoid HI-1. It has now upgraded to the Geminoid HI-6 – see Fig.42. It is an upper body only and is teleoperated. It is pneumatically operated with 16 actuators and has 16 degrees of freedom. The purpose of the android is to explore questions of “what exactly human presence is, whether human presence can be transmitted to remote locations, and whether androids can surpass humans through experimentation”. GR-1 & GR-2 www.fftai.com Fourier is a Chinese company that has developed the GR-1 and GR-2 humanoid robots. The GR-2 is a general purpose robot with sensors in its dextrous hands to make them touch-sensitive. It can be programmed with frameworks like ROS and NVIDIA Isaac Lab. See the video at https://youtu.be/N7qYcOuR7P8 HMND 01 https://thehumanoid.ai Humanoid has developed the HMND 01 robot, which they describe as a “labour automation unit” (see Fig.39). It is 175cm tall, weighs 70kg, has 41 degrees of freedom, a payload capacity of 15kg, a runtime of four hours and walking speed of 5.4km/h. Uses for the robot include goods handling, such as in warehouses, picking and packing for e-­commerce warehouses and parts handling in manufacturing operations. HRP-5P An experimental humanoid robot developed in Japan in 2018 by the National Institute of Advanced Industrial Science and Technology, it was demonstrated installing wall plasterboard sheeting similar to Gyprock. See https:// youtu.be/fMwiZXxo9Qg HUBO www.rainbow-robotics.com Rainbow Robotics is a Korean company that offers the HUBO2, shown in Fig.38, Fig.39: the HMND 01 robot. Source: TechNode – siliconchip.au/link/ac7p siliconchip.com.au which they claim is the world’s first humanoid robot to be commercialised. The first HUBO was released in 2004, with the HUBO2 going on sale in 2010. It has made commercial appearances, such as at the 2012 Philadelphia Phillies baseball game. iCub https://icub.iit.it iCub is an open-source research and recreational robot designed by a consortium of European universities for research into human cognition and artificial intelligence – see Fig.41. It is 104cm tall and weighs 22kg. It has been demonstrated with capabilities such as crawling, solving 3D mazes, performing archery, producing facial expressions, exercising force control, grasping small objects, and performing collision avoidance. It is said to use a neuromorphic processor. Fig.40: Gary, the hospital robot, enters a room. Source: www.hospital-robots. com/about Fig.41: iCub at the Center for Robotics and Intelligent Systems (CRIS). Source: https://w.wiki/Eotd Iggy Rob www.igus.com A partially humanoid robot; instead of having legs, it has a wheeled base (see Fig.43). Its suggested applications include performing wait staff duties, such as delivering food and drinks to restaurant customers, parts delivery on factory floors, and for education and research into robotics. It costs about US$54,500 ($83,500), is 1.7m tall and can carry 100kg. InMoov https://inmoov.fr The InMoov humanoid robot project is for hobbyists and universities, with a whole community of developers, including in Australia and New Zealand – see Fig.44. It is open source and can be 3D printed on any standard printer with a 12 × 12 × 12cm area. It utilises two Arduino Mega or Arduino Uno microcontroller boards, two Nervo Board shields and 28 servo motors. It has two cameras for object and face tracking, speakers for speech, one Kinect sensor (discontinued) or OAK-D-Lite-FF for 3D depth and gesture recognition, and a PIR sensor for presence detection. All of its fingers are motorised. Iron Iron from car manufacturer Xpeng (siliconchip.au/link/ac7i) uses their proprietary 40-core Turing AI chip with 3000 TOPS (trillions of operations per second) of processing power and their Tianji AIOS AI operating system, which is also used in their cars. It features 60 joints, 200 degrees siliconchip.com.au Fig.42: Professor Hiroshi Ishiguro with the android robot replica he made of himself, Geminoid HI-6. Which one is which? Source: https://drive.google.com/ drive/folders/1RN710FOs7r9KJ2TmXkh-W_0cqcHUtlNj Fig.43: the Iggy Rob robot. Source: www.igus.com/automation/news/ humanoid-robot Australia's electronics magazine Fig.44: the torso of InMoov. Its lower legs have not yet been developed. Source: https://inmoov.fr/gallery-v2/ December 2025  23 Figs.45, 46 & 47: the Kuavo 4.0 robot (source: www.lejurobot.com/en); the NASA Valkyrie or R5 robot, which is being tested in Australia (source: https://x.com/CWeezyeth/status/1643599326650986496/photo/1); and a range of humanoid industrial robots from Persona AI; a miner, builder, welder, fabricator and assembler (source: https://personainc.ai). of freedom, is 173cm tall and weighs 70kg. It is already being used to produce Xpeng cars – see Fig.48. Kuavo 4.0 www.lejurobot.com/en Kuavo is a product of Leju Robotics, designed as a general-purpose humanoid robot for applications such as personal assistance and industrial automation (Fig.45). It utilises Huawei’s Harmony­OS and PanGu multimodal large language model for AI. It is not clear if Kuavo uses it, but other products of this company are the world’s first to use 5G-A positioning, which uses 5G technology for high-precision location within indoor spaces. NASA The 2011 NASA Robonaut2 or R2 was the first humanoid in space, tested aboard the International Space Station (ISS). It was initially only a torso, but a mobility platform was added in 2014. It had significant technical problems, was not used and returned to Earth in 2018. The NASA Valkyrie (see Fig.46), also known as R5, is still under active development. It is also being tested in Australia by Woodside Energy “to develop remote mobile dexterous manipulation capabilities for uncrewed and offshore energy facilities”. It is 1.8m tall, weighs 136kg and runs on three Intel Core i7 CPUs. It is not currently deployed in space. Nurabot www.foxconn.com/en-us Foxconn, in cooperation with NVIDIA, has developed a robotic nurse with Tawian’s Taichung Veterans General Hospital called Nurabot (Fig.49). It has perception, navigation, understands language and has an ability to adapt. It is intended to address labour shortages, monitor patients’ vital signs, address caregiver burnout, move patients, deliver meals and medication, offer companionship, turn patients over in bed and learn patients’ habits. It does this with high-­ resolution sensors, autonomous navigation capabilities and NLP (natural language processing). In trials, the nurses love it because it takes over repetitive tasks and gives them more time for non-routine tasks. Patients like it as well, as the robot is always there for them. Optimus www.tesla.com/en_eu/AI Tesla has developed the Optimus Gen 2 general purpose humanoid robot Fig.48: Xpeng’s Iron robot making cars. Source: https://baa. Fig.49: the Nurabot nursing robot. Source: www.honhai. yiche.com/xiaopengP7jia/thread-51117011.html com/en-us/press-center/press-releases/latest-news/1605 24 Silicon Chip Australia's electronics magazine siliconchip.com.au Fig.50: the Optimus robot through various design iterations. Left to right, they are Bumblebee (September 2022), Optimus Gen 1 (March 2023), Gen 2 (December 2023) and Gen 3 (current). Source: https://x.com/ niccruzpatane/status/ 1937798034894762071/ photo/1 shown in Fig.50. The status of Gen 3 shown in that figure is unclear. Optimus uses Tesla’s in-house AI and uses parts of Tesla cars, such as the same computers, with custom-­ designed chips to provide the robot’s neural networks for tasks like locomotion, manipulation, navigation, vision processing and decision-making. Tthe same AI architecture as Tesla’s Autopilot and FSD systems. It uses the same cameras as Tesla cars for vision processing, actuators based on design principles developed for the cars, and the same 4680 batteries used in the Cybertruck. The neural networks used by Optimus are the same as used by Tesla cars to process visual inputs and adapt to environments. It also uses the same deep learning and auto-labelling techniques used by vehicles. Using its neural network, it can learn from videos of humans performing various tasks such as vacuuming, stirring food, disposing of rubbish or moving items on a factory floor. Elon Musk has previously shared a vision of collective learning and data sharing in Tesla’s AI and robotics initiatives, so the possibility exists that when one Optimus robot learns a new skill, they could all learn it, depending on the programming. Some critics have pointed out that early demonstration videos of Optimus involved teleoperation by human siliconchip.com.au operators. Tesla is currently using two Optimus robots in its factories for tasks like moving batteries, but they currently operate at half the efficiency of human workers. They intend to produce 1000 robots by the end of 2025, to be used by Tesla, with sales to other companies starting in 2026. The estimated cost per unit is US$20,000 to US$30,000 ($30,000 to $45,000). There has been discussion of controlling an Optimus robot via patients with the Neuralink brain-computer interface implant. This means a disabled person could ‘inhabit’ the body of an Optimus robot and control it to perform tasks by thought alone. It has also been proposed that such a person could control just parts of the robot, such as the arms, legs or hands, which could be attached to the body as artificial limbs. A patient named Alex has already demonstrated the ability to control an Optimus hand through Neuralink, via thought alone; see the video at siliconchip.au/link/ac7n Optimus has been integrated with X’s Grok AI, so it will be possible to have conversations with the robot and to get the Optimus to do anything as commanded by the operator. It may also be able to provide companionship. PAL Robotics https://pal-robotics.com This Spanish firm makes a range of Australia's electronics magazine robots, including bipedal humanoid types such as the REEM-C, TALOS and Kangaroo research platforms for robotics and AI research. They also make the ARI wheeled social robot for tasks like answering customer enquiries or running promotional campaigns. It uses AI, running on Ubuntu and the ROS framework, with facial recognition and other visualisation tools. Persona AI https://personainc.ai Persona AI in Houston are developing a range of heavy-duty industrial humanoid robots, based on one modular platform but customised for a variety of tasks including shipbuilding and welding – see Fig.47. One of the strengths of their robots is their advanced hands, derived from work by NASA. The firm was co-founded by ex-NASA staff. One usage model for these robots includes renting them out for specific jobs. For deployment for precision shipyard welding, they have a development partnership with HD Hyundai Robotics, Vazil and HD Korea Shipbuilding & Offshore Engineering, and are expected to be deployed in 2027. December 2025  25 Phoenix www.sanctuary.ai A robot from Canadian company Sanctuary AI (Fig.51), according to the company, Phoenix, running under their Carbon AI control system, “mimics subsystems found in the human brain, such as memory, sight, sound, and touch”. See the video at https:// youtu.be/-HizP4UQvug for more information. Fig.51: the upper body of the Phoenix robot. Fig.54: the OP3 robot. Source: https://en.robotis.com/sub/ business_platform_ op3.php Fig.52: the EngineAI PM01 in service as an experimental police robot. Source: South China Morning Post – siliconchip.au/link/ac7q Fig.55: a robot hand from Shadow Robot. Source: https://shadowrobot. com/dexterous-hand-series Fig.56: SoftBank Robotics’ NAO. PM01 www.engineai.com.cn EngineAI makes the PM01 general-­ purpose humanoid robot. Chinese police are using one in an experimental and demonstration role (see Fig.52). In police work, it can perform facial recognition and crowd scanning, although at this stage of development, it is not likely to be intelligent enough to be genuinely useful for police work. Still, it hints at a possible Robo­Coplike future. See the video at https:// youtu.be/vu930qj9CEI Poppy Project www.poppy-project.org An open-source platform for the creation, use and sharing of interactive 3D-printed robots for education, artists, scientists and hackers (see Fig.53). Porton Man Robotic Test System The Porton Man Robotic Test System (siliconchip.au/link/ac7j) is a humanoid robot for the US Army. Its purpose is to wear and test nuclear, biological and chemical (NBC) protective suits. It performs tasks that soldiers would perform, such as walking, running, kneeling, or laying prone while wearing the equipment and has 100 embedded sensors to test for leaks and to measure other parameters. Its main advantage is the ability to repeat movements precisely, enabling effective comparisons between different NBC ensembles. RoboPrime humanoid project RoboPrime is a very low-cost 3D-printed humanoid robot project for the enthusiast. The website at https:// github.com/simonepri/roboprime is no longer actively maintained; however, builders might still find some ideas there. Fig.53: the Poppy Humanoid v1.0.2 is 83cm tall, weighs 3.5kg, has 25 actuators and runs Odroid XU4 with Ubuntu 14.04. Source: www.poppyproject.org/en/robots/poppy-humanoid Fig.57: SoftBank Robotics’ Pepper. 26 Australia's electronics magazine Silicon Chip ROBOTIS https://en.robotis.com This Korean robot company makes open-source humanoid platforms, such as the OP3 (Fig.54), for research siliconchip.com.au Humanoid robots losing control There were recent incidents of humanoid robots losing control, which can happen, just like any other machine. There is obvious potential for this to harm people. That is why it is imperative that these robots are built with failsafe systems and some means for their handlers to deactivate them. You can see one such incident in the video at https://youtu.be/1eYZr9vdGl8 Legal concerns for AI There are no specific laws governing AI in Australia. In the event that AI and humanoid robots become sufficiently advanced to develop consciousness, it has been argued that they should be afforded rights as humans have. However, they are still man-made machines that mimic humans and still not human, just very advanced appliances. In regards to foundation models, polls of organisations have suggested that there is agreement that those who develop the models should be responsible for the risks, not those who use the models. and education purposes. You can find the relevant files at siliconchip.au/ link/ac7k Shadow Robot Company The Shadow Robot company (https://shadowrobot.com) specialises in making dextrous robot hands for other robotics manufacturers. One of their robot hands is shown in Fig.55. SoftBank www.softbankrobotics.com SoftBank Robotics offer two humanoid robots, NAO and Pepper. NAO (Fig.56) is described a teaching assistant. It is described as having a personality and an ability to inspire students from all ages, from preschool to university, including the ability to work with special-needs children. It is described as being able to connect “theory to practice with hands-on Fig.58: Sophia by Hanson Robotics. Source: www.hansonrobotics.com/ sophia siliconchip.com.au projects that encourage participation, teamwork, and creative problem-­ solving”. NAO can speak 20 languages, move naturally and runs on the Linuxbased NAOqi OS, a flexible framework that gives a lot of options for customising the robot. It is quite small at around 57cm tall, with a weight of 4.8kg. Pepper (Fig.57) is a robot designed to greet customers in a business. It can make personalised recommendations, help people find what they’re looking for, sell to and interact with humans. Pepper is 1.2m tall, weighs 28kg, runs for 12 hours on one charge, has a variety of sensors, runs on the NAOqi OS and can be customised with various SDKs (software development kits). carry on human-like conversations. The manufacturer describes Sophia as “a human-crafted science fiction character depicting the future of AI and robotics, and a platform for advanced robotics and AI research”. The robot has made appearances on many popular TV shows. Sophia utilises symbolic AI, neural networks, expert systems, machine perception, conversational natural language processing, adaptive motor control and cognitive architecture systems, among others. The manufacturer says that the robot has demonstrated rudimentary levels of consciousness under certain conditions using the Tononi Phi system of measuring consciousness (siliconchip. au/link/ac7l). StUWArt A 140cm-tall experimental humanoid robot from the University of Western Australia (UWA; siliconchip.au/ link/ac7m) – see Fig.59. It uses a commercial robot platform, with the focus of UWA research being on the development by students of autonomous control software enabling it to move and walk. The basic platform appears to be a Unitree G1, described below. Sophia www.hansonrobotics.com A humanoid robot by Hong-Kongbased Hanson Robotics (Fig.58). It can Titan www.roboforce.ai RoboForce has developed the Titan industrial robot, which can lift 40kg and place it with 1mm accuracy, running for eight hours on one charge – see Fig.60. The robots are modular and can be equipped with different hands Fig.59: the StUWArt humanoid robot with software under development by UWA students. Fig.60: the Titan-T industrial robot by RoboForce. Source: www.roboforce. ai/product Australia's electronics magazine December 2025  27 and bases. It is optimised to perform the five so-called primitive actions of all human labour: pick, place, press, twist and connect. Fig.61: the Unitree G1 robot. Source: www.unitree.com/g1 Fig.62: the soccer-playing version of the Unitree G1, the G1-Comp. Source: www.unitree.com/robocup Toyota Research Institute Toyota sees its future with large numbers of its workers being humanoid robots. The Toyota Research Institute has partnered with Boston Dynamics to integrate its ‘large behaviour models’ with Boston’s Atlas robot. Toyota has trained their in-house AI large behaviour model to perform 500 tasks, to be integrated with Atlas. Toyota trains their large behaviour models using humans to demonstrate the required tasks, with joysticks to control robot movement, or robots are trained using videos. The AI model then synthesises these experienced operations into the relevant actions. Toyota envisions using the robots for tasks such as transporting parts, assembling components and conducting inspections. They may eventually replace between 5% and 15% of human workers. There is a video about Toyota’s large language models at https://youtu. be/DeLpnTgzJT4 Unitree G1 robot www.unitree.com Standing at 130cm tall, and weighing 35kg, the G1 has a swappable battery pack with a life of two hours, three-finger hands with optional tactile sensor arrays, 3D lidar and depth cameras, a microphone, speakers, 43 degrees of freedom, eight CPUs and other features – see Fig.61. Another version of this robot, the G1-Comp, is designed for playing in robot soccer competitions (Fig.62). The G1-Comp uses the YOLOv11 algorithm for real-time object detection and recognition, pose estimation and image classification using convolutional neural networks. The H1 is similar to the G1 but is taller (180cm). See the videos at https://youtu.be/ GzX1qOIO1bE (G1) and https://youtu. be/M0KrTumJBFc (G1-Comp). Fig.63: the Walker S Lite (left) and Walker S. Source: www.ubtrobot.com/en/ humanoid/products/WalkerS Walker S www.ubtrobot.com The Chinese UBTECH Walker S Industrial Humanoid Robot assists on production lines with inspections and installing small parts. It comes in two models: the Walker S (170cm tall, 65kg, 41 degrees of freedom, 2.5 hour battery) and the Walker S Lite (130cm Australia's electronics magazine siliconchip.com.au 28 Silicon Chip What is AIoT? AIoT or artificial intelligence of things is like the internet of things (IoT), where devices can communicate with each other. However, each device also possesses artificial intelligence. The combination of IoT and AI in AIoT makes for an extremely powerful network, where AI-powered devices can interact and communicate with each other locally, area-wide, country-wide or even worldwide. It could conceivably be dangerous in the future, if not managed appropriately, with limitations to stop AI getting out of control. Just imagine an army of robots being programmed with malicious intent… Glossary of Terms AI – Artificial Intelligence; machines simulating human intelligence, such as learning, reasoning and problem-solving ANN – Artificial Neural Network; computational models inspired by human brains, used in machine learning ASIC – Application-Specific Integrated Circuit; a custom-designed chip optimised for a specific function or task CNN – Convolutional Neural Network; deep tall, 63kg, 41 degrees of freedom, two hour battery) – see Fig.63. Features of the Walker S include all-terrain autonomous adaptation, robust self-balancing, multi-modal large model-based decision making, hand-eye coordination and whole body manipulation, U-SLAM (UBTECH simultaneous location and mapping), 3D point cloud semantic navigation, human and environment comprehensive perception and multimodal human-robot interaction. It runs on Robot Operating System, Linux ROSA 2.0, supports teleoperation and AIoT (artificial intelligence of things). For more information, see the videos at https://youtu.be/UCt7qPpTt-g and siliconchip.au/link/ac7o carpentry, 3D concrete printing and other tasks – see Fig.64. It is intended to address a shortage of skilled construction workers and to reduce human injuries. Zyrex https://ricrobotics.com This 6m-tall AI robot for construction sites from the Californian company RIC Robotics is yet to be released, but is designed to perform welding, Further viewing The smallest humanoid robot The world’s smallest humanoid robot is 57.7mm tall and was built by Tatsuhiko Mitsuya at the Nagoya Institute of Technology in Japan – see Fig.65. Upgrading humanoid robots AI is progressing rapidly. Since AI is software-based, it is usually possible to upgrade a humanoid robot to make it smarter as new AI software is released, thus protecting the investment in robot hardware. A fascinating look at “The Proto-­ Robots of Antiquity” is available in the YouTube video at https://youtu. SC be/0QGkf13fVs4 learning models optimised for vision, detecting edges, shapes and patterns CPU – Central Processing Unit; a general- purpose processor that executes instructions & manages computing tasks DoF – Degrees of Freedom; independent movements a robot joint or mechanism can perform End Effector – a tool/device at a robotic arm’s end that interacts with objects FPGA – Field-Programmable Gate Array; a chip programmable for specific hardware tasks post-manufacturing GPU – Graphics Processing Unit; a processor specialised for highly parallel tasks like machine learning LLM – Large Language Model; an AI model trained on massive text datasets to generate or understand language Multimodal – An AI that processes and integrates multiple data types (text, images, audio, video etc) Neuromorphic Processor – a chip that uses artificial neurons to mimic the human brain NLP – Natural Language Processing; an AI’s ability to understand, interpret and generate human language Organoid – a simplified version of an organ designed to imitate it RTOS – Real-Time Operating System; an operating system that guarantees timely processing for critical tasks Fig.64: the giant Zyrex construction robot, compared to a human-sized robot at lower left. The robot is not fully humanoid like the others in this article – but it has some similarities. Source: Robotics & Automation News – siliconchip.au/link/ac7r Fig.65: the world’s smallest humanoid robot. Tactel – Tactile Element; a sensor element that detects touch, pressure or texture information Teleoperation – operating a machine remotely TPU – Tensor Processing Unit; a Google- designed chip optimised for accelerating machine learning workloads. Transformer – a neural network architecture that uses attention to process sequential data efficiently VLA – Vision-Language Action; an AI that combines visual input and language to perform actions or tasks VLM – Vision-Language Model; an AI that combines image understanding with text comprehension and generation siliconchip.com.au Australia's electronics magazine December 2025  29 By Andrew Levido Power Electronics Part 2: Controlling DC-DC Converters Last month, we introduced average value analysis and used it to analyse a range of DC-DC converter topologies at a high level. We then looked at a practical example of a buck converter. This time, we will delve into the control systems required to make these converters work correctly. D C-DC converters almost always operate in a closed-loop mode, where some kind of feedback is used to maintain the output voltage at the desired level, despite changes in load or source voltage. Often, the detail of the control loop is hidden from the user because it is incorporated in the driver chip, such as was the case for our buck converter example last month. Even when this is the case, it is handy to have at least a basic understanding of how they work. We will continue using the example of the buck converter we introduced last time. It was designed to convert a nominal 12V input (ranging from 10V to 14V) to a 6V output, to drive a 10W load for a maximum output current of 600mA. The switching frequency was 500kHz. We went through the process of selecting the filter components in some detail. If you want to know why we need to close the loop around our converter, take a look at Fig.1. This is the result of a simulation of the converter with no feedback and a fixed 50% duty cycle. The source voltage (blue trace) is programmed to be 12V for the first 30ms, dropping to 10V thereafter. In contrast, the load is programmed to be 10W for the first 10ms, increasing to 20W thereafter, with the resulting load current shown by the orange trace. The purple trace shows the output voltage. There is a significant transient when the load changes, but no change in output voltage, since the duty cycle is fixed. The transient peaks at about ±300mV, which is not great, and it takes about 5ms to settle. Things get even worse when the input voltage reduces at the 30ms mark. Not only does the output voltage drop because there is no regulation, but the ringing at the transition is horrendous. Again, it takes about 5ms for the result to settle into something close to the steady state value. A properly designed closed-loop control system should greatly reduce or eliminate these problems. Control theory is a huge topic, and one that can be very heavy on mathematics. I will therefore not be giving a control theory tutorial in the classical sense, but rather a practical exploration of some of the techniques that can be employed. If you are a control theory expert, you are hereby warned that I am going to skip some details in the interest of space and simplicity! Creating a mathematical model Fig.1: without a closed loop control system, the output of the DC-DC converter (purple trace) is unstable with changes in load (at 10ms) and input voltage (at 30ms). To create a stable and effective control system, we have to build a mathematical model to describe our DC-DC converter under dynamic conditions. We have already discussed two models for our converter: the very high-level periodic steady state (PSS) model we used for average value analysis, and the very low-level cycle-by-cycle simulation model we used to verify the ripple voltages and currents. We can’t use the periodic steadystate model for the control loop since it is dynamic behaviour we want to take care of. Also, we don’t need to model the cycle-by-cycle behaviour of the converter since the behaviour we are concerned with occurs over much longer time periods than a single 2µs cycle. Instead, we need an intermediate model. Fig.2 shows the block diagram of our converter with feedback presented in the classic control theory manner. Right in the middle is the modulator, which takes a control voltage proportional to the duty cycle (vc) and produces the switching waveform, whose average value, ‹vx›, is applied to the filter. The filter takes this voltage and produces the smoothed output voltage, vo. Each block has a gain, G, which relates its output signal to its input. Australia's electronics magazine siliconchip.com.au 30 Silicon Chip In front of these two blocks is Fig.2: this classic a compensator, which we will depiction of the describe in more detail below. DC-DC converter The input to the compensator is closed-loop an error, e, that is the difference system is useful between the reference setpoint vr to understand and the feedback signal vfb. The the circuit feedback signal is derived from the blocks we need output voltage we want to control to model. by some sensor, with a gain of H. Fig.3: the DCThese blocks are shown in a DC converter more familiar way in Fig.3. The arranged modulator includes the Mosfet and according to the diode switches, plus the circuitry block diagram that converts a control voltage to in Fig.2 is the pulse-width-modulation to drive starting point for the Mosfet. At its simplest, this is the development just a comparator with an output of a control that is high whenever the control model of the voltage exceeds a ramp waveform converter. The compensator is at the switching frequency. the missing piece The modulator therefore takes a of the puzzle at control voltage, vc, and produces this stage. the average voltage, ‹vx›. The modulator has a gain of Gm and it is easy to see that this must be the unitless negative input for the feedback. This quantity Gm = Vsrc/Vramp. If we assume anticipates that the compensator will the ramp voltage is 1V, the modulator probably involve op amps. gain simplifies to the source voltage, Simplifying the complexwhich is 12 in our example. It is worth noting that there is no frequency domain need for any switching to occur in this Describing the filter and compenmodel of the modulator. The modu- sator gains requires us to dip our toes lator produces an output equal to the into something called the complex-­ average voltage over one switching frequency domain, also known as the period. This voltage may vary with s-domain. Analysis of the behaviour time, but the time scale we are con- of capacitors and inductors involves cerned about is much longer than the differential equations, which we usu2µs switching period. ally describe in the time domain – for Because this modulator controls the example, the relationship between average voltage applied to the filter, voltage and current in an inductor is this converter is said to be employ- v = L di/dt. ing voltage mode control. There is We can describe the same equations another way to do it, which we will in the s-domain, which makes the come to later. mathematics much simpler, because The sensor gain is trivial to calcu- differentiation is reduced to multilate. It is simply the divider ratio rep- plication and integration is reduced resented by the two resistors, so Hs = to division. The conversion from the R1 ÷ (R1 + R2), or 0.204 in this case. time domain to the s-domain requires The buffer is there to isolate the com- something called a Laplace transform. pensator from the divider. You don’t need to know how to do We will ignore the compensator for this (unless you are an electrical engithe moment, but notice that I have neering student) because you can write absorbed the error summing junc- the s-domain expression for most comtion into the black box and added a mon components very easily without positive input for the reference and a having to do any maths. Table 1 – passive component complex impedances Component Time Domain s-Domain Complex Impedance Z(s) Inductor v = L di/dt v = iLs Ls Capacitor i = C dv/dt i = vCs 1/Cs Resistor v = Ri v = ir R siliconchip.com.au Australia's electronics magazine The “s” in s-domain is a complex frequency, which is just a way of describing a unit-amplitude sinewave with a given frequency and phase. We won’t, however, have to deal with any complex numbers in this explanation – we can simply treat s like any other algebraic variable. To transform the inductor mentioned above to the s-domain, the time differential (d/dt) is simply replaced by s to get the s-domain equivalent relationship v = i • L • s, which I hope you can see is kind of analogous to Ohm’s law. The complex impedance of the inductor, indicated by Z(s), is therefore simply Ls. Using a similar logic, we can arrive at the s-domain expressions for the voltage/current relationships of common parts, and hence their complex impedances, shown in Table 1. To show just how useful this transformation is, we will use these complex impedances to calculate the transfer function of a simple RC low-pass filter shown at the top of Fig.4. If you substitute in the complex impedances from the table and use the normal voltage divider equation, you get the s-domain transfer function 1 ÷ (RCs + 1) after a little bit of algebraic manipulation. It’s that easy. Poles and zeros You can see that there must be a particular value of complex frequency s that makes the denominator of the December 2025  31 expression equal zero, and therefore the transfer function becomes undefined (but very large). This value, s = -1 ÷ (RC), is called a “pole” because you can visualise it as a spike rising to infinity on the two-dimensional s-plane. The complex frequency domain uses radians per second as the unit of frequency, but converting to Hertz (via the relationship that one cycle is 2π radians), this pole is at a frequency of 1 ÷ (2πRC)Hz. This should be familiar as the corner frequency of an RC filter. If we turn to the RC high-pass filter at the bottom of Fig.4 and go through the same process, we arrive at the s-­ domain transfer function RCs ÷ (RCs + 1). This too has a single pole, at –1 ÷ RC, but the s in the numerator means there is also a value of s where the numerator of the fraction becomes zero, so the transfer function is equal to zero. It may come as no surprise to know that this is called a “zero”. In this case, the zero occurs at s = 0, so the LC highpass filter has a transfer function with one pole at 1 ÷ (2πRC) and one zero at the origin. By origin, we really mean the origin of the s-plane, but you can think of this zero as being at “0Hz”. So, you can easily work out the poles & zeroes of any network by calculating the transfer function in the s-domain as a fraction. Poles are the values of s (frequency) when the denominator is zero, and zeroes are the values of s when the numerator is zero. Poles & zeros have a specific meaning in the frequency domain. When a signal encounters a pole, the slope of the gain reduces by −20dB/decade and the phase shifts by −90°. Conversely, when a signal encounters a zero, the slope of its gain increases by 20dB/ decade and the phase shifts by +90°. Fig.5 shows this in action for the two filters we just analysed. In the case of the low-pass filter, the signal is unattenuated and has a phase shift of zero until it encounters the pole at which point it turns down to a slope of −20dB per decade. At the same time, the phase shifts by −90°. Fig.4: finding the s-domain transfer functions for these filters is as simple as substituting the expressions from Table 1 into the voltage divider equation and performing some elementary algebra. In the case of the high-pass filter, the zero at the origin means that the gain is increasing by +20dB per decade from ‘zero’ and the phase is already shifted by +90°. When the pole is encountered, the −20dB/decade shift puts the gain back to flat, and the −90° shift brings the phase back to zero. In practice, this means that you can work out the transfer function, the pole and zero locations and therefore the gain/phase versus frequency characteristic of any network of resistors, capacitors and inductors with a bit of high-school algebra. No wonder engineers like the s-domain. Thanks, Monsieur Laplace! Buck converter output filter Armed with this new knowledge, we are ready to work out the transfer function, Gf, of the buck converter’s output filter to fill out our control system model. This is shown on the lefthand side of Fig.6. If you have a go at this yourself, you will see I have made a small approximation in the denominator, where an RCs term is dropped because it is very much smaller that the dominant RLs2 term. This kind of simplification is common in complex frequency analysis, so watch out for them. The resulting transfer function has an s2 term in the denominator. This signifies that the transfer function has two poles; in this case, both are at 1 ÷ (2π√LC). This means the roll-off at this frequency is −40dB/decade, and the phase shifts by −180°. There is also one zero at 1 ÷ 2πRC due to the ESR of the of the capacitance. This pulls the gain back up to −20dB/ decade and the phase back up to −90°. This double-pole is a problem for our converter. Gain ‘peaking’, like that shown dotted due to the Q of the LC filter, and the rapid phase transition can cause instability like the ringing we saw in Fig.1. If we compare this plot to the simulation on the righthand side of Fig.6, you can see that the peaking extends to about 15dB, which is potentially quite serious. Designing a compensator Fig.5: the poles and zeros obtained from the s-domain transfer functions in Fig.4 can be easily transferred to the frequency response plots. A pole introduces a −20dB/decade slope change and a −90° phase shift. A zero 32introduces Silicon Chip Australia's electronics a +20dB/decade slope change and a +90° phase shift. magazine Finally, we come to the design of the compensator. There are several ways to go about this, but they all come down to manipulating the poles and zeros of the closed-loop transfer function. Their number, position and relationship can be tweaked to ensure the siliconchip.com.au Fig.6: the DC-DC converter output filter has two poles at a frequency determined by the inductance and capacitance, and a zero at the frequency of the capacitance and its ESR. The resulting frequency response contains a nasty peak & steep phase shift responsible for the ringing in Fig.1. Fig.7: we create a compensator characteristic (▪) to transform the filter response into the dominant pole characteristic (▪). The compensator requires two poles & two zeroes. system is stable and to optimise the transient response. For example, if we are most worried about overshoot, we can set up a highly damped system that will eliminate it at the expense of response time and settling time. On the other hand, we might be concerned about getting to the new voltage quickly, and can therefore tolerate a bit of overshoot. I’m not going to dig into all of that here. Instead, I will demonstrate a simple technique called dominant-pole compensation that focuses on the open-loop poles and zeros. This is the same technique used to ensure op amps are stable in closed-loop applications. The aim is to make the open-loop frequency response look like a single-­ pole filter that rolls off the gain at a steady −20dB/decade until it reaches 0dB at some crossover frequency, fc. This roll-off is required to ensure the gain drops to unity well before the phase shift reaches −180°. The difference between the phase shift at the crossover frequency and −180° is the ‘phase margin’, a measure of the stability of the circuit and its tolerance to external disturbances. We can achieve this roll-off by adding a compensator with the characteristics shown in Fig.7. The modulator siliconchip.com.au and the sensor gains are not frequency-­ dependent, so they don’t impact the shape of the gain/frequency characteristic of the open-loop converter, and can be ignored. They do impact the absolute value of the gain, but we are not concerned with that here. If we want to transform the filter gain/frequency characteristic (in red) into the dominant pole characteristic in blue, we require the compensator to have the green characteristic. It should have a pole at the origin, to set the roll-off to −20dB/decade. We then need two zeroes at the corner frequency of the LC filter to cancel the double-pole in the filter response. The Fig.8: this Type III compensator has the necessary poles and zeroes. The transfer function is calculated in exactly the same way as for the filter; the component values are discussed in the text. Australia's electronics magazine compensator should have another pole at the frequency of the output capacitance/ESR zero to cancel the corresponding zero in the filter. So in total, we need a compensator with two zeroes and two poles, one of which is at the origin. This is known as a Type III compensator, and a suitable circuit is shown in Fig.8. The reference voltage is applied to the non-inverting input of the op amp, and the feedback signal is applied to the inverting input via R3/C3. You can work out the compensator’s transfer function in the s-domain in exactly the same way as we did for the filter. This time, we just assume that vr is zero, and use the usual equation for the gain of an inverting amplifier: G = −Zf /Zi, where Zf is the impedance of C2 in parallel with R1 plus C1, and Zi is the impedance of R3 in parallel with C3. The resulting transfer function (again with a minor simplification) is shown below the circuit diagram. There are two zeros, one formed by R1C1 and one by R3C3, and two poles, one at the origin and one formed by R1C2. The component values shown are calculated by setting the frequency of both zeroes, 1 ÷ (2πR1C1) and 1 ÷ (2πR3C3), to be the same as the LC December 2025  33 seems likely the chip uses a similar compensator internally. Current-mode control Fig.9: compare this closed-loop response to that of Fig.1. The disturbances when the load or supply voltage change are much smaller and settle much faster. The lower graph shows the open loop gain and phase with the compensator included. Contrast this with the filter characteristic in Fig.6. filter’s 1 ÷ (2π√LC), which is 1.56kHz. I chose 100nF and 1kW for nice, round values. The non-origin pole at 1 ÷ (2πR1C2) should be at the same frequency as the zero formed by the output capacitor and its ESR (20.4kHz). Since we have already chosen R1, we can calculate C2 to be 7.8nF. This is not a standard value, so I used 8.2nF, the nearest one. This small error won’t make a huge difference, since the values for output capacitance and ESR we are using (102.4µF and 76mW) are hardly precision values themselves. Simulation I simulated this control circuit to see how it will perform. The result is shown at the top of Fig.9. The stimulus is exactly the same as for Fig.1 – the load resistance changes from 10W to 20W at the 10ms mark, and the source 34 Silicon Chip voltage drops from 12V to 10V at the 30ms mark. The difference is dramatic. There is just a small blip at the 10ms mark, which peaks at about +20mV (+0.3%), and lasts for just 150µs. At the 30ms mark, the voltage dips about 130mV when the input changes, but it recovers without overshoot in around 600µs. I ran a second simulation to plot the open-loop gain of the entire circuit, which is shown at the bottom of Fig.9. While you can clearly see the expected -20dB/decade roll-off from 100Hz to 100kHz, a small amount of gain peaking is still visible, but it is much less pronounced. The simulation also calculates the crossover frequency, which is 4.36kHz, and the phase margin, which is 66°. You may recall that the crossover frequency we calculated from the TPS5410 data sheet was 4.5kHz, so it Australia's electronics magazine We mentioned above that there is a second type of modulator that can be used in DC-DC converters. This is known as current-mode control, and it is shown in Fig.10. In this case, the modulator controls the average current through the inductor, ‹il›, instead of the average voltage at the filter input. At the beginning of every switching period T, the latch is set, so the Mosfet switches on and the inductor current begins to increase. This current is monitored by a current sensor, usually a resistor, and the resulting voltage is compared to the control voltage, vc. When the current reaches the level defined by the control voltage, the latch is reset and the Mosfet switches off until the next cycle. The result is that the peak current is determined by the control voltage and the value of the sense resistor. If the current ripple is small, this turns out to be a reasonable approximation to the average current. This approximation is not perfect, so most real-life implementations use something called slope compensation to avoid a potential instability at very high duty cycles. This is not relevant for the discussion below, so I will not go into it here. Editor’s note: see the LED Dazzler project (February 2011; siliconchip. au/Article/899) for a practical implementation of slope compensation in a switch-mode regulator. The transfer function of a current mode modulator is therefore approximated by Gm = 1 ÷ Rsense. Note that this gain is a transconductance (a voltage to current conversion). Because the output of the modulator is now a current, we have to revisit the transfer function of the output filter and the compensator, since the filter is no longer the voltage divider we modelled earlier. Fig.11 shows the calculation of the filter transfer function, which is performed in exactly the same way as before. What is different is that the inductor is effectively irrelevant, since the modulator current passes through it (you can think of the inductor as part of the modulator if that helps). The output voltage is now dependent on the impedance of the filter capacitor, including its ESR, and the impedance of the load. The upshot of this is that the transfer siliconchip.com.au function is simpler, with a single pole at a frequency dependent on the output capacitance and the load resistance, and a zero at the frequency related to the capacitance and its ESR, like before. It follows that our compensator can be simpler, as it now needs only two poles and one zero, as shown at the bottom of Fig.11. This is known as a Type II compensator, and an example is shown in Fig.12. The component values of R1 & C1 are calculated so that the frequency of the zero (1 ÷ 2πR1C1) is the same as the zero formed by the output capacitor and the load, at 1 ÷ 2πRloadC, which is 156Hz. The pole at 1 ÷ 2πR1C2 should be at the same frequency as the zero formed by the output capacitor and its ESR (20.4kHz). Given that R1 is 10kW, the closest standard capacitor value comes out to be 820pF. The value of R3 should be very low compared to R1, and can even be zero. I simulated this converter as well, although in this case, a change in input voltage was not modelled. Fig.14 on page 36 shows the results with a much magnified scale. The peak excursion and recovery time are very similar to the voltage mode controller, as we would expect. Discontinuous current Before we finish with control theory for now, we have to cover one more topic. So far, we have assumed that the current through the converter is continuous. That is, we have assumed the peak-to-peak current is small compared to the average. But what happens as the average inductor drops to the point it is close to the peak-topeak ripple? Fig.13 shows our old friend, the buck converter, where we have plotted the inductor current and the voltage vx that we have used in all of our analyses. On the left is the case when the inductor current is high enough that the negative excursion of the (exaggerated) current ripple remains positive. This is known as the continuous current case, and has been the assumption we have used to derive the modulator models for both voltage mode and current mode examples above. We could, however, imagine a situation when the load and therefore inductor current reduces to the point that the inductor current ramps down siliconchip.com.au Fig.10: a current-mode modulator controls the average inductor current instead of the average voltage as in Fig.3. This change flows on to the filter and compensator transfer functions. Fig.11: the current-mode filter transfer function is simpler, with just one pole set by the capacitor and the load resistance and one zero due to the capacitor and its ESR. The compensator therefore only needs two poles and one zero. Fig.12: this Type II compensator has the requisite poles (one at the origin) and one zero to compensate the current-mode controller. See the text for the component values. Fig.13: discontinuous mode occurs when the current reaches zero before the end of the switching period. This changes the modulator transfer function, and thus the compensator is required for optimum performance. Australia's electronics magazine December 2025  35 Fig.14: the resulting closed-loop response to a load change is similar to that of the voltage mode controller (albeit with a different scale). Silicon Chip kcaBBack Issues $10.00 + post $11.50 + post $12.50 + post $13.00 + post $14.00 + post January 1997 to October 2021 November 2021 to September 2023 October 2023 to September 2024 October 2024 onwards September 2025 onwards All back issues after February 2015 are in stock, while most from January 1997 to December 2014 are available. For a full list of all available issues, visit: siliconchip.com. au/Shop/2 PDF versions are available for all issues at siliconchip.com.au/Shop/12 We also sell photocopies of individual articles for those who don’t have a computer Compact HiFi Headphone Amplifier Complete Kit SC6885: $70 December 2024 & January 2025 siliconchip.au/Series/432 This kit includes everything required to build the Compact HiFi Headphone Amplifier. The case is included, but you will need your own power supply. 36 Silicon Chip Australia's electronics magazine to zero at some time (Zt) before the end of the switching period, as shown on the right. This is known as discontinuous current operation. When the inductor current falls to zero, the inductor voltage must also be zero, so the point vx will be at the output voltage, Vload. It’s pretty easy to see that the modulator transfer function will not be the same as for the continuous conduction case. The fact that the transfer function changes in discontinuous mode has implications for our control loop. You can design an optimised control loop for a switching converter operating in either mode, but compromises are required if the converter will operate in both modes. Converters are therefore generally designed to work in one mode or the other. High-power converters usually use continuous current mode, and thus have a specified minimum output current. Low power converters (say, ≤50W) often operate exclusively in discontinuous current mode. Of course, depending on what you are powering, it may be necessary to operate in both modes, in which case compromises much be made to ensure stability while keeping reasonably good load and line regulation. Conclusion Whew! This has been a pretty heavy article, but I think it is important to have a basic understanding of the principles of control theory as applied to DC-DC converters. You may never have to completely design a compensator from scratch, as the manufacturers do a lot of the heavy lifting for us, but it is important to understand what’s going on for those occasions when things don’t go to plan. At the very least, we have added yet another tool to our growing set of ways to look at and understand power electronics systems. As the number of models at our disposal grows, I am reminded of the saying, “all models are wrong, but some models are useful”. Knowing just which model or tool to use in what circumstance is the mark of a true expert, and comes only with experience. Next time, we will look at isolated DC-DC converters and reverse-­ engineer a commercial converter to see what we can learn from the proSC fessionals. siliconchip.com.au Gifts + ! s t e g d ga altronics.com.au NEW! NEW! 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B 0012 Nicholas Vinen Adjustable brightness and ambient auto-dimming using an LDR Adjustable pattern cycle time Mostly pre-assembled; can be up and running in under an hour Power supply: 12V DC recommended <at> 1-2A (operating range: 6-16V DC) SC7535 Kit ($80) Includes a pre-assembled PCB with nearly all parts fitted, except for IC1, REG2 etc (see the parts list) 24cm tall and wide white star-shaped PCB 80 onboard bright WS2812B RGB LEDs in a star/circle pattern 12 different LED light patterns, each with four possible colour palettes Manual or auto-cycling patterns and palettes Jazz up your Christmas tree (or just about anything else) with this luminous RGB LED Star. It has 80 “NeoPixel” LEDs that create an array of dazzling, colourful patterns. You can choose which patterns and colour schemes you like, adjust the brightness and more. It’s quick and easy to build, too! siliconchip.com.au Australia's electronics magazine December 2025  41 Y ou can watch a video of some of the available patterns and colour schemes at siliconchip. au/Video/RGBStar It has been a few years since we’ve published a Christmas ornament project, despite those generally being very popular. Partly it’s because people were still building our previous designs. However, while making more kits for the November 2020 RGB LED Christmas Star, we had two realisations. Firstly, it had been five years since we published that design. Secondly, it’s a lot of work to build, but people obviously think it’s worthwhile as they continue to order kits for it. It uses 30 individual RGB LEDs, with four pins to solder on each, plus 90 current-limiting resistors, 13 driver ICs, more than 20 bypass capacitors and some other sundry parts. Assembling the board would take most of a day. That got us thinking: was there an easier way? Our first thought was WS2812B “NeoPixel” LEDs. These devices have GND and Vcc pins plus serial data input and output pins (four in total). The Vcc and GND pins are all joined in parallel, while the output of one LED goes to the input of another, forming a daisy chain (you may well have seen these on RGB LED strips). That means you only need one pin on a microcontroller to drive many – possibly even hundreds – of these devices. They’re bright and can display one of 16,777,215 different colours. By writing clever software on a microcontroller, we could generate all sorts of cool patterns using a string of these devices. So what’s not to like? Our concerns were that they are SMD-only and, if you buy them individually and put a lot on a board, the cost can add up. However, there is a solution to that: get the PCB manufacturer to solder the LEDs for you. That saves a lot of work and gives a neat, professional result. Also, because they have access to huge quantities directly from the manufacturer, it can be cheaper than buying individual parts and soldering them yourself. Down the rabbit hole So we set to work designing a PCB for this, with four questions immediately popping up. What shape, size and colour should the board be, and how many NeoPixel LEDs should we put on it? We liked the star concept, but weren’t sold on the five-pointed star used in the November 2020 project we mentioned earlier. Partly that was out of a desire to do something different, but also, five-pointed stars don’t look right to us. Of course, real stars don’t have points; they are distant, bright spheres that should look like a point source. It’s the optical imaging system (a telescope, or perhaps our eyes) that makes them appear to have points. Usually, those points are arranged symmetrically, meaning there are an even number of them. Fig.1 shows an image from the James Webb Space Telescope where you can see that its optics generate six lines that appear to emerge from the star’s point source. To cut a long story short, we decided that a more realistic and cool-looking star would have four long and four short points, with the short points offset by 45°. We also decided to make the board white, for two reasons. Firstly, to differentiate it from the earlier star, which was black, and secondly, because the original WS2812B LEDs came in white plastic packages. (Black ones are now available, but let’s put them aside for another day.) As for the size, we were able to design a board with that shape that wouldn’t be too expensive to manufacture by making it 238mm tall and 238mm wide. The trick is to rotate the design by 45° so that it fits inside a 168 × 168mm square, as that’s the basis on which the manufacturers charge (238mm ÷ 168mm ≈ √2). Having drawn the board shape, we arranged a string of LEDs around the edge of the star shape, plus two rings inside, ending up with 80 LEDs in total. That seemed like a reasonable number to generate some interesting patterns. So the mechanical side of things was sorted out, and it was time to turn to the electronics! Power delivery and control Fig.1: stars look like more than just points because of the optics viewing them (eg, a telescope or our eyes). The lines that appear to radiate from them are normally symmetrical, meaning an even number, and some are longer than others. Hence the shape of our Star, with four long points and four short ones. At full brightness, set to produce white light, each LED draws around 50mA. Multiply that by the 80 LEDs and we can see that we need to deliver around 4A at 5V to run them all. That’s a substantial amount of power: 20W. Of course, this isn’t a torch, so we would never actually drive them all white at once. The actual average power required would be a more modest 10W or so; 2A at 5V. And that’s assuming you would run it at full brightness, which, to get a bit technical, is eye-searingly bright. Most people would run it close to 5W of total LED power. Still, the 10W figure could be delivered by a reasonably efficient 2A buck (step-down) regulator running from a higher voltage, like 12V. The same 5V supply (or a separate one) could then run a microcontroller to generate the patterns. Besides the LEDs, the micro and power supply, we would also need a decent number of bypass capacitors spread around the board. The WS2812B LEDs control brightness Australia's electronics magazine siliconchip.com.au 42 Silicon Chip using pulse-width modulation (PWM), meaning there will be constant switching spikes distributed around the board. So we’ll want a few ceramic and perhaps tantalum polymer capacitors to keep the 5V rail nice and stable. All that would be left would be a few buttons and perhaps trimpots to do things like adjust brightness, change the patterns, set up pattern auto-­cycling and such. Add an LDR to monitor the ambient light level for auto-dimming, and the design was complete. Getting it assembled As mentioned earlier, our plan was to order some prototype PCBs and have the manufacturer fit the WS2812B LEDs for us. We would then add the power supply, microcontroller and a few other bits and pieces to finish it off. The power supply and microcontroller could go on the back of the board, so they wouldn’t mar its appearance, meaning only one side of the board needed to be pre-populated. We figured we could also get them to place the bypass capacitors for the WS2812Bs on the same side, as they would not ruin the appearance as long as we placed them symmetrically. But we’d leave fitting all the other parts for ourselves (or someone else building this later). As we went through this process, we discovered that the WS2812B LEDs require “special handling” because they have a high moisture-­sensitivity level (MSL5). Automated/contract PCB assembly is usually done using solder reflow, where solder paste is melted in an infrared (IR) oven, or wave soldering, where a wave of molten solder passes over the surface of the board and some sticks to the exposed pads and pins. In both cases, the components rapidly heat up from room temperature to about 250°C over a few minutes. They sit at the high temperature for just long enough to let all the solder melt and reflow, then they are cooled back down to close to ambient temperature. The whole process takes about 4-6 minutes. If any components on the board contain moisture (water), that water will flash boil and rapidly expand. The result can be exploding components – not what you want. So they have to be dry before you start; either removed from a hermetically sealed package just before soldering, or baked in an oven at a lower temperature (90-125°C siliconchip.com.au Parts List – RGB LED Star Ornament 1 white 168 × 168mm double-sided star-shaped PCB coded 16112251 1 12V DC 1A+ (2A recommended) power supply × 1 2-pin header or right-angle header (CON1; optional) • 1 5-pin header or right-angle header (CON2; optional, for ICSP) × 1 3-pin header or right-angle header (CON3; optional, for testing) × 2 6A 18mW (120W <at> 100MHz) SMD M3216/1206 ferrite beads (FB1, FB2) [Tai-Tech HCB3216KF-121T60] 1 2.7A 33μH shielded SMD inductor, 12 × 12mm (L1) [Sunltech SLH1204S330MTT] 1 GL5528 20kW-1MW LDR (LDR1) • 3 6 × 3mm two-pin SMD tactile pushbutton switches with white actuators (S1-S3) 2 10kW Bourns TC33X-2-103E SMD trimpots (VR1, VR2) Semiconductors 1 PIC16F18126-I/SL 8-bit 14-pin microcontroller programmed with 1611225A.HEX, SOIC-14 (IC1) • 1 LDL1117S50R or AMS1117-5 low-dropout 5V linear regulator, SOT-223 (REG1) 1 AP5002 20V 2A 500kHz integrated buck regulator, SOIC-8 (REG2) • 80 WS2812B-V5 serial RGB “NeoPixel” LEDs, SMD 5050 (LED1-LED80) 1 AO3400A 30V 5.8A N-channel Mosfet, SOT-23 (Q1) 1 BZX84C5V6 5.6V ±1% 250mW SMD zener diode, SOT-23 (ZD1) 1 BZG05C-5V6 5.6V ±6% 1.25W SMD zener diode, DO-214AC (ZD2) 1 B340A or S3A 40V 3A SMD schottky diode, DO-214AC (D1) Capacitors (all 50V SMD X7R MLCC, M3216/1206 size unless noted) 1 220μF 25V polymer aluminium electrolytic capacitor, 6.3×6mm SMD [Shengyang SM227M025E0600] 6 220μF 6.3V polymer tantalum electrolytic capacitors, SMB case [Panasonic 6TPE220MAZB] 12 22μF 25V X5R 1 22μF 10V SMA tantalum [Kyocera AVX TAJA226M010RNJ] 31 100nF 1 4.7nF 1 1nF Resistors (all ±1% SMD M3216/1206 size) 2 100kW 1 10kW 2 6.8kW 1 1.3kW 1 1kW 1 330W RGB LED Star Kit (SC7535, $80 + P&P): comes with a pre-assembled PCB with all parts fitted except those marked with • or ×. The parts marked • are included in the kit but must be fitted by the constructor. for 24-48 hours) to drive out all the moisture before soldering. So, we had to pay a little extra to get the WS2812B LEDs for our prototype boards baked. That meant we needed to choose the ‘standard’ assembly service rather than the ‘economic’ one. The standard service also allows you to put components on both sides of the board. Hmm. It was starting to look like we might as well get virtually the whole thing assembled! One of the parts we chose to use, the buck regulator controller, is no longer being manufactured (we’ll explain why we chose it a bit later). We have hundreds, but didn’t feel like sending them to the manufacturer, so we decided to solder it ourselves. It’s only an 8-pin device in a relatively large Australia's electronics magazine SOIC package, so not difficult for us or any other constructor to add. We also left the PIC16 chip that produces the patterns off the assembled board. It’s also in an SOIC package (with 14 pins, though) and we figured we could supply programmed chips that constructors could easily fit to the board. We also didn’t have them put any headers on it, since we figured people might have different preferred arrangements for wiring up the power supply. Other than that, though, all our prototype boards – and the ones we’ll supply to readers – come with almost all the parts fitted. So assembly is quick and easy, despite the large number of LEDs! This also helps to keep the total cost to build it down, since the December 2025  43 manufacturer can source these parts in bulk from their partner warehouse. As a result, we can supply a kit that includes the mostly assembled board, programmed microcontroller and switch-mode chip, ready to assemble, for around $80 + P&P. That is somewhat more than the $45 we charge for the November 2020 RGB LED Star kit, but considering the time savings in not having to assemble the whole thing yourself from well over one hundred parts, the larger PCB, larger number of LEDs, better patterns and brightness, it ends up being a pretty good deal. But for those intrepid constructors, the blank PCB will be available separately, if you want to assemble the whole project yourself. Circuit details The full circuit of the RGB LED Star is shown in Fig.2. By showing only some of the long chain of LEDs, we manage to keep it relatively simple. It can be broken into three main blocks: the LEDs, the microcontroller and the power supply. Microcontroller IC1 drives the chain of LEDs (LED1LED80) via its RA2 digital output and a 330W series resistor, as recommended in WS2812B data sheets to reduce overshoot and ringing from trace inductance and limit fault current into the first LED. Each LED’s output drives the subsequent LED input until LED80, which is loaded with a resistor to ground to reduce the chance of any ringing or EMI from an unterminated output pin. A total of 42 bypass capacitors scattered around the board, in three different values, provide local bypassing for these 80 LEDs. In total, they can draw up to 4A (!), and they dim the LEDs using PWM, so we want to ensure they see a low 5V supply source impedance. Test header CON3 gives us a way to drive the LEDs before IC1 is soldered to the board, should we need that. Most constructors will not need to fit it. The user controls are three pushbuttons, S1-S3, and two trimpots, VR1 & VR2; the buttons connect to digital inputs RC0-RC2 (pins 10-8). The software in IC1 enables a weak pull-up current on these pins so they are normally held high, at around 5V. When a button is pressed, it pulls that pin to 0V, transitioning to a digital low, so the software can sense that. Debouncing is performed in software. When VR1 & VR2 are rotated, they vary the voltages at pins 3 & 2 of IC1 between 0V (fully anti-clockwise) and 5V (fully clockwise). IC1 uses its internal analog-to-digital converter (ADC) to convert these voltages into numbers between 0 and 4095, to control the overall brightness and the duration of each different pattern, respectively. Another analog input, ANC5 at pin 5, connects to the junction of a light-dependent resistor (LDR1) and a fixed 100kW resistor. The voltage at this pin will be higher The front of the LED Star includes two buttons to change patterns. 44 Silicon Chip in bright ambient lighting conditions and closer to 0V in darkness. Again, the ADC is used to sample this voltage to provide auto-dimming, so the LEDs are not so bright at night, but bright enough to see clearly during the day. At 100% brightness, the RGB LED star is eye-searing! So we definitely need both manual and automatic brightness control. Five-pin header CON2 can be used to program IC1 in-circuit. We used this during development, but since kits/boards will come with a pre-­ programmed microcontroller, you won’t need to fit it unless you want to design your own patterns or otherwise make changes to the firmware. Power supply Power comes in via a two-pin header (CON1) or soldered wires shown at upper left. Mosfet Q1 and zener diode ZD1 provide reverse polarity protection without dropping much voltage. If the polarity is correct, Q1’s gate is pulled high, switching it on and providing a low-resistance path between the negative conductor and ground. If the polarity is reversed, Q1’s gate is pulled negative. It remains off, and its intrinsic body diode is reverse-­ biased, so no current can flow. The Mosfet is rated at 30V, and ZD1 clamps its gate voltage at a safe level, so nothing will happen with a negative voltage up to -30V applied to the circuit. The maximum positive voltage is limited to 18V by REG1. One 220μF aluminium polymer electrolytic capacitor and two 22μF ceramics provide bulk storage and bypassing for the input of REG1, an LDL1117 5V low-dropout regulator. This is a true low-dropout regulator, and it will provide a regulated 5V rail for the microcontroller as long as there’s at least 5.5V at its input. It also has a 22μF tantalum output filter capacitor, required for stability. Its 5V output is only used to power the microcontroller and connected components like the LDR and trimpots. This means that the switching noise from the LEDs doing their PWM brightness control won’t affect the microcontroller’s analog measurements. The remainder of the power supply is a DC/DC converter that supplies the high-current 5V rail for the LEDs. It is based around REG2, an AP5002 ‘2A siliconchip.com.au Fig.2: the PIC16F18126 microcontroller at lower left controls the 80 LEDs by sending serial data from its RA2 digital output. The LEDs have plenty of bypass capacitors of various sizes to provide them with the peak current they draw during operation. The power supply at the top includes reverse-polarity protection (Q1/ZD1), a linear regulator to power the micro (REG1) and a switch-mode step-down buck regulator (REG2) to power the LEDs from a ~12V DC source. buck’ converter. This chip is obsolete now, but we like it so much that we bought several hundred, so we will supply them with boards/kits (or separately, if you really need one for some other reason). Some of the reasons we are still using this is that it is easy to solder, coming in an 8-pin SOIC package with no thermal pad underneath; it has a useful range of voltages (up to 20V) and currents (it says 2A but can actually deliver 4A or more in some cases), can go to 100% duty cycle (meaning it’s a ‘low dropout’ buck regulator!) siliconchip.com.au and it’s up to 90% efficient. We also find that it ‘just works’. One of its nice features is an external compensation network that requires just two or three components. This is one of the keys to its stability in a wide range of situations. We’ve used it in a few projects before, such as the Simple 1.2-20V 1.5A Switching Regulator (February 2012; siliconchip.au/ Article/774) and the CLASSiC DAC (February-April 2013; siliconchip.au/ Series/63). Now, while in theory the LEDs could draw up to about 4A if they were all Australia's electronics magazine at 100% brightness and set to white; this is not a torch, so that’s unlikely to happen. In general, this regulator will see a load below 2A <at> 5V; perhaps a little higher in extreme cases. So, the internal switch current limit of 3.5A and L1’s rating of 2.7A are not really of concern (especially since L1’s current rating is saturation-based, not thermal). Since we’re recommending a 12V DC input and we’re producing a regulated 5V (more like 4.9V, actually) for the LEDs, REG2 is operating near its sweet spot, around 90% efficiency (its December 2025  45 Fig.3: the star has been rotated 45° to fit on the page; the upper-left corner is intended to be the top. All the components you see here come pre-soldered to the board except for LDR1, in the centre. Some holes at lower- right allow you to attach it to a stick or the top of a Christmas tree. 2A headline rating is more of a ‘worstcase scenario’, thankfully). We have ferrite beads at the input and output of the DC/DC converter section to try to reduce the amount of switching hash radiated from either end. FB2 is also convenient in that if you run into problems, you can remove it to disconnect the output of the DC/ DC converter from the LEDs, allowing you to test them in isolation. Zener diode ZD2 is not strictly necessary; it’s a ‘belts and braces’ protection measure. Should there be positive spikes from the output of REG2 for some reason (eg, the load suddenly goes from 4A to 50mA and it can’t switch off fast enough), ZD2 will 46 Silicon Chip conduct once the LED supply exceeds about 5.5V, limiting the supply rail to a safe level of about 6V in the short term. PCB design The top side of the assembled board is shown in Fig.3. The only part you need is add here is the LDR at the centre. The first LED in the chain, LED1, is towards upper left. You can follow the snaking path of the data from there clockwise around the star, back to LED56 just below LED1, then to the circle formed by LED57-LED72, and finally the smaller concentric circle formed by LED73-LED80. Note how there is a polymer tantalum capacitor on each of the longer Australia's electronics magazine ‘arms’ of the star, plus two in the middle, for distributed bypassing. The eight points of the star also feature a 22μF bypass capacitor, plus numerous 100nF capacitors scattered throughout so that all LEDs have a low source impedance. Most of the copper on the top of the board is a +5V distribution plane, with the underside copper being GND. Hence, there are pairs of vias near the GND end of most components to connect them to the ground plane. The star is intended to be rotated so that LED5 is at the top. There are holes at each long point (eg, for hanging it), plus some extra holes on the bottom one so that a stick or similar can be siliconchip.com.au Fig.4: the control and power supply circuitry is located in the centre of the back of the Star. The DC/DC converter delivers 4.9V DC at up to several amps to the centre of the board, where it’s conducted out to the LEDs arrayed around it by copper planes; the front of the board has the +5V (4.9V) plane while the GND plane is on the back side. attached for holding it up (shown with M2 machine screws in them in some of the photos). Turning to the underside (Fig.4), you can see that the power supply and control components are clustered in the centre, divided by fivepin header CON2. The control components are above and to the right of CON2, while the power supply is below and left. The power connections (CON1) are near the bottom of the star, so any attached wires can hang down behind it. A slight revision We made just a couple of minor changes to the design between the siliconchip.com.au prototype and the final version. Firstly, we added a 220μF 25V low-ESR aluminium polymer capacitor across the input of the linear regulator, REG1, visible on the left in Fig.4. This is to overcome any lead inductance from the power supply, suppress ringing and provide a local charge reservoir for the switch-mode regulator. While the prototype worked without it, we felt that it was worthwhile adding, especially since the part only costs about 10¢. We also added another 22μF ceramic capacitor in parallel with it for higher-frequency stabilisation of the input supply. Finally, we decided to replace the boring old AMS1117-5 regulator Australia's electronics magazine (REG1) on the prototype with the more modern LDL1117S50R. This is a direct drop-in replacement, as it’s in the same SOT-223 package and has the same pinout. However, it is a true low-dropout regulator that can withstand a higher input voltage (18V vs 15V), with a lower quiescent current (0.25mA vs 5mA), so we thought it was a worthwhile upgrade (the required tantalum output capacitor upgrade costs more than the regulator). Construction When you receive the board, it should have all the top-side components fitted besides LDR1, and all the underside components fitted besides December 2025  47 the three pin headers (all of which are optional), IC1 and REG2. It will also have a pair of ‘rails’ attached to it, which were used to hold it during the assembly process (see the photo below). There is a series of holes drilled between the rails and the PCB edges (‘mouse bites’). These allow you to easily snap the rail off by flexing the junction back and forth a few times. The breaks should be fairly clean, but if you want to, you can clean them up with a file. Just make sure you don’t breathe the resulting fibreglass dust (eg, do it outdoors and ideally while wearing a mask, or at least with the wind blowing it away from you). Testing Our prototypes worked straight away, so really the easiest way to proceed is to solder LDR1, IC1 and REG2 to the board, making sure IC1 & REG2 are orientated correctly, then apply power and check that it lights up and start producing some nice LED patterns. However, if you want to test it more methodically, you can (as we did with our prototypes initially, just to be safe). Firstly, you can solder CON3 to the board. If you don’t want it visible on the front of the Star, you can either solder wires to the pads on the rear, or remove it later. Connect CON3’s terminal marked + to the output of a bench supply. Short the The LED Star has some M2-sized mounting holes, for if you want to attach it to something solid. other two pins together and connect them to the negative output/ground. Set the current limit to 100mA, the voltage to 5V and apply power. Our board drew 40mA in this condition, exactly what was expected when powering 80 LEDs with 0.5mA quiescent current each. This verifies that there are no short circuits or faulty components among the LEDs or their bypass capacitors. Next, solder to the pads of CON1. You can use a right-angle header on the rear of the board if you have the type of right-angle header you can solder from the same side, such as a polarised right-angle header. Alternatively, solder the wires to the board. Apply 7-12V DC and you should see a current draw of around 5mA if you have an early board with the AMS1117, or 0.5mA if you have the LDL1117 regulator. Use a multimeter to check the voltage between pads 1 & 14 of IC1. You should get a reading close to 5V. Now remove power and solder REG2 to the board (remember what we said earlier about getting its orientation right!). One side is quite close to D1, but we managed to solder the pins on that side without bridges by bringing the iron in from above. Good news: pins 5 & 6 are connected electrically, as are pins 7 & 8, so as long as you don’t get a bridge across the middle two pins on that side, it’ll be fine! Place a jumper on CON3 between the middle and ground pins (ie, not on the + side). This holds the input to LED1 low. Now apply 6-12V via CON1 and measure the voltage across the pins on either side of CON3, being careful not to short anything. You should measure close to 4.9V DC, and the current draw should be around 50mA. If all is good, switch it off and solder IC1 to its pads, again checking its orientation carefully. This should be easy as there aren’t any components too close to its pins, but check for solder bridges and if you find any, remove them with flux paste and solder wick. Because parts are soldered to both sides of the PCB, it needs to be manufactured with rails, as shown here. They are used to hold it in place during soldering. When you receive it, you can flex them back and forth a few times carefully and they will snap off. If you want to clean up the rough edges left behind, it’s easy to do with a file, but don’t breathe the dust. 48 Silicon Chip Australia's electronics magazine siliconchip.com.au The finished underside of the RGB LED star is shown on the right, with a close-up shown inset. This inset photo shows the missing parts that you need to fit yourself, but are included in the kit. If IC1 hasn’t been programmed, you can attach CON2 and a PIC programmer and upload the firmware (1611225A.HEX). But since you most likely bought an assembled board, it will come with a pre-programmed chip, so you are ready to power it up properly using a high-current (capable of at least 1A) 12V DC supply at CON1 and you should be rewarded with colourful patterns. Using it By default, it will cycle through all 12 possible patterns and four colour palettes at an interval determined by the setting of trimpot VR2. Fully anti-clockwise will make it cycle roughly once per second, while fully clockwise will give you around five minutes per pattern. VR1 will adjust the overall brightness, and the light level on LDR1 will also affect the brightness. You’ll need a jeweller’s slotted screwdriver or a similarly slim tool to rotate the small screws of the two trimpots. Note that there are no stops; they rotate through 360° but they only work properly over about ¾ of that travel. So if you get flickering when adjusting VR1, or odd behaviour when adjusting VR2, you’re probably off the track. Pressing S1 briefly will switch to manual pattern selection mode. Each further press of S1 will cycle to the next pattern. Once pattern 12 is reached, pressing S1 will go back to pattern 1. siliconchip.com.au Similarly, pressing S2 briefly will switch to manual palette selection mode. Each further press of S2 will cycle to the next palette. Once the fourth palette is reached, pressing S2 will go back to the first. The auto-cycling works mostly independently for patterns and palettes, meaning you can have both change automatically periodically, or just one or the other, or neither. To resume auto-­ cycling the pattern, hold down S1 for a couple of seconds. To resume auto-­ cycling the palette, hold down S2 for a couple of seconds. If both are set to cycle, when it’s time to cycle, the unit will select the next enabled pattern with a random palette. That way, you get to see different patterns with different palettes. You can disable some patterns or palettes if you don’t like them. To disable the current pattern (regardless of whether it’s auto-cycling or manually selected), hold S2, then quickly press and release S1, then release S2. It will go to the next enabled pattern, and the last pattern will be skipped from now on. You can’t re-enable individual patterns since you can’t select them, but you can re-enable them all by holding down S2, then pressing and holding S1 for a few seconds, and then releasing S1 before S2. Similarly, to disable the current palette, hold S1, then quickly press Australia's electronics magazine S2, release S2 and then S1. It will go to the next enabled palette, and the last palette will be skipped from now on. You can’t disable the last pattern or palette, though. To re-enable all palettes, hold down S1, then hold down S2 for a few seconds, then release S2, then S1. There is a third switch on the back of the board labelled S3. It can be used to change the LDR setpoints or disable automatic brightness control. Usually, VR1 sets the maximum brightness, but it will be reduced if the LDR senses a low ambient light level. By default, it will be reduced to a very low setting, around 5% of maximum, in complete darkness. To change that, adjust VR1 to get the minimum brightness you want, then press S3 briefly. You can disable auto brightness adjustment by holding S3 for a couple of seconds; then only VR1 sets the brightness. Setting the minimum re-enables it. You may want to temporarily disable auto brightness when adjusting VR1 to set the minimum since it’ll remove the effect of ambient light while you are making the setting. If you find that too much of the light from the LEDs shines on the LDR, making its brightness control unstable or ineffective, you could shrink a short section of black heatshrink tubing around it to shield it from light from the sides. That way, it should only react to ambient or reflected light. We December 2025  49 didn’t need to do that, but since the LDR calibration is automatic, it must be exposed to both light and dark before it will sense properly. Firmware operation The firmware is written in C and split into five files: • neopixel.c is the main program that includes pin configuration, peripheral configuration, button, potentiometer and LDR sensing, user interface logic, LED update code and the main loop. • LEDs.c contains 320 bytes of data, stored in flash, on the locations of the 80 LEDs on the physical PCB for use by the effects code. • rgb.c contains helper functions that generate the four colour palettes and perform RGB colour mixing. • trig.c contains an integer sine table and sine/cosine functions for trigonometric effects. • effects.c contains the logic to implement the 12 separate patterns that can be displayed on the LEDs. Some effects are based on the locations of the LEDs in the chain, some on their Cartesian (X/Y) coordinates on the PCB, and some on their polar (distance/angle coordinates) on the PCB. For example, effect #1 is a gradient that snakes its way along the chain of LEDs from LED1 to LED80. It is a nice-looking effect but the simplest to implement. Effect #3 is coloured circles that radiate from the centre of the star, out the points, and then end. It uses the polar coordinates, comparing the distance of the LED from the centre of the board to the current radius of the circle. Effect #5 is a colour spectrum that rotates around the display like a spinning wheel. It is similar to effect #1, but it uses the polar angle of the LED rather than its position in the chain. Effect #10 is coloured bubbles that grow from random points within the star and then burst. It calculates the distance of an LED from the centre of a bubble using the formula distance = √(x1 – x2)2 + (y1 – y2)2. The effects are double buffered, meaning that the processor is calculating the next state of the LEDs to show (into buffer x) simultaneously with transmitting the last update to the LED string (from buffer y). The buffers are swapped on each run through the SC main loop. 50 Silicon Chip Updating Neopixels using the CLC hardware We got this idea from Microchip Application Note AN1606 “Using the Configurable Logic Cell (CLC) to Interface a PIC16F1509 and WS2811 LED Driver”. Unfortunately, the CLC peripheral in modern PICs is configured quite differently from the PIC16F1509, so the code in that App Note is no longer very useful. Also, we realised it’s unnecessarily complex – using two CLCs when the job can be done with just one (possibly due to improvements in the CLCs since it was written). The difficulty in driving WS2812B chips is that they use a somewhat unique scheme that encodes 0s and 1s into different-length positive pulses. Interestingly, different versions of the WS2812B, even within the same manufacturer, have different specifications for what is required. As shown in Fig.a, the specific version we are using (V5) requires: Zero bit: 220-380ns high, 580-1000ns low One bit: 580-1000ns high, 580-1000ns low Reset frame: low for at least 280μs Note how this scheme is difficult to encode with a system that emits bits at consistent intervals. A zero bit lasts for 800-1380ns and a one bit lasts for 11602000ns. So, to have consistent bit intervals, the interval must be between 1160ns (862kHz) and 1380ns (725kHz). If you select an interval in the middle of, say, 1250ns (800kHz), that means you could have a one bit with a 50% duty cycle, ie, 625ns high and 625ns low. The high time for the zero bit would need to be at least 250ns to avoid the low time exceeding the 1000ns threshold, giving you a range of 250-380ns. We use the MSSP peripheral to generate an 800kHz SPI stream as the basis for this signal. This gives us an SCK signal that’s high for 625ns and low for 625ns, and an SDO signal that’s either high or low for 1250ns depending on the value of the bit being transmitted. We feed both of these to a CLC cell, along with a synchronised PWM waveform that has a high time of around half the SCK high-time at ~312.5ns – see Fig.b. Fig.a: there are quite a few WS2812B variants, and they all seem to have slightly different timing requirements (sometimes compatible with each other, sometimes not). We’re using the current V5 variant; its timing is shown at upper right here, with the data formats (common to all variants) below. The timings in the lower part of the diagram are nominal. Australia's electronics magazine siliconchip.com.au Fig.b: we’re using the MSSP serial peripheral, CCP PWM generator and CLC configurable logic cell in the PIC to generate the WS2812B signal with minimal CPU overhead. Writing a byte to a buffer triggers the MSSP to generate eight bits of data. We combine its clock (SCK), serial data output (SDO) and synchronised PWM signal with the CLC to make the orange waveform the LEDs expect. That time of year is nearly here... CHRISTMAS Spice up your festive season with eight LED decorations! Luckily for us, it’s possible to synchronise the MSSP to a PWM peripheral by setting the SSPM bits in the SSPxCON1 register to a value of 3 (SPI Host Mode: Clock = TMR2 output/2). TMR2 is also the default clock source for a Capture/Compare/PWM (CCP) Module, which can be set in PWM mode. So we can synchronise the leading edge of the PWM pulses with the leading edge of the SCK pulses. We can then use the CLC to perform the simple logic operation (SDO & SCK) | PWM. The SDO & SCK operation produces 625ns positive pulses if the data bit is a one or no pulse if the data bit is a zero. By ORing this with the PWM pulses, which overlap with the start of the SCK pulses, we either get a 625ns pulse if the data bit is a one or a ~300ns pulse if it’s a zero – see Fig.b. Unfortunately, there’s no good way to stream multiple bytes of data out using the MSSP module; it only has a single buffer register (SSPxBUF) and you can only initiate a transfer by writing a byte to it when the peripheral is not already transmitting data. We solve this by setting up a timer (Timer 4) to run from the main oscillator with a period of 82, which is just a tiny bit longer than it takes the MSSP peripheral to transmit one byte. By initiating a series of byte transfers in the interrupt service routine (ISR) triggered by Timer 4, we create an effectively uninterrupted stream of data to update all the LEDs with minimal processor overhead. Excluding the setup code, that means we only need the following code to update all 80 RGB LEDs. ## Additional code required to update all LEDs void __interrupt() isr(void) { SSP1BUF = *RGBBufPtr; if( ++RGBBufPtr == RGBBufEnd ) PIE2bits.TMR4IE = 0; PIR2bits.TMR4IF = 0; } static void StartTransmission (unsigned char* start, unsigned char* end) { RGBBufPtr = start; RGBBufEnd = end; while( PIE2bits.TMR4IE ) ; TMR2 = 0; TMR4 = 0; PIE2bits.TMR4IE = 1; } This is the basic code used but it lacks the 280μs reset timer after each transmission, implemented with a separate timer peripheral. siliconchip.com.au Australia's electronics magazine Tiny LED Xmas Tree 54 x 41mm PCB SC5181 – $2.50 Tiny LED Cap 55 x 57mm PCB SC5687 – $3.00 Tiny LED Stocking 41 x 83mm PCB SC5688 – $3.00 Tiny LED Reindeer 91 x 98mm PCB SC5689 – $3.00 Tiny LED Bauble 52.5 x 45.5mm SC5690 – $3.00 Tiny LED Sleigh 80 x 92mm PCB SC5691 – $3.00 Tiny LED Star 57 x 54mm PCB SC5692 – $3.00 Tiny LED Cane 84 x 60mm PCB SC5693 – $3.00 We also sell a kit containing all required components for just $15 per board ➟ SC5579 December 2025  51 HOW TO DESIGN Printed Circuit Boards Part 1 by Tim Blythman The cost of professionally made printed circuit boards (PCBs) has dropped dramatically over the last decade, while the available options have expanded. While we offer PCBs we design for our projects, anyone can make their own custom boards. So, how do you go about turning an idea into a printed circuit board? W e have published several articles about PCB design and manufacturing before. However, we haven’t really explained the entire process of designing and ordering circuit boards from scratch. We wanted to create a series of articles that would be helpful for beginners as well as those who already have some PCB design experience. PCB design is a discipline that is both a science and a bit of an art. This series will offer some techniques and strategies, but PCB design is something that benefits from practice. This article will also talk about processes and terminology. It’s much easier to find information when you know the terms to use. Our Making PCBs article from the July 2019 issue (see siliconchip.au/ Article/11700) mentioned the data and processes that are needed to create a PCB. If you’re not familiar with how PCBs are manufactured, it could 52 Silicon Chip be helpful to have a look at this article to know about the underlying technology and processes. Our review of Altium’s CircuitMaker software from January 2019 (see siliconchip.au/Article/11378) touched on the steps involved in using a software package to turn a circuit into a printed circuit board. But it is not just circuits that can become PCBs. Their price and versatility means that custom PCBs also make excellent panels and enclosure lids; we are sure that they have other uses, too. Electronics design automation (EDA) programs generally include tutorials and guides for getting started. EDA software is more than just PCB design, but that is a large part of it. This series is more about the details that such guides might not cover, including ideas and processes that are involved in good PCB design. Our frequent reviews of the Altium Designer software, most recently in Australia's electronics magazine June (siliconchip.au/Article/18307) reveal that EDA software is continually evolving. Altium Designer We are going to work through the process of PCB design using Altium Designer. It’s the software that we use, but most EDA packages work similarly. So even if you plan to use something else (for example, KiCad), the concepts we discuss will be useful. To keep things simple, we’ll stick to basic two-layer (double-sided) designs. Simple boards like this make up the vast majority of our designs. Two-sided boards are so commonplace now that it isn’t worth the trouble of making a single-sided design. The PCB manufacturers are set up for two-sided processes, and there are unlikely to be savings unless you order thousands of boards. It’s possible to design for boards to be etched by hand, but we have not done that for many years because commercial boards are so inexpensive. There is a free trial option for Altium Designer (www.altium.com/ altium-designer/free-trial/roadmap). The related CircuitMaker package is also free to use, but projects are stored in Altium servers (in “the cloud”) and are visible to other CircuitMaker users. Helpfully, its interface is similar to Altium Designer. See www.altium. com/circuitmaker Both Altium software packages work through two distinct phases. First, there is the process known as ‘schematic capture’. This amounts to drawing the circuit diagram within the software. Internally, the software records the components that are used and how they should be connected. The second stage is PCB layout. This mainly involves placing the components and traces to connect them together. This can be likened to drawing the final PCB design. The many circuit diagrams and PCB overlays in our project articles are taken from these two phases of the PCB project. While it is possible to design a PCB without schematic capture, it’s a far more error-prone process, and we don’t recommend it. The small amount of time invested in drawing the circuit is repaid in the much increased likelihood of your PCB working first time. Before we get to the nitty gritty of PCB design, let’s first look at what we’re trying to achieve. siliconchip.com.au Fig.1: there are lots of options possible when ordering a PCB, but the defaults are usually the quickest and cheapest (and fine for many uses). We suggest sticking with these defaults if you haven’t designed and ordered a PCB before. Source: PCBWay – www. pcbway.com/orderonline. aspx ▶ Fig.2: Altium Designer offers many layer stack options, but the actual parameters shown here are only critical for simulation in cases like RF or high-speed design. It’s simply to show how a PCB is structured and the information here is not processed into the Gerber files. The apparently overly-precise figures are simply imperial values converted into metric. Typical Gerber files for a two-layer board Layer Extensions Board shape/outline .gm, .gm1 or .gbr Top copper .gtl or .gbr Top solder mask .gts or .gbr Top silkscreen overlay .gto or .gbr Bottom copper .gbl or .gbr Bottom solder mask .gbs or .gbr Bottom silkscreen overlay .gbo or .gbr PCB manufacturing Once the PCB layout phase in Altium Designer is complete, “Gerber” files are exported. These are what the PCB manufacturer uses to create the physical boards. In addition to uploading those files (usually as a ‘ZIP’ archive), there will be various options to choose from, such as the colour of the solder mask on the PCB. Fig.1 shows the ordering page, including options, of PCBWay. It looks like a lot of choices, but most of our designs use the defaults as shown. These settings will typically be the cheapest and fastest PCBs to produce. However, it’s worth noting critical siliconchip.com.au parameters like minimum track width and spacing and minimum hole size. These are part of the so-called manufacturing capabilities. For most designs that will be manually assembled, there should be no problem adopting much more generous guidelines. An example can be found at www.pcbway.com/capabilities.html It’s important to set up ‘design rule checks’ that match (or are at least similar to) the manufacturer’s capabilities. This way, the software will flag anything that will cause a manufacturing problem and allow you to fix it before finalising the design. Most of our designs are laid out with margins at least double what is listed Australia's electronics magazine regarding things like trace width and spacing; even the cheapest boards are comfortably within our requirements. However, there are cases, such as when you’re working with fine-pitch SMD ICs, that you have to push close to the standard manufacturing limits. It’s handy to know what PCB manufacturers are capable of producing. If you’re looking to create panels, lids and the like, there is a good range of colours available too. Some even offer full-colour printing on the PCB surface now! Fig.2 shows a so-called layer stack and its visualisation as seen in Altium Designer. The green layers that are not labelled are the solder mask layers; December 2025  53 these are what actually give PCBs their colour, as well as reducing the chance of short circuits while soldering. At left is a ‘via’ connecting the top and bottom copper layers. These seven layers roughly correspond to seven of the eight files in a set of Gerber files. The dielectric layer marks the fibreglass (FR-4) core, and its file indicates the shape of the PCB. An eighth file defines the holes that need to be drilled in the PCB. Most Gerber files are equivalent to a monochrome image, with the colour indicating whether the material on that layer (copper, solder mask or silkscreen printing) is present or not. Due to the way PCBs were historically made, the actual data consists of shapes, called apertures, which are combined to create the final image. Computer technology has changed the way these steps work. The PCB Manufacture panel gives a bit more detail about the different files and layers, and how they relate to the processing steps involved in PCB manufacture. See also the diagram in the panel overleaf showing how various PCB features are created in the various layers. Design overview Fig.3 shows the steps that are required to create a PCB and the associated file types (in the square blocks) that are used in Altium Designer. There will probably be a set of generic component libraries available from your EDA software, but if you are using any components with more than three leads, there is a good chance that you will have to add your own entries. We will cover that too. Schematic capture and PCB layout are fairly manual processes, while the library files and design rules are a major part of the ‘automation’ in EDA. They help to make sure that you end up with a valid and functional result. Tools such as automated design rule checking, automatic routing of traces and hierarchical design can help to speed up the process. The design rules and libraries you use might change depending on how you intend to assemble your PCB. This is part of the field known as ‘design for manufacture’ (DFM). A PCB intended for manual assembly (such as our project PCBs) could be very different from a design intended for mass production through pick-and-place and reflow or wave soldering. For example, we have created library files with large pads and clear silkscreen labels to make manual assembly easier. A mass-produced PCB might cram all the components on one side to avoid a two-pass process. The pads will be small to require a minimal amount of solder and may completely omit silkscreen labels due to lack of space. Consider that the ideal pad size and shape for any given component will be different depending on how it is soldered, what solder is used and even what other components and tracks are nearby. We expect most readers will produce designs for manual assembly like us, so we will concentrate on that. It’s possible to create multi-sheet or multiboard projects with Altium Designer, but we’ll keep it simple, using just a single SchDoc (schematic/circuit) file and a single PcbDoc (board) file. We find that this is sufficient for most our designs. One of the great things about going to the trouble of drawing the circuit in this way is that if you later have to make changes to either the circuit or Fig.3: these are the steps involved in the design and manufacture of a PCB. This article concentrates on setting up libraries, while next month we will discuss design rules and the schematic capture and PCB layout steps, followed by Gerber file exporting and ordering. 54 Silicon Chip Australia's electronics magazine the board, the software will check that they match up. So, for example, if you remove a bunch of tracks to move components, then forget to add one back, it will tell you. You don’t want to find out about it when your new board revision doesn’t work! That’s a lot of background, but we hope that it will provide some insight into why things are done as they are. It’s also the case that there is no single correct way to lay out a PCB; we shall describe the workflow that we find works best for us, but you may decide to vary it once you have some experience. Libraries Good libraries are the foundation of a solid PCB project, so we should start with how to create and use libraries. The Altium Academy channel has a video titled “How To Create Your Own Libraries in Altium Designer” (https:// youtu.be/bOi45nshqP8). Altium can use integrated libraries, but commonly, you will see separate SchLib (symbol) and PcbLib (footprint) files. The schematic library consists of the circuit diagram symbols for various components. They are usually also be linked with specific footprints in the PcbLib file. A footprint corresponds to a physical component package so, for example, the same TO-220 footprint could be used for many component types, such as bipolar transistors, Mosfets, voltage regulators, diodes and so on. There can also be multiple footprints for a given package. Our libraries have variants of the TO-220 package footprint for vertical mounting, horizontal mounting, and with the tab affixed to the PCB using a screw. There are also variants that make provision for a heatsink to be added (possibly even including pads to solder heatsink retaining pins into). The visual representation is important for helping to understand the circuit diagram, but the pins and external connections (such as exposed pads and mounting points) are probably the most critical part. As we go along, keep the Properties tab open so that you are aware of all the different parameters that relate to the elements that you are working with. Fig.4 shows a 74HC595 shift register IC in our SchLib file. The main image shows how it would appear in the circuit during schematic capture. siliconchip.com.au ▶ Fig.5: you can configure your schematic (circuit) symbols however you like. You might see many components laid out like this to neaten the resulting diagrams. Real ICs often have pins in odd places due to silicon limitations; while you can make the symbols reflect that, it isn’t necessary. Fig.4: setting up your libraries will also give you practice in working with the schematic capture and PCB layout tools, since they use similar environments. At lower left are three linked footprints (in a separate PcbLib file); note how they correspond to three different package types. Pin 15 is selected in the main window, and at right are the pin properties, with the mouse pointer selecting from the pin type menu. By making sure this property is set correctly, you can avoid improper connections during schematic capture. The rules will flag cases when a conflict is created, such as when two outputs are connected together, or if an input is not driven by anything. You can also see that the pins on the part show directional arrows for inputs and outputs. A bidirectional pin (such as a microcontroller’s general purpose I/O pin) will show arrows both in and out. The pin names are shown inside the rectangle, and the pin designators (numbers in this case) outside. The designators are what connects a specific pin to a pad within a footprint. This example shows the pins arranged identically to their physical layout. This is not a requirement; other arrangements can be used. Fig.5 shows a CD4017 decade counter with inputs on the left and outputs on the right. The positive supply is at the top and the negative supply at the bottom. siliconchip.com.au This arrangement will almost certainly make the circuit diagram in the SchDoc file neater, but it means that the circuit is less like the PCB layout. However, remember that the point of a schematic diagram is to most clearly represent the function of a circuit, and often that’s quite distinct from the best physical layout on the circuit board. The various components are referenced by their names (design item IDs), so it is important to give unique names to each component. We have adopted the convention of prefixing each name with an underscore to ensure that they are distinct from Altium’s included libraries – see Fig.6. The numerical pin designators that you see are typical for ICs and indeed most components, but this field is a character string, so could be “A” or “K” for the anode or cathode of a diode. “A1” or “A2” could be used for the two anodes of a common-cathode diode. You can also see another very handy feature in Fig.6: components can also be broken into various parts (Part A, Part B etc). In the case of this hex inverter, it makes it easier to swap between using different inverter elements (using a dropdown menu) if this is needed to simplify PCB routing. Parts can also be used to separate logically distinct parts of a component, such as a relay’s contacts and its coil. Altium Designer can handle Fig.6: multi-part components allow flexibility in swapping equivalent sub-parts. Being able to separate the sub-parts can also help in creating a tidy schematic. Australia's electronics magazine December 2025  55 Fig.7: reading and interpreting engineering drawings such as these is a handy skill if you need to create your own footprint libraries. Source: Infineon Technologies – www. infineon.com/assets/row/public/ documents/10/49/infineonbtn8962ta-ds-en.pdf automated pin swapping if you do not wish to do it manually. The schematic editor always works on a 50mil (50/1000in or 1.27mm) grid, so you should make sure that all pins fall on this grid spacing by setting the grid snap. This will make it easier to drag-and-drop wires while editing the schematic. It’s easy to switch between metric and imperial units with the ‘Q’ hotkey. Even components that aren’t connected electrically can be created as components, since the schematic editor can be used to generate a bill of materials (BoM); for example, screws & spacers for mounting. Having a component that can be dropped into the SchDoc file means that it is automatically included in the bill of materials. It’s also a good idea to make sure the names and descriptions are apt, so that the BoM is clear. Something with no electrical connections, like an enclosure, could have a footprint with a 3D body associated, and it would be visible in the 3D view of the PCB and generate an item in the BoM. The enclosure’s footprint could also include a template of the PCB outline so that the PCB’s features can be aligned to the enclosure. 56 Silicon Chip Similarly, components can be marked as ‘no BoM’, meaning they do not generate an entry in the bill of materials. An example of this would be a PCB trace antenna or inductor; there is no separate item that needs to be purchased as the component is effectively part of the PCB. You don’t need to worry about all these ideas right away, but it’s handy to be aware of them when you are creating and editing your library files. This will help make PCB layout go smoothly. Footprints Critical parts of the footprints in the PCB library are the copper layer pads that correspond to the pins in the schematic library. You can also include objects on any layer of the final PCB, and even 3D bodies, which can be used to create quite realistic 3D views of the PCB. By default, the pad designators are matched to the pin designators, but this can be changed if necessary. For simplicity, we try to keep our libraries so that the pin and pad designators match. Creating a footprint from scratch generally requires the component data Australia's electronics magazine sheet to provide the recommended pad geometry. Fig.7 shows the relevant page from the data sheet for the BTN8962 half-bridge driver, with the manufacturer’s suggested pad layout diagram at lower right. Manufacturer-recommended footprints are usually quite compact; we would consider lengthening some of the pads to ease soldering. We’ve found that these sort of diagrams need to be studied quite carefully. The pad sizes are clear enough, but because Altium Designer’s coordinate system is based on the centre of the pads, the pad coordinates will need to be deduced. Note that the pad pitch (which will match the distances between pad centres) is not shown at lower right, but it is marked elsewhere. The Properties tab comes in handy here, since you can directly enter the coordinates that you have calculated. Multiple pads in a row can be selected together and lined up by entering a coordinate that is used for all of them. Although many programs use Ctrl+click to add items to a selection, Altium Designer uses Shift+click for this purpose. For example, we can easily lay out the lower row of pads by putting the centre pad at a zero X-coordinate. The other pads can be placed at multiples of the 1.27mm (50 thou/mil or 0.05in) pin pitch. Knowing metric and imperial equivalents can come in handy. Setting the grid snap to match the pin pitch (or a fraction of it) can also be helpful. You might find an existing footprint in another library that will work, especially if it is a reasonably common type. Altium Designer also has two different footprint creation wizard tools, found under the Tools menu. These work quite well for packages that have a regular pin arrangement. Otherwise, the process of creating a footprint is similar to that for a symbol. The pads are placed as needed, with their shape, size and hole diameter defined. There is no need to manually lay out the solder mask layer, since the pads include both copper and solder mask elements. Silkscreen designators will be added by Altium Designer automatically during the PCB layout stage. Adding just the eight pad elements here will be sufficient to create a functional part. Our libraries include additional component outlines on the silkscreen siliconchip.com.au The PCB Manufacturing Process This is a brief overview of the files in a Gerber set and how they are used to physically create a board. This is for a two-layer PCB; boards with more layers simply have extra files. Often, the PCBs will be laid out in a large panel, possibly with other different designs, with the final step being to separate the PCBs from the panel before shipping them to customers. Our explanation below will necessarily be simplified. The steps shown adjacent are from PCBWay’s website. They also have videos showing the details of each of these steps. Note that steps 3-6 only apply to multi-layer (more than two-layer) boards. You can see the videos at www.pcbway.com/ pcb-service.html The process starts with a large panel of copper-clad FR-4 fibreglass laminate. The most common PCBs use a copper thickness of 0.025mm, which is equivalent to 1oz (28g) of copper per square foot bonded to 1.5mm thick fibreglass (resulting in a nominally 1.6mm finished PCB). Thicker copper layers can be created by electroplating. The first step is to drill and plate the plated holes. These are holes that connect the two sides of a PCB, and might be part of a via or through-hole pad. Thus, they will connect copper areas on both sides of the PCB. The drill locations are stored in an ‘NC drill file’; NC stands for numerical control and the format is quite similar to the GCODE commands that are used to drive CNC machines, including 3D printers. Altium exports these with a TXT file extension, and they are readable with a text editor (like the Gerber files). The drill file itself makes no distinction between plated and nonplated holes. Instead, it is assumed that holes that end inside a copper area at both ends are plated, while those that end in areas without copper are Ground Pour: polygon on top of copper layer This shows the numerous steps involved in manufacturing a PCB. Source: PCBWay – www. pcbway.com/pcb-service. html non-plated. Holes that are ambiguous will probably be flagged as errors by the PCB engineers. (Sometimes people put plated and non-plated holes in separate files.) The design rules typically specify a minimum annular ring (around a hole) that ensures the distinction is clear. Some holes, especially those that aren’t round, such as slotted or irregular shapes, might be defined in the outline layer (GMx file). These are also made at this stage, using a milling machine. The holes are then plated. A chemical process adds a thin layer of copper, which is then thickened by electroplating. Electroplating is much easier to do while there are solid conducting areas of copper on both sides of a PCB. The next step is to etch the copper layers. First, resist layers are applied to the top and bottom of the board using a dry film process similar to DIY PCB etching. Transparency masks are computer-­ generated from the GTL (top layer) and GBL (bottom layer) Gerber files and printed. The resist is cured using the transparencies, and the uncured resist is removed to allow the etchant to act on the copper. Then, the solder mask layers are applied. The apertures in the solder mask Gerber files (GTS and GBS) are used to mark where there are holes in the solder SMD Pad: usually square on top copper and solder mask layers TH Pad: shapes (circles, rectangles or stadium) on copper and solder mask layers possibly with a throughhole VIA: like a TH pad, PCB ID: Non-Plated Hole: drill hole Designators: text on but with no solder text on top not surrounded by copper silkscreen layer mask opening copper layer The anatomy of a printed circuit board. Various items on the PCB are created from elements in different layers here. Fortunately, most EDA programs will manage all the elements when a pad is placed on the PCB. siliconchip.com.au Australia's electronics magazine mask, so it works in the opposite (negative) fashion to the other layers, which normally mark where the material should remain. The etched PCBs are coated with a film of liquid solder mask ink. The ink is selectively cured by exposing it to ultraviolet (UV) light from a projector. This technology is similar to that used in resin 3D printing. The uncured resin is cleaned off, and the remaining ink may be cured further. It’s at this stage that a surface finish may be applied to the bare copper to prevent it oxidising and to improve solderability. We use the HASL (hot air solder level) treatment on the majority of our boards. This involves dipping the entire board into liquid solder, where it adheres to the exposed copper. The excess solder is then blown off using hot air. A different process called ENIG (electroless nickel immersion gold) uses a chemical process to plate a thin layer of nickel onto the copper. Another chemical step then plates gold over the nickel. The nickel layer is needed so that the reaction that deposits the gold can proceed. The silkscreen overlays are added next (GTO and GBO layers). These also use a UV-reactive ink that works like the solder mask layer, except that it is exposed and printed in a positive fashion. The liquid silkscreen is applied and selectively cured. Excess ink is washed away, and the remaining ink is fully cured. The remaining unplated holes and slots are then cut, drilled or routed as needed, using the TXT and GMx layers. This will include the board outline, and will thus result in the board being removed from its panel. There may be a number of different test and inspection steps that occur during the process. AOI (automatic optical inspection) can be used to compare the board appearance to that expected from the Gerber files, while various electrical tests can be used to ensure that the traces have no breaks or short circuits. Once the board has passed testing, it is complete, and it will be packaged and shipped. December 2025  57 layer to ease assembly of the PCB. They also make it easier to see conflicts, such as where components would overlap, for example. You can add unconnected mounting or clearance holes. Like the symbol pins, these are usually marked as such by using the ‘0’ designator. Manufacturer Part Search Many manufacturers now supply library files, so it is easier to use their parts. Thus, it’s worth mentioning the Manufacturer Part Search feature of Altium Designer. It can be opened from the Panels button. Some, but not all, components can be added to your libraries by simply downloading them from the internet. Fig.8 shows this panel, along with a search for BTN8962 half-bridge driver IC. You can see that the listed part name has its full suffix shown, since we need to be specific about which particular package we wish to import. The dropdown labelled “12 SPNs” refers to supplier part numbers; you can click through here to see the listed part at a supplier like Mouser or DigiKey. The search has two results, and the top result includes the models we need for our libraries. The second result with the red struck-out icon does not contain models, and will not help us. Models are visible below that, with the schematic symbol followed by the footprints. You can download the models by right-clicking on the listed item and selecting the option to ‘Download as File Library’. The download is a ZIP file containing several library items. We find it easy to simply open each library file and to copy and paste the items into our own libraries, where they can then be edited as needed. For example, we would change the symbol name to fit our naming scheme. You can also see that the footprint library contains three different footprints. If in doubt, choose the “L” low-density variant, since this will be the easiest to manually solder. The “N” variant is what would likely be used for a mass-produced PCB. You should also check the footprints on downloaded models to be sure that the layers correspond to your conventions. We typically use mechanical layer 15 (GM15) for the board outline, but some models use this for the component courtyard (outline). If this were not changed, we might end up with holes all over the PCB, since we 58 Silicon Chip also use that layer for unplated slots (routed out of the PCB). Making footprints Items like connectors will probably need to be made from scratch unless a downloadable model is available, since they can vary so much. Many SMD connectors also include throughhole mounting pads, so they will need a mix of pads. Most surface-mounting pads simply exist on the top layer (and can be flipped onto the bottom layer with the component). They are automatically allocated a matching solder mask outline, which is expanded slightly to ensure that the pad is fully uncovered, even if there is a slight misalignment. Thus, you don’t need to worry about creating separate items in the copper and solder mask layers. Through-holes are created as a ‘Multi-layer’ item, which includes a drill hole (with settable diameter) and top and bottom layer pads, which can be various shapes including square, round or lozenge-shaped. The two layers are matched in size by default, but each can be set separately. You can also offset the hole centre from the pad centre if needed. We prefer to work with through-hole parts in imperial dimensions, since many components are on a 100mil (0.1in or 2.54mm) pitch. A non-plated hole can be created by setting the pad (copper) layers to have an X-Size and Y-Size of zero. Such a hole could be used for a mounting screw or LED to pass through. It’s a good idea to centre your footprints on the origin (0,0) and arrange them symmetrically if possible. By all means, copy and paste existing footprints (and symbols), then modify them. It will make things easier and will ensure that the style of your components remains consistent. Altium Designer also supports many layers beyond the ones that are needed for PCB manufacturing. Layer Mechanical-1 is often used for so-called 3D bodies, which are the entities used to create 3D views of the components. The simplest of these are extruded shapes, such as a rectangle extruded vertically to form a cuboid, or a circle extruded into a cylinder. Adding even a single 3D body to each component will allow you to better visualise a completed board. In Fig.8, our downloaded model of the BTN8962 includes a simple cuboid 3D body. We often use Mechanical-14 for reference marks and ‘fiducials’. For designs that are mounted in an enclosure, we can mark its walls so that things like jacks and sockets can be aligned correctly. They can also be used to help create the cutting and drilling diagrams, since dimensions and coordinates can be directly read from the Properties. Summary Fig.8: the Manufacturer Part Search can remove the need to do a lot of the work in creating components and footprints within your libraries. However, not all components are supported, and we recommend thoroughly checking the imported files for correctness and consistency. Next month, we will look at some of the strategies we use for designing PCBs, both during schematic capture and PCB layout. This will also include aspects such as PCB design rules. We will follow with the steps involved in finalising your Gerber files and sending them away to be manufactured. In the meantime, you can start SC building your libraries. Australia's electronics magazine siliconchip.com.au X 0213 X 3260 SAVE 20% NEW! 40 24.95 $ $ X 0232 SAVE 37% 50 $ 5m Flexible Camping Strip Light Kit Great for setting up temporary lighting in tents, awnings, gazebos etc. Simply secure it to your tent poles and plug it into a 12V power source. 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X 2385 SAVE 29% 39 $ Your Yourelectronics electronicssupplier suppliersince since1976. 1976. Shop in-store atat one ofof our 1111locations around Australia: Shop in-store one our locations around Australia: WA » PERTH » JOONDALUP » CANNINGTON » MIDLAND » MYAREE » BALCATTA WA » PERTH » JOONDALUP » CANNINGTON » MIDLAND » MYAREE » BALCATTA VIC » SPRINGVALE » AIRPORT WEST » VIRGINIA » AUBURN » PROSPECT VIC » SPRINGVALE » AIRPORT WESTQLD QLD » VIRGINIANSW NSW » AUBURNSASA » PROSPECT Or Orshop shoponline online24/7 24/7<at><at>altronics.com.au altronics.com.au ®® Build BuildItItYourself YourselfElectronics ElectronicsCentre Centre © Altronics 2024. E&OE. Prices stated herein are only valid until 31/12/24 or until stocks run out. Prices include GST and exclude freight and insurance. See latest catalogue for freight rates. © Altronics 2025. E&OE. Prices stated here in are only valid until 31/12/25 or until stocks run out. Prices include GST and exclude freight and insurance. See latest catalogue for freight rates. Image source: https://unsplash.com/photos/aerial-photography-of-flowers-at-daytime-TRhGEGdw-YY Earrth Ra Ea Rad dio John Clarke’s Parrt 1 : w� Pa w�ispe isperrs of of the sk sky With this ‘natural radio receiver’, you can listen to solar and atmospheric disturbances, like storms or auroras. These create electromagnetic waves in the VLF (very low frequency) and ELF (extra low frequency) range. I ntriguing natural sounds such as whistlers, tweeks and the chorus can be heard using this simple receiver. Naturally produced electromagnetic waves are abundant throughout the world, and are there for the listening with the right equipment. Not only can you hear the sounds that are created in your local region, but even from other parts of the world! These low-­frequency electromagnetic waves, often created in the Earth’s atmosphere, are guided by the ionosphere that encircles the globe. The waves are reflected or refracted off the ionosphere, and can travel halfway around the world, all just waiting to be received and listened to. Our portable Earth Radio is powered by an internal or external battery and can receive most of the VLF and ELF frequencies covering the 3Hz to 30kHz bands. These are at the lower part of the electromagnetic spectrum, as shown in Fig.1. We only show frequencies on the spectrum down to 1Hz, although electromagnetic waves can be even lower in frequency than that. The electromagnetic spectrum covers a huge range of frequencies, including much higher frequencies such as broadcast radio waves (~1100MHz), microwaves (300MHz+), infrared (300GHz+) and visible light (400-790THz). The higher-frequency waves have more energy, which is why UV, X-rays and gamma rays can cause skin and cell damage. To pick up the naturally produced VLF and ELF signals, we use a loop antenna. Its advantage is that it is small enough to be portable despite the low wavelengths involved. Outputs on the Fig.1: electromagnetic waves span a huge range of frequencies, from 1Hz and below (ELF) up to 1026Hz (gamma rays). The range we’re interested in here is from 20Hz to 22kHz, within the ELF and VLF bands. 60 Silicon Chip Australia's electronics magazine siliconchip.com.au Fig.2: there are several ionised layers in the ionosphere, and they change between day and night. Without particles from the sun to keep them ionised, some layers disappear at night, while others move and/or become thinner. receiver include one for recording, with another suitable for listening via headphones or earphones. The Earth Radio receives signals due to atmospheric phenomena and also from the sun. The most significant atmospheric source is lightning. Lightning generates a wide range of electromagnetic waves, including light, radio waves, X-rays and gamma rays. The radio waves they produce extend into the VLF frequency range (3-30kHz) and some in the ELF range (3Hz-3kHz). Another source is geomagnetic storms. These are disturbances created by the coupling of the Earth’s magnetic field with the solar wind. The solar wind is produced by solar flares and other sun irregularities, such as coronal mass ejections. They can cause changes in the Earth’s magnetosphere (magnetic field), and as a result, induce currents into a receiving antenna. Apart from electromagnetic wave production, these events can also result in auroral (visible) displays. They are known as the Aurora Borealis (Northern Lights) in the northern hemisphere and Aurora Australis (Southern Lights) in the southern hemisphere. siliconchip.com.au Earth Radio Kit (SC7582, $55) this kit includes all non-optional parts, except for the case, battery, timber and cable/wire. Practical uses of the ELF and VLF electromagnetic bands are transmitters for communications with submarines and around the world (as described in our article on Underwater Communication in March 2023 – siliconchip. au/Article/15691). Other man-made sources of these low-frequency radio waves aren’t necessarily wanted. These include 50/60Hz mains hum and somewhat higher-frequency signals produced by electronic equipment like computers and electric motors. One of the main reasons that the signals from lightning and solar activity can be interesting is due to the ionosphere that surrounds the Earth. This layer of the atmosphere provides a waveguide for the VLF and ELF waves to travel within. They effectively bounce off the ionosphere and the Earth as they travel around the globe. For example, signals produced by lightning start out as a static noise. But by the time they are received, the waveforms may have morphed into different sounds such as tweeks, whistlers and the chorus. The original static sound is altered by the pathways they take before being received. Australia's electronics magazine Changes in the ionosphere between night and day, and the strengthening of ionospheric regions due to solar flares and coronal mass ejection events, also contribute to variations in the sounds received. So the ionosphere has a large impact on the sounds produced. The ionosphere The ionosphere (Fig.2) is the region of the atmosphere where the gases are ionised (split up into positively and negatively charged particles) by solar and cosmic radiation. It ranges from 70-1000km above the Earth’s surface, and is generally considered as being made up of three regions: D, E, and F. The F region splits into two layers (F1 and F2) during the daylight hours, but merges into a single layer during the night. Ionisation is strongest in the upper F region, and weakest in the lower D region; the latter basically exists only during daylight hours. During daylight hours, VLF and ELF signals generally pass through the D region and are refracted by (or reflect off) the E region, leading to a weakened signal. The D region is stronger during a solar flare event and acts as December 2025  61 a waveguide for VLF and ELF signals. This is because the wavelength of these signals is a significant fraction of the height of the D region. For example, a 20kHz electromagnetic wave has a wavelength of 15km, while a 2kHz wave has a 150km wavelength. With a strong D region in the ionosphere, these signals refract off it, and less loss is experienced, since they no longer pass through the D region to refract in the E region. This generally leads to a sudden increase in received signal, called ‘sudden ionospheric disturbance’ (SID). Some ELF and VLF signals manage to exit the ionosphere, where they will follow the magnetic field lines of the magnetosphere. They can reach 10,000km or more above the Earth before re-entering at a different location. There are several types of emissions possible, which are characterised as static, tweeks, whistlers, the chorus and hiss. Spectrograms Signals from our Earth Radio or from recorded sources can be visualised with spectrograph software. Three examples of audio files and spectrograms are shown in Figs.3, 4 & 5. Figs.3 & 4 are spectrograms of the audio files at www.spaceweather.com/glossary/ inspire.html, while Fig.5 was captured using our Earth Radio. Fig.5 consists mainly of close-by lightning statics. The horizontal red line at just under 20kHz is from the VLF radio station in Exmouth, Western Australia, under the call sign of NWC. It transmits on a 19.8kHz carrier. The expanded view of the NWC radio station signal in Fig.5(a) shows how the encoding uses a variation of frequency-shift keying (FSK) modulation called minimum-shift keying. Here, there is a 50Hz change between a ‘0’ and a ‘1’. Static Lightning strike statics (sometimes called sferics, short for “atmospherics”) are the signals from lightning that most people will be familiar with. This is the sound you will hear on an AM radio during an electrical storm – constant crackling and popping. Static signals are from nearby lightning strikes, within about 1,000km. They are seen on a spectrogram (with frequency on the vertical axis and time on the horizontal axis) as vertical lines. This indicates that all frequency components in the signal arrive at the same time. Tweeks Tweeks are lightning-caused electromagnetic emissions that have travelled around 2000km or more within the waveguide between the Earth and the ionosphere. The ionosphere varies in its properties throughout its thickness, The Earth Radio can run off an internal 9V battery or external 12V source. 62 Silicon Chip Australia's electronics magazine so higher-frequency components travel faster than others and thus will be received sooner than the others. Fig.3 shows a spectrogram of both static and tweeks. The tweeks are characterised by frequencies around 2kHz being delayed compared to other frequencies. These sounds are reminiscent of the Australian bell miner bird call. This spectrogram was produced using the Raven Lite 2 spectrograph software. The top waveform is the approximately 14-second-long audio signal. The lower spectrogram shows how its frequency components change over time. The tweeks have a vertical line at high frequencies, but at lower frequencies, they curve off to the right a little at around 2kHz. This indicates that the lower frequencies arrive later at the receiver compared to other frequencies. This results in a somewhat musical twang quality to the sound. Whistlers Like tweeks, whistlers have a musical quality. This is due to the longer propagation delay of their lower-­ frequency components compared to the higher frequencies. This leads to different frequency components of the signal becoming offset in time. It is the interaction of the signal with both the ionosphere and magnetosphere that causes the longer time delay for the different frequency components. Whistler signals travel along the magnetic field lines of the Earth, and can go to the opposite side of the Earth before returning. Since the path along magnetic field lines is very long (as much as three Earth diameters), the time delay differences are large, and the signal begins as a high-pitched tone, reducing to a lower pitch over time. It is likened to that of a falling bomb. Each whistler sound can last for as long as a couple of seconds. They are seen as long descending arcs on a spectrogram in the Fig.4 spectrogram. Again made using Raven Lite 2, the waveform shows whistlers intermixed with static. Each static signal is visible as a vertical line covering most of the frequency spectrum. Whistlers have a more musical quality than the tweeks in Fig.2 due to the propagation differences of more frequency components of the signal. siliconchip.com.au Different frequency components of the signal become offset over time. Note how the lower the frequency, the longer the time delay. There is about two seconds between the arrival of frequencies above 10kHz before the 1kHz component of that signal is received! Chorus Two types of ‘choruses’ can occasionally be heard: the dawn chorus and the auroral chorus. The dawn chorus is best listened to at sunrise, and can resemble crickets or a chorus of birds, or it may sound like dogs barking or squawks from flocks of birds. It comprises a range of overlapping sounds. The signals on a spectrogram show quick-rising arcs of less than a second each in duration. Its presence is dependent upon geomagnetic activity, such as the emission of a solar flare from the sun. The auroral chorus is generated within the aurora, and can be heard in areas close to where the aurora occurs. It is strongest during periods of high geomagnetic activity. A recording of a “VLF auroral chorus” (plus other natural radio sounds) is at www.youtube. com/MindOverMatter55/videos Fig.3: a spectrogram of an atmospheric recording that includes ‘tweeks’, sounds produced by the static from lightning crashes travelling around the globe, with the higher-frequency components travelling faster and thus arriving earlier. Source: www.spaceweather.com/glossary/inspire.html Hiss These sounds are typically emitted via the aurora and are high-pitched sounding. Hiss can also originate in the magnetosphere. When to listen Natural Radio signals can be heard at any time but are most prevalent before dawn. Tweeks are most common at night, and the chorus can be heard within several hours of sunrise. The results are usually better when there is strong geomagnetic activity. Sferics can be heard constantly at any time. If you want to find out the best times to be listening, there are websites such as www.abelian.org/vlf/index. php?page=live where natural radio events and solar and sunspot activity are logged. For events that occur within Australia, there is a page at www.facebook. com/groups/1953353338413426/ plus alerts and warnings for solar and geomagnetic activity and auroras on the Bureau of Meteorology website (Australia; www.sws.bom.gov.au/Space_ Weather). siliconchip.com.au Fig.4: this shows whistlers, which are similar to tweeks, but they have travelled further. As a result, their component frequencies are more spread out over time. Fig.5: some lightning statics captured with our Earth Radio prototype, along with the 19.8kHz signal from NWC in Exmouth, WA. The zoomed-in view shows its FSK/MSK modulation scheme, with the carrier varying by 50Hz to indicate different bits. Fig.6: this block diagram shows the stages of the radio. Current from the loop antenna is converted directly into a voltage signal, then higher frequencies and 50/60Hz hum are removed. It is further amplified and can be listened to using headphones/earphones or recorded with an audio recording device. Other countries should have similar indicators of space weather conditions. Natural radio receivers There are several well-documented receivers that are designed to receive the VLF and ELF electromagnetic bands. These include the interactive NASA space physics ionosphere radio experiments (Inspire) VLF receiver (https://theinspireproject.org). This is available as a kit of parts and is meant to ‘inspire’ school and university students to take an interest in and study science, technology, engineering and mathematics (STEM) subjects. The American Radio Relay League (ARRL) magazine QEX also has several articles and designs on VLF receivers; the January/February and the March/ April publications in 2010 are of interest. The first is entitled Radio Astronomy Projects by Jon Wallace and the second Amateur Radio Astronomy Projects; A Whistler Radio by Jon Wallace. These two publications are available at www.qsl.net/w/wb4kdi/AROL/ ARRL/QEX/ There is also Renato Romero’s home page at www.vlf.it with articles and designs exploring the ELF and VLF radio bands. Everyday Practical Electronics magazine (EPE) in the UK had an Atmospherics monitor that was a receiver for the VLF band in their April 2003 issue (www.pemag.au/ projects-legacy.html). One of the difficulties with VLF and ELF receivers is that the wavelengths are so long (15,000km for 20Hz and 15km for 20kHz). This makes using a ¼-wave whip antenna impractical; any antenna of any practical length will be so much shorter than this that it will only provide a low signal level. An alternative to a ¼-wave whip is the Marconi “T” antenna. One suitable for ELF to VLF waves comprises an 11m high antenna with a 45m beam at the top. However, this is still very large and is certainly not portable. See siliconchip.au/link/ac8q for more information on these antennas. We envisaged using a loop antenna that gave comparable reception to a Marconi ‘T’ antenna but without the huge size. It is based upon “AN EASY VLF LOOP, 200Hz-20kHz reception without transformers” by R. Romero & M. Bruno (see www.vlf.it/easyloop/_ easyloop.htm). Another advantage of a loop antenna is that it is directional, so interference noise, especially mains-derived noise, can sometimes be nulled out to a large extent. Radio design Natural radio VLF and ELF band receivers are rather unique because most of the frequency range of the VLF to ELF bands is within the audio frequency range of 20Hz-20kHz. This allows the signals to be directly heard by simply converting the electromagnetic waves to sound using headphones or earphones after the signal from the antenna has been amplified sufficiently. Radio receivers designed for the AM broadcast band (530-1700kHz) and higher frequencies are well above the audio band. In these cases, audio signals are used to modulate the high-­ frequency carrier. Modulation varies the carrier level for AM (amplitude modulation) or the frequency for FM (frequency modulation). The receiver demodulates the received signal to recover the audio. Receiver block diagram The loop antenna is held on a timber frame and is intended to be portable so you can find an ideal place to use it. Fig.6 shows the block diagram of our Earth Radio. The loop antenna receives Australia's electronics magazine siliconchip.com.au 64 Silicon Chip Parts List – Earth Radio Fig.7 shows the full circuit. It uses seven op amps in three packages (one single, one double and one quadruple). Some provide amplification, some active filtering, with another to drive headphones or an earphone via current-­boosting transistors. The receiver is powered either by a 9V battery or an external 12V DC supply. When a DC plug is inserted into barrel socket CON4, its internal switch disconnects the 9V battery’s negative terminal from circuit ground. Without the plug inserted, the 9V battery is 1 double-sided, plated-through PCB coded 06110251, 97 × 70mm 1 105 × 80 × 40mm Hammond RM2005LTBK, Multicomp MP004809 or RS Pro ABS translucent enclosure [RS 198-1379, Mouser 546-RM2005LTBK] 1 9V battery with matching snap (BAT1) 1 9V battery holder clip [Altronics S5050] 3 3.5mm stereo PCB mount jack sockets [Altronics P0092, Jaycar PS0133] (CON1-CON3) 1 2.1mm or 2.5mm inner diameter PCB-mount DC power socket (CON4) 2 SPDT right-angle PCB-mount sub-miniature toggle switches (S1, S2) [Altronics S1421] 2 8-pin DIL IC sockets 1 14-pin DIL IC socket 1 M3 × 5mm panhead or countersunk screw and nut 1 100mm-long cable tie 2 1mm PCB pins (optional) Potentiometers 3 100kW top-adjust 3296W style trimpot (VR1, VR2, VR8) 2 50kW top-adjust 3296W style trimpot (VR3, VR5) 1 2kW top-adjust 3296W style trimpot (VR4) 1 10kW top-adjust 3296W style trimpot (VR6) 1 10kW logarithmic taper 18-tooth spline 10mm horizontal PCB-mounting potentiometer (VR7) [Altronics R1935, Jaycar RP8756] 1 13mm knob for VR7 Semiconductors 1 OP07CP low-noise precision op amp, DIP-8 (IC1) [Jaycar ZL3974] 1 TL074 quad JFET-input op amp, DIP-14 (IC2) 1 TL072 dual JFET-input op amp, DIP-8 (IC3) 1 BC337 45V 0.8A NPN transistor, TO-92 (Q1) 1 BC327 45V 0.8A PNP transistor, TO-92 (Q2) 2 1N4148 75V 200mA silicon diodes, DO-35 (D1, D2) 1 1N5819 30V 1A schottky diode, DO-41 (D3) 1 3mm LED (LED1) Capacitors 4 470μF 16V PC electrolytic 3 100μF 16V PC electrolytic 1 22μF 16V PC electrolytic 1 2.2μF 16V PC electrolytic 1 1μF 16V PC electrolytic 3 100nF 63/100V MKT polyester 4 47nF 63/100V MKT polyester (with closely matched values; see text) 1 33nF 63/100V MKT polyester 2 1.5nF 63/100V MKT polyester 1 1nF 63/100V MKT polyester 1 15pF NP0/C0G ceramic Resistors (all ¼W ±1% axial unless noted) 1 100kW 2 30kW 2 4.7kW 2 1kW 1 150W 1 82kW 7 10kW 1 1.5kW 1 620W 2 1W ½W (±5% OK) Parts for loop antenna 4 20 × 12 × 2400mm dressed all-round timber batten (hardwood for outdoor use) 1 8mm diameter, 1.2m-long timber dowel 1 3.5mm stereo jack line plug 1 3m length of twin-core shielded audio cable 3 0.63mm-diameter, 36m-long enamelled copper wire spools (105m total length) [Altronics W0406, Jaycar WW4018] OR 2 0.5mm-diameter, 57m-long enamelled copper wire spools (114m total length) • [Altronics W0405, Jaycar WW4016] 1 50mm length of 1mm diameter heatshrink tubing 1 wire clamp or cable tie (see text) • will likely result in reduced performance compared to 0.63mm-diameter wire siliconchip.com.au Australia's electronics magazine low-frequency electromagnetic waves, and IC1 converts the current from the antenna to a voltage. The signal is amplified using a low-noise operational amplifier chip (op amp). It also includes signal roll-off above 22kHz. This removes signals that could otherwise overload the receiver. The signal is then sent to a low-pass filter to further remove signals above 22kHz (as the first filter is not perfect) before passing through a notch filter. This notch filter removes the mains frequency and prevents mains hum from dominating the received signal. The notch can be set to attenuate either 50Hz or 60Hz, depending on the local mains frequency. Switch S2 selects the signal from before or after the notch filter. You can also minimise mains hum by adjusting the orientation of the loop antenna. This may not be successful in built-up areas, where there are sources of mains radiation in almost every direction. A further gain stage based on IC3 provides more gain, up to 51 times, set using VR5. The recording output signal level can be adjusted with VR6, while the volume to the headphone amplifier is adjusted with VR7. Circuit details December 2025  65 connected to and powers the circuit. Diode D3 prevents reverse polarity connection current flow, while power is switched via toggle switch S2. LED1 lights via the 1.5kW series resistor when power is on. A half-supply rail (around 4.5V with a 9V battery) is used to bias signals so that they can swing symmetrically about this reference before clipping to the supply rails. This half-supply rail is derived by two 10kW resistors across the main supply, with the junction decoupled to ground with a 100μF capacitor. Op amp IC2d buffers this half-­ supply voltage, and this buffered reference (+4.5Vb) goes to non-inverting pin 3 of op amp IC1 via a 4.7kW resistor. IC1 is a low-noise op amp that is used to convert the alternating current in the loop antenna to a voltage. The loop antenna is AC-coupled by a 470μF capacitor at one end to the non-inverting input of IC1, with the other end connecting directly to the inverting input (pin 2). The signal level is set by the 4.7kW feedback resistance connected between the op amp output at pin 6 to the inverting input (pin 2), in conjunction with the current generated in the loop. The op amp output level adjusts so that the inverting input voltage matches the non-inverting input. The op amp input offset voltage and input offset current will affect how close they are. Both are low due to IC1 being a precision op amp. A 1nF capacitor between pins 2 and 3 of IC1 shunts high-frequency noise, while the 1.5nF capacitor across the 4.7kW feedback resistor provides a high-frequency roll off at 22.6kHz. A roll-off is also inherent in the op amp itself, as it has limited bandwidth beyond audio frequencies, so is not capable of providing a signal output at AM broadcast frequencies. This also applies, to a lesser extent, to other op amps used in the circuit. The OP07 has a typical noise specification of 9.6nV/√Hz. The noise current specification for this op amp is also very low, typically below 1.7pA/√Hz. For this design, having a low noise current is important since we are amplifying the loop antenna current rather than the voltage, and we don’t want noise to swamp the signal. The 470μF capacitor that AC-­ couples the antenna loop to IC1 prevents large DC shifts in the op amp output when the loop antenna is moved or when the loop wires move in wind. These small antenna movements can otherwise generate very low-frequency signal due to movement within the Earth’s magnetic field; enough to result in signal clipping unless prevented by the low-frequency roll-off of the capacitor. The output of IC1 is fed to IC2c, which provides the active part of the 22kHz low-pass filter. It attenuates signals above 22kHz so that higher-­ frequency signals do not swamp the wanted VLF and ELF waves. The filter is a third-order multiple-feedback arrangement. The filter components were chosen to produce a steep roll-off above 22kHz, but this is at the sacrifice of having a small amount of ripple in the passband, below 22kHz. This is called a Chebyshev filter. The ripple in the design is minimal, though, at a maximum of just ±2dB. For our design, we obtain an overall 85dB per decade roll-off due from IC1 and the 22kHz low-pass filter combination. The third-order filter itself provides a much steeper roll-off compared to a first- or second-order filter. Designing these filters is made easier using the filter design tools from siliconchip.au/link/ac8r Following the low-pass filter is the active Twin-T filter used to notch out and severely attenuate the mains frequency, based on IC2a & IC2b. This can be tuned to 50Hz or 60Hz to match Fig.7: the full circuit of the Earth Radio, which is laid out similarly to the block diagram (Fig.6). IC1 is a low-noise precision op amp, IC2 is a quad JFET-input general-purpose op amp and IC3 is a similar dual op amp. 66 Silicon Chip Australia's electronics magazine siliconchip.com.au the mains frequency in your location. The twin-T comprises two T sections, with one half being VR1, VR2 and the two parallel 47nF capacitors. The other half consists of the two series-connected 47nF capacitors and VR3. The notch frequency in Hz for the first tee is 1 ÷ (π × [VR1 + VR2] × 47nF). VR1 and VR2 are set to the same value: 68.1kW for a 50Hz notch, or 56.2kW for a 60Hz notch. For the second tee, the frequency is 1 ÷ (4π × VR3 × 47nF), so VR3 is set to 34kW for the 50Hz notch, or 28.1kW for 60Hz. Typical 47nF capacitors are rated at ±5% or even ±10%, so unless you buy special ±1% (or better) 47nF capacitors, they need to be chosen so that the values are all within 1% of each other. More on selecting them later. If the average is above or below 47nF, VR1, VR2 and VR3 can be adjusted to set the notch to the correct frequency. The main thing is that they are all close in value. You can find the Twin-T filter calculations are at siliconchip.au/link/ac8s The depth of the notch filter is adjustable using VR4. It can set the notch so that the rejection level is deep, with VR4 clockwise for a value of 220W. It is less deep with VR4 adjusted fully anti-clockwise (2kW). The PCB is relatively compact but still easy to assemble. We provide this adjustment since it is easier to adjust the notch frequency when the notch is not too deep. Once the frequency is set, increasing the notch depth will reduce the 50Hz or 60Hz hum further. This will also narrow the notch so that frequencies on either side of the notch frequency will be less affected by attenuation. Switch S1 switches the notch in or out. In the ‘out’ position, S1 selects the signal directly from the output of the 22kHz filter at pin 8 of IC2c. In the ‘in’ position, the signal is selected from the notch filter output at the output of IC2a. The frequency response is shown in Fig.8, giving a high-frequency rolloff at around 85dB/decade, with the 50Hz notch around 80dB down. More gain IC3a provides more signal gain, which is set using VR5. The maximum gain of 51 times is with VR5 set at its maximum resistance. The amplifier’s input signal is already biased at half supply by its source; the 22μF capacitor coupling the 1kW resistor from pin 2 to ground charges to the average DC voltage of pin 3 and adds a low-­ frequency roll-off at 7.2Hz. The output from IC3a is also AC-coupled to prevent DC current flowing in VR6 and VR7. This prevents DC voltage shifts when making adjustments with these potentiometers. VR6 sets the recording level at the 3.5mm output jack socket, CON2. VR7 sets the volume of the signal level Earth Radio Kit (SC7582, $55): includes the PCB and everything that mounts on it, plus the antenna jack plug. The case, battery, power supply and antenna parts are not included. siliconchip.com.au Australia's electronics magazine December 2025  67 applied to headphone amplifier IC3b. The wiper of VR7 is AC-coupled to IC3b’s non-inverting input and biased to half supply via a 100kW resistor connecting to IC2d’s output. The bias voltage from IC2d is additionally low-pass filtered by a 1kW resistor and 100μF capacitor to prevent oscillation of the overall circuit. This can occur when the half-supply reference for the circuit varies with the signal level. Without the filtering, the half supply at IC1’s pin 3 input from the IC2d output could be modulated by the signal at VR7’s wiper, causing feedback and oscillation. IC3b works in conjunction with buffer transistors Q1 and Q2 to drive headphones or earphones. The output of IC3b drives Q2 directly, in emitter-­ follower mode, with Q1’s base set at approximately two diode voltage drops higher. Diodes D1 and D2 ensure that both transistors are conducting some current even with no signal by applying sufficient bias voltage to their base-emitter junctions. There is always a small voltage across the 1W resistors, which minimises crossover distortion during the period when output drive current hands over from one transistor to the next, as the signal passes the half-rail voltage. Feedback to the inverting input of IC3b also minimises distortion by correcting the signal output to match that of the signal applied to the non-inverting input. The presence of the 1W resistors also stabilises the quiescent current via local negative feedback. Current through the bias diodes is set by 10kW resistors in series with them from both supply rails. Trimpot VR8 is included to reduce this bias current should the total diode voltage be significantly higher than the sum of the transistor base-to-emitter voltages. This could otherwise cause high quiescent current and transistor overheating. VR8 is normally set to its maximum unless the quiescent current of Q1 & Q2 is too high. Adjusting VR8 for a lower resistance will bypass some of the diode current and reduce the resulting forward voltages. This can be used to account for differences between different batches of diodes and transistors. The output from the headphone amplifier, at the junction of the two 1W resistors, is AC-coupled to headphone socket CON3 using a 470μF capacitor to remove the half-supply voltage, preventing a direct current flow through the headphones. Another 470μF capacitor bypasses the power supply. Several 100nF capacitors and a 470μF capacitor also bypass the supply for the op amps throughout the circuit for stability, providing a low supply source impedance for each device. Selecting the 47nF capacitors As previously mentioned, the 47nF capacitors for the Twin-T filter need to be selected so their values are within 1% of each other. Typically, if you buy 5% plastic film capacitors on a bandolier (cardboard tape/belt), the adjacent components will have similar values. We found that four capacitors of the same marked value in a row weren’t within ±1% of the actual 47nF rating, but whatever value we measured for the first one, the other three would all measure within 1% of that. You may need to get more than four capacitors so that at least four will be of a similar value. That’s still a lot cheaper than purchasing 1% capacitors. If you have a capacitance meter, the values can be measured directly. Alternatively, if you have an oscilloscope or frequency meter, the capacitors can be tested using a standard astable oscillator made with a 555 or 7555 timer. The frequency of oscillation is related to the capacitance. Fig.9 shows the circuitry required. Using 10kW for RA and RB, the frequency of oscillation would be around 1023Hz (ie, just over 1kHz) for a 47nF capacitor. Note that the oscillator frequency doesn’t accurately tell us the capacitance value. However, if you select capacitors that give the same frequency to within 1%, the capacitor values will be within 1%. This means you need a spread of less than 10Hz for the configuration shown. The easiest method is to measure the frequency of all four capacitors and then subtract the lowest reading from the highest. If the number you get is no more than 10, you’ve found a set of capacitors that’s close enough. Otherwise, measure a fifth and then remove whichever value is the furthest from the others and repeat until you get a spread of no more than 10Hz. Next month The follow-up article next month will start with the PCB assembly instructions for the Earth Radio. After that, we’ll describe how to build the loop antenna, then testing the Earth Radio, followed by some advice on SC getting the best out of it. Fig.8: the frequency response of the Earth Radio, as determined by simulation. You can see the 50Hz notch and the high-frequency roll-off. Fig.9: if you don’t have an accurate capacitance meter, this simple circuit can be used to check how close a set of capacitors are in value, using a frequency meter. Australia's electronics magazine siliconchip.com.au 68 Silicon Chip SAVE 15% Great for crafts and hobbies too! 50 $ T 1463 NEW! 49.95 $ T 2181 SAVE 21% 5 In 1 Plier, Cutter & Crimper Interchangeable magnetic heads with ultra secure fit for even the toughest of jobs. 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Shop in-store at one of our 11 locations around Australia: WA » PERTH » JOONDALUP » CANNINGTON » MIDLAND » MYAREE » BALCATTA VIC » SPRINGVALE » AIRPORT WEST QLD » VIRGINIA NSW » AUBURN SA » PROSPECT Or shop online 24/7 <at> altronics.com.au Build It Yourself Electronics Centre® © Altronics 2025. E&OE. Prices stated here in are only valid until 31/12/25 or until stocks run out. Prices include GST and exclude freight and insurance. See latest catalogue for freight rates. By Tim Blythman Decoder Base Station Using DCC Remote Controller DCC Booster DCC Decoder for model locomotives DCC is a great way to control model trains in a realistic fashion. You need a DCC Decoder for every locomotive and a base station to produce DCC signals on the track. This DCC Decoder project provides the first essential part. We will follow up with a DCC Base Station project and a guide to getting started with DCC using these in our next issue. Image source: https://pixabay.com/photos/steam-locomotive-model-series-23-3627896/ F or many years, model railways were limited to a simple DC system. The two rails formed conductors that carried power from a controller directly to the motor of a model railway locomotive. The controller would simply provide a voltage and polarity to control the loco’s speed and direction. It’s simple and elegant, but only allows one train to be operated at a time. Of course, real railways run more than one train simultaneously and, over time, different technologies have been used to allow this. One wellknown example divides the track into sections called blocks. A switch panel routes power between the blocks and multiple controllers. As you can imagine, this can get very complex. In the late 1980s, Lenz Elektronik of Germany developed the early versions of what would later become digital command control or DCC. The US National Model Railroad Association (NMRA) took an interest and developed DCC as a standard under its auspices. The NMRA had previously worked on standards relating to matters such as wheels and trackwork for model railways, intending to foster 70 Silicon Chip compatibility and interoperability between manufacturers. With its promotion as a standard, DCC continues to be one of the most popular systems for operating model railways. The NMRA continues to develop and publish updates to the DCC standards and these, along with their other standards are available to download from siliconchip.au/link/ac7w We have published several articles related to DCC, including feature articles and projects for boosters, programmers and even a computer-based base station. However, this is the first DCC decoder we have published, which means that all you need to build a complete DCC system can now be found within Silicon Chip. The Complete Arduino DCC Controller can round out a working system (January 2020; siliconchip.au/ Article/12220), but it relies on a computer, so might be excessive for readers looking for a straightforward starter system. Thus, we have also designed a standalone Base Station based on a Raspberry Pi Pico 2 microcontroller and touch panel LCD, which we will introduce next month. Australia's electronics magazine That unit can control up to five locomotives when paired with a suitable power supply, and can also perform decoder programming; more than enough for most small layouts. If you are not familiar with DCC terminology, refer to our glossary later in this article. DCC operation The Arduino-based DCC Programmer from October 2018 (siliconchip. au/Article/11261) included a panel on how DCC works. Fig.1 is a comparison between so-called DC operation (using a controller providing an analog DC voltage) and DCC operation. Fig.1(a) shows DC operation. This project deals with the decoder shown in Fig.1(b). Its main role is receiving power and data as a varying voltage sent from the DCC base station, and decoding the data to drive the motor and function outputs. The latter might be incandescent globes or LEDs. Note that the motor and lights are not part of this project; they will be part of the locomotive to which you are adding the Decoder! Our Decoder and Base Station can siliconchip.com.au Features & Specifications 🛤 Suitable for N and HO/OO scales 🛤 Has support for a keep-alive capacitor 🛤 Long, short and consist addressing 🛤 Acceleration control and speed curve adjustment 🛤 Configurable lighting outputs and flashing effects 🛤 Voltage compensation option 🛤 Supports operations, paged, physical and direct programming modes 🛤 Supports 14-step, 28-step and 128-step speed control 🛤 Robust design with current limiting on all outputs Fig.1: (a) shows the typical wiring inside a DC-only (non-DCC) locomotive, while (b) shows how this changes when a DCC decoder is fitted. If two locomotives were on the track in (a), their motors would run together. The decoder allows the lights and motors in the two locomotives to run independently. The vertical red lines show where wires need to be cut to install the Decoder. be the electronic basis of an economical but complete DCC system. The Decoder design is robust, with current limiting provided for both the motor and function outputs. Commercial DCC decoders include a multitude of features, including sampled and synthesised sounds and complex lighting effects. Our Decoder provides some basic lighting effects and is an inexpensive option if you assemble it yourself. We’ve also designed the Decoder with the idea that our readers might want to extend or modify it by adding circuitry. Thus, the Decoder provides connections for rectified DC from the track as well as regulated 3.3V, something not commonly seen on commercial decoders. Alternatives These days, RF (radio frequency) controls have become more accessible and microcontrollers more powerful. We have published a few projects relating to RF control of model railways, including Les Kerr’s Battery-Powered Model Train from the January 2025 issue (siliconchip.au/Article/17607). siliconchip.com.au We have also seen systems based on WiFi. These typically use low-cost ESP8266 or ESP32 WiFi processor modules. Still, we are yet to see uniform standards for these sorts of systems, so you will likely be committed to building all the parts yourself if you choose one of these options. Our DCC Decoder and Base Station have both been designed according to the DCC standards, and tested with their commercial counterparts. So they can be used as part of an existing DCC system, or further expanded in the future with other products. The Decoder A locomotive decoder (also known as a mobile decoder) is intended to be used with a model railway locomotive or perhaps a motorised railcar. It could be fitted into the tender behind a steam locomotive, if there is space, or if that is where the motor is installed. 🛤 Size: 28 × 14 × 4mm 🛤 Input voltage: up to 17V peak 🛤 Motor current: up to 500mA 🛤 Function outputs: four, up to 100mA each There are other types of decoders, such as accessory decoders, that can operate line-side equipment, such as points motors and signal lights. An accessory decoder could even control lights or other effects in model buildings and such. A function-only decoder is another type of decoder that does not drive a motor, but can operate lights or other effects. A typical use would be for internal or marker lights in a carriage, or perhaps to control a smoke generator in a guard’s van to emulate a wood stove. Since a locomotive decoder can also control these functions, there is no reason our Decoder cannot be used as a This tiny DCC Decoder board can be fitted to a model railway locomotive, allowing it to be controlled by a DCC Base station. It’s only 14mm wide (shown here at twice actual size), so is suitable for HO/OO and N scale models. Australia's electronics magazine December 2025  71 function-only decoder in other rolling stock. Still, we expect that many readers new to DCC will be most interested in adding the Decoder to their locomotives as a starting point. As a rough guide to getting started, you will need a locomotive decoder for each locomotive or other item of motorised rolling stock, plus a base station to coordinate them all. This article will present the Decoder design; for those who already operate a DCC system, we’ll provide a wiring guide and some information about its operating parameters; particularly its configuration variables (CVs). This information should be enough to allow our Decoders to be customised and integrated into an existing DCC layout. For those new to DCC, we’ll provide a more complete user guide alongside the DCC Base Station in the next article. Circuit Fig.2 shows the circuit of our four-function DCC mobile Decoder. Some of the connections are marked with colours; these correspond to the wire colours used by the DCC standards. Each connector on the Decoder is just a PCB solder pad. With space at a premium, the expectation is that wires are soldered directly to the Decoder PCB. The DCC track signal is an AC square wave of about 12V-15V in amplitude (24V-30V peak-to-peak) with its frequency varying around 6kHz. The actual voltage is not too important, although our design limits it to about 7V minimum and 17V maximum. Just over 12V is typical, since this is the nominal operating voltage of the motors in many locomotives. The track connections to the Decoder (red and black) feed into bridge rectifier BR1 and then REG1, an MCP1703 3.3V LDO (low-dropout) regulator, providing our nominal 12V and 3.3V rails. The MCP1703 has a maximum operating input voltage of 16V; our 17V maximum track voltage is due to this, with a drop of at least 1V due to the bridge rectifier. Note that some DCC systems operate at up to 22V peak, so you should check this before using our Decoder with an existing system. Some systems can provide a wide range of supply voltages, so it may be possible to simply reduce it. The 3.3V rail powers IC1, a PIC16F18126 microcontroller, at its pins 1 and 14. This is the 14-pin sibling of the 20-pin PIC16F18146 micro we have used in numerous other projects. The firmware we have created will work in a PIC16F18124 or PIC16F18125 (with less memory), but we will supply the PIC16F18126 parts in the kit as it’s only slightly more expensive. The three capacitors provide the necessary bypassing for the regulator and microcontroller, while a pair of 100kW resistors connect pins 10 and 11 of the micro to the track connections. The 100kW resistors limit current flowing into the pins, and allow the micro to safely detect the polarity of the higher track voltage and thus receive the DCC signals. An 11:1 divider (10kW/1kW) on the 12V rail connects to pin 9 on IC1, allowing the voltage on this rail to be measured by the 3.3V micro. IC2 is a DRV8231 3A full-bridge motor driver IC, powered at its pins 1 and 5 from the 12V rail. Digital Fig.2: despite its simple design, the Decoder includes current limiting on its motor and function outputs. The 100W resistor provides charging current for an external capacitor that can feed power back to the circuit via D1, allowing the Decoder to continue operating through brief contact losses, as might occur with dirty track. 72 Silicon Chip Australia's electronics magazine siliconchip.com.au signals from IC1 (pins 5 and 6) feed control pins 2 and 3 on IC2. Pins 6 and 8 are the outputs, which are connected directly to the motor pads on the PCB (orange and grey); it is expected that a brushed DC motor is connected here. IC2 sets our lower voltage operating limit. At about 4.5V, it disables its motor outputs, so we can guarantee operation down to 7V as required by the DCC standards. In practice, we expect that everything will work down to about 6V on the track. The 3.3V rail also connects to pin 4 of IC2 (Vref). This is used by the circuitry inside IC2 to provide a current-­ limiting feature. The motor current flows to ground via pin 7, so a 0.68W current measuring resistor (shunt) between pin 7 and ground develops a voltage proportional to the motor current. An amplifier internal to IC1 multiplies this by 10 and compares it to the Vref voltage. If Vref has been exceeded, the power to the motors is shut off briefly to limit the current. A current of 485mA develops 0.33V, sufficient to trip the current-limiting circuitry, hence the nominal 500mA motor current limit specification, which is well within the limits of this chip. The DRV8231 can also detect short circuits and, to limit damage, it provides a longer shut-off time in that case. The circuitry around N-channel Mosfets Q1-Q4 is for the function outputs. Q1-Q4 are in an open-drain configuration for switching 12V loads under the control of a 3.3V microcontroller. The 10kW gate pull-down resistors ensure that the Mosfets are switched off any time the micro is not driving the control signals from its pins 2, 3, 7 and 8. A typical load would be a light of some sort, such as a string of LEDs and their dropping resistors, or a small ‘grain of wheat’ incandescent globe, connected between the 12V pad (blue) and the function output (white, yellow, green or purple). The 10W resistor between each Mosfet source and ground provides the current-limiting feature. The 2N7002 Mosfets require a source-to-gate voltage of about 2.3V to fully switch on, so if more than 100mA flows through one of the 10W resistors, there is no longer sufficient voltage (out of the 3.3V logic level signals) to turn on the Mosfet, and the current is limited to this level. The Mosfet will not handle a short circuit to 12V indefinitely, even with current limiting, but will survive longer than if it were exposed to a direct short circuit. With four 100mA outputs, a 500mA motor plus some tens of milliamperes consumed by the microcontroller, the Decoder is also kept under the 1A limit of BR1. The remaining circuitry is a so-called keep-alive that allows another larger capacitor to be connected, so that the Decoder can operate over dirty track or in other cases of intermittent This Decoder is fitted with a full complement of the wires that have colours set out in the DCC standards. There is also a small capacitor fitted to the keep-alive connections. DCC power. The capacitor is charged via the 100W resistor, limiting the peak charging current. The power is returned to the circuit when needed via D1, a schottky diode. We haven’t specified a capacitor here, since the circuit is fully functional without it. If needed, the capacitor can be chosen based on the space available in the locomotive. It is completely optional and unnecessary in most cases. This is also a handy location to pick off 12V if needed for other circuitry. The pins needed for in-circuit serial programming (1, 4, 12, 13 and 14) are connected to the ICSP header pads, with the standard 10kW pullup resistors between pins 4 (MCLR) and 1 (Vdd); pin 14 is ground. The 3.3V power and ground connections on the ICSP header can also be used to power other circuitry if required. Connection diagrams Figs. 3 & 4 show these locations on the PCB. Firmware operation 1 double-sided 14 × 23mm PCB coded 09111241, 0.8mm thick 1 PIC16F18126-I/SL 8-bit microcontroller programmed with 0911124A.HEX, SOIC-14 (IC1) 1 DRV8231DDAR motor driver IC, SOIC-8 (IC2) 1 MCP1703A-3302 3.3V SOT-23 LDO regulator (REG1) 4 2N7002 SOT-23 N-channel Mosfets (Q1-Q4) 1 1A SMD bridge rectifier BR1 [MBS4 or CD-MMBL110S] 1 1N5819WS SOD-323 schottky diode (D1) 1 3cm length of 20mm diameter heatshrink tubing Capacitors (all SMD M2012/0805 size MLCC) 2 10μF 25V X5R 1 100nF 50V X7R Resistors (all SMD 1%, M2012/0805 size, ⅛W unless noted) 2 100kW 6 10kW 1 1kW 1 100W The DCC Decoder 4 10W shown at actual size. 1 0.68W ¼W The main role of decoding the incoming DCC signal is performed by an interrupt service routine (ISR) that is triggered every 22μs. DCC uses pulse widths of 58μs (nominal) to encode a binary ‘1’ bit, with widths over 100μs encoding a ‘0’ bit. By counting the time between DCC polarity changes, the bits can be decoded. The Decoder then assembles the bits into packets, looking for a long string of 1s (‘preamble’) as the start marker. Since each byte is preceded by a zero bit, the firmware can easily determine its position within a packet. Each packet also includes a checksum; if this doesn’t match what the Decoder calculates, the packet is rejected as the data has been corrupted. Finally, the packet address (encoded as the first one or two bytes) is used to determine how the packet should be treated. siliconchip.com.au Australia's electronics magazine DCC Decoder Kit (SC7524, $25): includes everything in the parts list December 2025  73 Parts List – DCC Decoder DCC Glossary Accessory Decoder: A DCC decoder that is designed to be used in a fixed location; for example, to control points or signals, or fixed lights like streetlights. They operate within a separate address space to the decoders used in locomotives. Analog: Describes the traditional mode of model railway operation, where a voltage is applied to the track that directly drives the locomotive’s motor via wheel pickups. The analog controller sets the voltage and polarity to determine the speed and direction. Base Station: A device that drives tracks with a DCC signal and has the means to accept user input to control the decoders. Many also incorporate programming features as well. Booster: A device that can drive tracks with a DCC signal but has no user input. It might take its signal from a separate Base Station or other system. A booster is often used to increase the amount of power that can be delivered to the track. Consist: In full-scale railway parlance, a consist is a grouping of rolling stock that operates together and might include locomotives, passenger cars and freight cars. In DCC, it describes a grouping of mobile decoders with a common consist address, allowing them to be operated as though they were a single unit. CV (Configuration Variable): A non-volatile variable (typically stored in EEPROM) that determines a decoder’s operating behaviour. This includes the address (fleet number) that it responds to and how its motors and functions react to user input. DC: See Analog. Decoder: A device that receives power and data from a DCC signal and drives lights, motors etc. Mobile decoders can be fitted to rolling stock, while accessory decoders can control trackside equipment. Keep-Alive: A means of providing power to a mobile decoder if the track contact is poor or intermittent. It usually consists of a capacitor and some circuitry to keep the capacitor charged, feeding power back to the decoder as needed. Locomotive: For DCC, this typically includes any item of rolling stock that is fitted with a motor and mobile decoder, including passenger railcars and motorised tenders. Long Address: A 14-bit address with valid values from 0 to 10239. This range includes all four-digit numbers. Mobile Decoder or Multifunction Decoder: Also known as a locomotive decoder, it is a decoder designed to operate the motor and lights of a mobile item of rolling stock, such as a locomotive, although it could be fitted to a passenger railcar or other powered unit. Operations Mode: A form of CV programming that can occur on an operating track. The programming packets are addressed and can target a specific decoder. This is the only form of programming that can occur while the locomotive is active. Programmer: A device that can perform CV programming. This might include a Base Station that includes programming features or a standalone device like our Arduino-based Programmer from October 2018 (siliconchip.au/Article/11261). Service Mode: Address-only, Physical, Paged and Direct modes do not program decoders by address, so they must occur on a segregated track in service mode to ensure that only the target decoder receives the programming commands. Short Address: A 7-bit address in CV1. It is the only CV that is mandatory. Addresses up to 127 are valid, but values above 111 conflict with the addresses used for service mode programming packets and are best avoided. As well as commands addressed to specific locomotives, there could also be broadcast packets on address 0. These allow all trains to be stopped quickly. Some packets are used to program configuration variables. The 22μs timer is also divided down into 7ms intervals to manage the acceleration and deceleration of the motor. This is a feature of DCC that allows the locomotive speed to be ramped up or down at a rate to simulate inertia. A separate 4Hz timer is used to allow the function outputs to be modulated in various flashing patterns. An 74 Silicon Chip eight-bit CV is used to set whether or not a function output should be switched on, so flash rates from ½Hz to 2Hz are possible, as well as other sequences. Using these timers, the Decoder also monitors how long it has been since it has received a valid packet addressed to it. This can be used to stop the loco if a packet has not been received within a certain period. If the firmware sees a correctly addressed packet, it is decoded and the internal state is updated. This could involve changing the speed or Australia's electronics magazine direction, or switching some of the function outputs on or off. IC1’s internal analog-to-digital converter is used to measure the voltage on the 12V rail. This can be used to compensate the motor and function outputs, to even out variations on the 12V rail. This can happen with dirty track or if the Decoder needs to source current from the keep-alive capacitor. The motor driver is provided with a PWM (pulse-width modulated) signal to vary the motor speed. This raw speed value received is altered by the acceleration, deceleration and compensation features and can also be adjusted by means of CVs that can be used to alter the speed mapping profile. There are also CVs that can be used to remap the function commands (as received in the DCC packets) to specific function outputs, so this is another processing step that occurs, as well as applying any flashing patterns that have been set. CV programming CV programming involves modifying the configuration variables (CVs) to change the Decoder’s behaviour. The CVs are kept in non-volatile EEPROM, so are not lost during a power cycle. The list of CVs supported by our Decoder is shown in Table 1. These operate as described in the DCC standards, except for those set aside for manufacturer-specific features. For example, CV1 is used to hold a 7-bit value that is known as the short address. Thus, changing CV1 allows the Decoder to respond to a different address. This is key to how DCC allows multiple locomotives to operate on the same tracks. Typically, the address will correspond to the fleet number painted on the locomotive or item of rolling stock. The firmware also looks for the specific sequences needed for CV programming and updates the EEPROM and all live operating parameters. Since CV programming can make permanent changes, extra checks are made to validate these commands. One check that is performed (and required by the standard) is to only make changes when two consecutive, identical packets are received. There are several different programming modes, but our Decoder fully supports Address-only, Physical, Paged and Direct modes. These fall siliconchip.com.au Table 1: configuration variables supported CV Notes Default 1 A 7-bit value for the short address (the eighth bit is ignored). Values above 99 are not recommended because some of them conflict with the addresses used for service mode programming. Many systems restrict this to the range 0-99 and use the long address (CV17 and CV18) for all 3- and 4-digit addresses. The default address of 3 is standard. 3 2 Start voltage. This value (as a fraction of 255) dictates the proportion of voltage that is applied at the lowest speed setting. 0 3 Acceleration. A value of 0 means acceleration is disabled (the speed is updated instantly). Different systems are in use, but this Decoder applies a speed step every (7ms × value of CV3). Higher values mean slower acceleration, so a setting of 3 will result in one speed step every 21ms. 0 4 Deceleration. Works the same as Acceleration, but allows a different rate to be applied for deceleration. 0 5 High voltage. This value (as a fraction of 255) dictates the proportion of voltage that is applied at the highest speed setting. If it is 0 or 1, it is treated the same as 255, ie, full voltage. 0 6 Mid voltage. This value (as a fraction of 255) dictates the proportion of voltage that is applied at the middle speed setting. If it is 0 or 1, it is ignored (only the start and high voltages are used in calculations). 0 7 Manufacturer version number (read only). 0x5C (‘SC’) 8 Manufacturer identification number (read only). The value of 13 has been allocated by the NMRA for the identification of DIY decoders. 13 11 Packet timeout. This value dictates how many seconds a decoder will maintain speed without receiving a validly addressed packet. When set to 0, the decoder will not time out and will continue to maintain speed. 0 17 Most significant bits of the long address. The long address is nominally 14 bits and the top two bits of CV17 are set. The range of valid values for CV17 is restricted to 192-231, allowing the meaningful bits to represent the values 0-39. This gives an address range from 0-10239, which includes all four-digit decimal numbers. 192 18 The least significant bits of the long address. 0 19 Consist address and direction. Consisting allows multiple locomotives to be allocated to a train under the control of a single address, the consist address. The lower seven bits form a short address that can be used for consist addressing. When these seven bits are set to 0, consist mode is off; otherwise, the Decoder will respond to its consist address instead of its short or long address. When the upper bit is set, the locomotive operates in the opposite of its normal direction (eg, for running back-to-back with another locomotive). 0 29 Basic configuration. The bits in this CV each correspond to a basic configuration setting. Our Decoder implements: Bit 0: When set, the locomotive reverses its direction of travel. Bit 1: When set, the Decoder interprets baseline speed packets as 28-step; when clear, as 14-step. The Decoder will also respond to 128-step packets at all times. Bit 5: When set, the Decoder responds to its long address; otherwise, it uses its short address. 2 The default values for CVs 33-37 are 1, 2, 4, 8 and 0 respectively, meaning the functions are mapped across each row. It is possible to configure multiple commands to control a single output or for a command to control multiple outputs. 33 Headlight forward command → F0F output (RA5) 1 34 Headlight reverse command → F0R output (RA4) 2 35 F1 command → F1 output (RC3) 4 36 F2 command → F2 output (RC2) 8 37 F3 command → Mapped to none 0 CVs in the range 47-64 are free for custom use by designers. We have implemented them as follows. 47 Voltage compensation. When enabled, the Decoder adjusts the PWM duty cycle to counteract changes in the supply voltage. Bits 0-3 (values 1-15): when set, the Decoder sets the function outputs to match the voltage to the setting. For example, if the Decoder’s supply voltage (measured at the bridge output) is 14V and bits 0-3 are set to 7 (0111), then the functions will be driven with a 50% duty cycle output. Bit 4: when set, the decoder compensates the motor PWM to make the full speed value correspond to 16V. When either value is set to 0, the corresponding compensation is disabled. 0 For CVs 49-52 each bit corresponds to a time slice of ¼ second over a two-second period, with the MSB occurring first. A set bit means the function output is on and thus different values will give different flash patterns. Where two different values are provided, they will operate out of phase. All values 0-255 are valid, see the right-hand column for what some example values will output. 255: solid on 85 or 170: 2Hz 51 or 204: 1Hz 15 or 240: ½Hz 49 F0F output (RA5) → Flash pattern 255 50 F0R output (RA4) → Flash pattern 255 51 F1 output (RC3) → Flash pattern siliconchip.com.au 52 F2 output (RC2) → Flash pattern Australia's electronics magazine 255 December 255 2025  75 Figs.3 & 4: there is little room on the PCB for silkscreen markings, so you will have to pay close attention to these overlay diagrams, shown at ~400% scale. The Decoder will typically be wired into a locomotive as shown here. If using lamps, the polarity doesn’t matter & series resistors are not needed. into the category of what is known as service mode programming. These modes do not use the Decoder address, so are typically done on a segregated programming track to target a single Decoder. They would most often be used during initial setup of the locomotive after the Decoder is installed, before it is placed on the mainline track. Our Decoder also supports the long form of operations mode programming, which is intended to be used on a mainline track. It thus includes an address parameter, to ensure that the commands are directed to the correct decoder amongst the many that might be on the main track. This mode could be used to tweak 76 Silicon Chip the speed or acceleration profile and then perform testing without having to shuffle the locomotive to and from the programming track. As such, this mode does not permit changes to critical CVs like addresses. Decoder assembly The Decoder is built on a double-­ sided PCB coded 09111241 that measures just 14 × 23mm. It has been deliberately kept as small as possible, to fit in tight spaces (it’s even thinner than a normal PCB at 0.8mm), so there isn’t much room for designators on the silkscreen. You will have to closely follow the overlay diagrams (Figs.3 & 4) to ensure all parts are fitted correctly. Australia's electronics magazine The passive components are all M2012/0805 size (2.0 × 1.2mm), while the rest are in SOT-23 or SOIC packages. None are too difficult to solder, but it would be ideal for prospective constructors to have some SMD experience. We recommend that you have on hand flux paste (ideally in a syringe), solder-wicking braid, a magnifier, finetipped tweezers and the usual SMD gear. We found it easier to start assembly with the top side of the PCB; this has the track and motor connections (M and T) and the SOIC-8 chip, IC2. A PCB holder or some Blu-Tack will be handy to secure the board and prevent it from moving around. IC2 has an exposed pad on its underside. We have placed a large hole under that pad so that you can flow solder through from the other side, or melt solder paste via that hole. To ensure that solder will flow through this hole from the other side of the board, add plenty of flux paste to the pad under the IC and inside that hole. That should help to draw solder through from the other side of the board. Solder this first so that you can check that this joint is secure before starting on any other pins. This could also be soldered with a reflow oven, if you have one. We secured the chip flat against the PCB with polyimide (Kapton) tape and then flipped the board over to feed solder through the pad hole on the other side. Flow solder into the hole and apply heat for a few seconds. If the chip is properly soldered, after removing the tape, the chip remains in place. Then solder the remaining leads. After that, fit REG1 as shown. The single diode, D1, has its cathode towards the bridge rectifier and track connections. The two capacitors on this side are both 10μF parts. Once they are in place, solder the bridge rectifier, being sure to orientate it correctly. Follow with Q4 and the resistors on this side, being careful to match their values. Flip the PCB over and solder Mosfets Q1-Q3 and IC1. Note that both ICs have their pin 1s towards the centre of the PCB. The sole 100nF capacitor is in one corner of the PCB. The seven remaining resistors can then be fitted. Clean off the excess flux with an appropriate solvent and allow the PCB to dry, inspect it carefully and rectify any siliconchip.com.au We have wired up this compact N-scale motorised chassis (Kato 11-107) using our Decoder, allowing us to run tests on a short length of track. We removed two folded metal strips from inside the chassis to break the electrical connection between the track pickups and the motor. For clarity, we have left off the heatshrink, but we recommend you fit it to prevent short circuits. For views inside the mechanism, see www.hookstonemodels.co.uk/dcc-conversion/kato/11-107-chassis solder bridges or dry joints before powering up the Decoder. Programming IC1 If you have purchased a kit or programmed IC, IC1 will be programmed and programming will not be needed. Otherwise, you can use a Snap, PICkit BASIC, PICkit 4 or PICkit 5 to program the 0911124A.HEX file onto the chip. We were able to program one of our prototypes by fitting a 5-way pin header strip to a PICkit 5’s header socket and then pressing that against the Decoder’s ICSP header pads. We used the PICkit 5 to provide 3V power. If that doesn’t work, then you may need to temporarily solder a header strip to the Decoder’s ICSP header pads. The pad spacings are slightly narrower than 0.1in (2.54mm), but a standard 0.1in header can be made to fit without too much trouble. Testing If you would like to run some tests on the Decoder, you can apply power from a 9V battery to the ‘T’ pads that feed into the bridge rectifier. Because of the bridge, the polarity does not matter. This should power up the Decoder and you should see about 7.5V across the keep-alive pads. The output of the 3.3V regulator should be available on pins 2 and 3 of the ICSP header. You can see these connections in Figs. 3 & 4. If these voltages are present and nothing is getting hot, all is as well as can be expected. Connections The connections to the Decoder are shown in Figs.3 & 4, with the wire colours chosen to match those set out in the DCC standards. The other siliconchip.com.au connections are not part of the standard, but could be useful. If you have previously installed a decoder in a locomotive, these colours should be familiar. There is a lot of detail in fitting a Decoder to a locomotive. We can only provide some general advice, but anyone with a reasonable understanding of electronics should be able to figure it out. It’s a good idea to check the voltage and current demands of the locomotive and any additions before making any changes. We expect that LEDs and most small, modern motors will satisfy the requirements, but incandescent globes and older motors may draw more current than the Decoder can provide. One of the most important things is to ensure that all existing connections between the wheel pickups (to the track) and the motor or lights are broken. An existing connection may conflict with a connection made by the Decoder, causing a short circuit. In Fig.1(a), the red lines show the wires that might typically need to be cut; note that the diodes will probably be discarded in this case, as the Decoder provides rectified DC to the function outputs. Check the polarity of any LEDs carefully as well, since they may need to be reversed. In practice, the red and black (track) wires will go to the wheel pickups on the locomotive. They are effectively interchangeable, but the standards specify that the red wire should connect to the right-hand rail when the locomotive is travelling in the forward direction. The grey and orange wires go to the motor and, again, they are interchangeable in practice. The standards say that it should be wired up so that the locomotive moves forward when the orange wire has the more positive voltage. We suggest you connect these wires in whatever way is easiest. If it turns out that the locomotive drives the wrong way, that can be fixed by simply changing a configuration variable. The blue wire effectively provides a positive supply, with the various function outputs connecting to negative when active. Thus, LEDs should have their anodes connected to the blue wire; common-anode LEDs are typical. There is usually enough voltage to allow two or maybe three LEDs to be connected in series with a single ballast resistor. The default configuration makes the white wire active when the locomotive is going forward and the yellow Fig.5: wire up a Pico 2 and a 9V battery like this to use the Arduino sketch to test the Decoder. The resistors are simply provided for protection against damage to the Pico 2 or a connected computer. You should also connect some lights or a motor as per this diagram to see the outputs working. Australia's electronics magazine December 2025  77 Silicon Chip PDFs on USB wire active when the locomotive is in reverse. Again, it might make sense to use whatever wire is convenient and change the function mapping using the configuration variables. Remember to thread heatshrink tubing over the wires before soldering them, then shrink it into place once everything is tested and operational to prevent accidental short circuits. If you don’t plan on wiring an external capacitor to the keep-alive pads, you might be able to fit another surface-­ mounting capacitor (similar to the 10μF parts already on the PCB) directly across these pads. That will give the Decoder a small extra power reserve. Arduino test sketch The USB also comes with its own case ¯ 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). Conclusion THE FIRST SIX BLOCKS COST $100 OR PAY $650 FOR ALL SEVEN (+ POST) NOVEMBER 1987 – DECEMBER 1994 JANUARY 2005 – DECEMBER 2009 JANUARY 1995 – DECEMBER 1999 JANUARY 2010 – DECEMBER 2014 JANUARY 2000 – DECEMBER 2004 JANUARY 2015 – DECEMBER 2019 OUR NEWEST BLOCK COSTS $150 → JANUARY 2020 – DECEMBER 2024 WWW.SILICONCHIP.COM.AU/SHOP/DIGITAL_PDFS 78 Silicon Chip We are providing an Arduino test sketch (DCC_ADD3_TEST) that can be used if you have a locomotive (or motors and lamps) and Decoder, but not a base station. It is written for a Pico or Pico 2 using the arduino-pico board profile. It is based on the code we have developed for the upcoming Base Station. The sketch produces a logic-level DCC signal between GP0 & GP1 pins and expects power to be provided by other means. We can use the keep-alive connections for this purpose; something like a 9V battery would work and provide a limited current. Note that the polarity is important when connecting to the keep-alive pads. Fig.5 shows the wiring needed to run a test with the Arduino sketch. You will also need to connect a motor or lights to see any effect. The sketch produces commands with the default address of 3, and drives the motor outputs in both the forwards and reverse directions. The function outputs are activated in turn, with the actions shown on the serial monitor. Australia's electronics magazine Readers who already have a DCC system will quickly integrate DCC Decoders into their layout and have their own ideas about how to set them up. A DCC Decoder is not much fun on its own, so we will be following up with the DCC Base Station next month. We will also provide more background with a feature article for getting started with DCC in case you have not worked with a DCC system SC before. siliconchip.com.au 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. Pulse-Counting Logic Probe This Probe was designed for debugging vintage computers. It’s useful for systems such as the PET’s character address generation (CAG) system, which has lines that carry numerous, short pulses. Due to the very narrow pulses, there is a chance you could miss one when probing the line with an oscilloscope, and it’s also quite annoying having to count many pulses. I assembled it on protoboard, as shown in the photo. The TIL311 hexadecimal LED displays are siliconchip.com.au readily available on eBay, for around $7-20 each from China. The probe simply consists of two binary counters (74HCT161) that are clocked by the pulse chains being probed. The counting is gated; in the PET, this is typically done using either the HORZ DISP ON pulse or the VIDEO ON pulse. However, the probe can be gated by many other signals in other systems. When the PET’s circuitry is functioning normally, a specific number Australia's electronics magazine of pulses appear in specific time windows throughout the circuitry. So this probe is very handy for checking that everything is working properly. A simple gated logic probe counts pulses during the gate window. At the end of the count, the data is latched in the displays, then shortly after, the counter ICs are cleared. The display is blanked during the counting time, so the visible display appears clean and stable. With two single-digit hex displays, the count can range from 0 to 255, presented as a hexadecimal value from 00 to FF. That ‘byte’ can be readily converted into a decimal value if required. Decimal counter ICs such as the 74HCT160 could be used, but that would limit the count to 99, unless another counter IC and display module were added. Since the probe could be misused by accidentally powering it in reverse or by applying excessive voltages, some protections are provided. If an excessive power supply voltage is applied (say, connecting it to 12V by mistake) or it is applied with reverse polarity, the power zener diode (ZD1) conducts. There is a sacrificial 1/8W 1W resistor that will vaporise and behave as a fuse in that case. It pays to mount that resistor a little clear of the PCB. It drops only around 100-150mV, depending on the display brightness. Diodes D1 and D2 and the resistors across them allow selective delays of leading or trailing pulse edges. This is because the outputs of the 74HCT132 gates have a low resistance and can charge or discharge the small added capacitances quickly via the diodes, but the charge or discharge pathway is slowed down with the resistors to create specific delays. In this case, 500ns and 150ns delays are provided to allow the counter to pick up (count) a pulse at the end of the GATE time, and to latch the count shortly before the counters are cleared for the next pulse counting cycle. The probe has two optional inputs, GATE and GATE, depending on the December 2025  79 polarity of the pulses available to use as the gate pulse. For example, to count pulses at the probe tip while the VIDEO ON signal is LOW, the GATE input would be used. Alternatively, to count pulses while the gating signal is high, such as for the HORZ DISP ON pulse, the GATE input would be used. Because the displays are blanked during counting, they do not run at full brightness. The brightness depends on the duty cycle of the GATE/GATE pulses. Even with a 5-10% duty cycle, the LEDs in these modules are easy to see. This photograph shows me using it to probe a PET computer for debugging purposes. For more detail on how this probe is used in computer repairs, see: https://worldphaco.com/uploads/ PETREPAIR1.pdf Dr Hugo Holden, Buddina, Qld. ($120) 10A ammeter using a low-cost digital DVM (digital voltmeter) Digital LED voltmeters capable of reading up to around 33V are available on sites like AliExpress and eBay for just a few dollars. For example, I found a 5-digit voltmeter available in various LED colours that obviously autoranges around 4.3V, and fairly seamlessly as well. They are advertised as being suitable for automotive use. I acquired one and checked its accuracy with my trusty Fluke 87V on the high DC accuracy setting. I was astounded by the accuracy of this unit – the DC voltage matched the Fluke 87V almost digit for digit from 30V down to 0.0001V! I decided this 80 Silicon Chip voltmeter could be the basis for a simple but accurate 10A DC ammeter, to be powered by a stable 12V DC source. One bonus feature I wanted to incorporate was a voltage output that’s proportional to the monitored current. The circuit supply input voltage should be a regulated 12V DC supply, maximum 13V. The power section is connected through CON2 to a 7805 5V linear regulator. Its 5V output is directed to an ICL7660 switched-­ capacitor voltage inverter, producing a -5V rail. That gives a total rail-to-rail voltage of 17V for the OP07 precision op amp IC. The external circuit is connected through CON2, with its high side on pin 2, and current flowing through series-connected resistors to the low side at pin 1. It is independent of and isolated from the power supply to the circuit, although both pins of CON2 must remain within the range -14V to +80V with respect to the GND terminal on CON1. The three low-value resistors have a combined series resistance of 0.21W, and the voltages generated across them are detected by IC1 and IC2 and amplified 50 times by each. These two ICs provide two current ranges, and they are switched by relay RLY1 under the control of LM358 op amp IC4. At power-on, the non-inverting pin 3 input of IC4 is held to 0.5V (TP3) due Australia's electronics magazine to the voltage divider formed by trimpot VR4 and the 12kW fixed resistor. With no external load at CON1, output pin 5 of IC2, and hence pin 2 of IC4, remain near 0V. The pin 1 output of IC4 therefore goes high, activating DPDT relay RLY1 and lighting LED1. This connects IC1’s output to IC3, an OP07 precision op amp set with a gain of approximately two times. The total gain will therefore be around 100 times. With zero input, an INA282 can output go almost to the negative supply rail but not quite – approximately 40mV minimum remains. This is cancelled out by a 4.7MW resistor to the -5V supply rail (its value may need to be slightly lower, eg, 3.9MW, to get 0 output with no current flowing). This is then followed by another 4mV of precise trimming of IC3’s offset voltage by VR2 to measure zero volts on the voltmeter. There is a certain satisfaction in measuring 0.0000V correctly to four decimal places! IC1 detects the voltage generated across the two 0.1W 5W series-­ connected resistors for the range 0-1A. The maximum power dissipated by these at the full 1A is only 200mW. IC2 detects the voltage across the 0.01W Welwyn current sense resistor (Jaycar RR3420). It is a very stable (extremely low temperature coefficient), low-inductance 1% resistor siliconchip.com.au and covers the range 1-10A. Its maximum dissipation at 10A is 1W, and it just passes the ‘finger test’. Of course, it is well within its 3W rating. With 1A flowing through CON1, the two 0.1W resistors in series drop a total of 200mV, which is amplified by IC1 by 50 times to produce 10.0V. Its pin 5 output voltage is reduced by VR3 and its series resistor, to 0.5V, and fed through the contacts of RLY1 to IC3 (if selected). At the same 1.0A current, the 0.01W resistor drops just 10mV. IC2 amplifies this by 50 times, again to 0.5V, and it flows through the other relay contact to IC3 (if selected). The pin 5 output of IC2 is also connected to the inverting input of op amp IC4 (pin 2), and IC4 controls the switching of the relay. With 0.5V on pin 3 of IC4, the range change occurs seamlessly around 1A. The 1MW resistor provides feedback hysteresis so that the relay doesn’t chatter if the current is hovering around 1A. Once it passes the threshold to switch, it must change by a few tens of milliamperes in the opposite direction before the relay will switch back. At the changeover from the low to high range, the other pole of the relay shorts out the two 0.1W resistors. The relay’s contact resistance is unimportant as only the current through the siliconchip.com.au 0.01W resistor is displayed in this case. Shorting out those resistors is important, though. Otherwise, the pair would dissipate a total of 20W at 10A! Calibration is easy once the 4.7MW resistor has been varied (if necessary) to get a 0V output at 0A. Series connect a power supply, a variable resistive load and an accurate DMM on the amps range through CON1. First, apply a current of approximately 2.0A continuously. The relay should switch off, as indicated by LED1. Adjust VR1 until the numerical output of the DMM is matched by the voltmeter connected to CON3. Next, reduce the current to approximately 200mA, changing your DMM to the mA range if required. The relay and the LED should switch back on. Adjust VR3 until the readout on your meter again matches the DMM output. Do not reverse these two sequences. I am impressed with the linearity and accuracy of the INA282 ICs. At 10A exactly, the Fluke 87V started flashing its maximum warning display, and the meter under test registered 9.98A. With a 12V supply, the current displayed actually reached 11.3A before plateauing. At the low end, the display remained accurate down to about 40-50mA, with a 4mA difference apparent below about 25mA. Australia's electronics magazine One distinct advantage of my circuit is that you can run up to 10A continuously without overheating or damage if the circuit is free to air as an unenclosed panel meter. Most multimeters, including my Fluke 87V, can only handle 10A in about 30-­second bursts to prevent overheating or other damage. The current consumption in the circuit is low, about 10mA on the high range, rising to 40mA on the low range due to the power consumed by the relay coil. Colin O’Donnell, Adelaide, SA. ($90) December 2025  81 PART 3: PHIL PROSSER Digital Preamplifier and Crossover Last month, we assembled and tested the three PCBs used in our new Digital Preamplifier, which uses digital signal processing (DSP) rather than analog techniques. It isn’t just a preamp – it can also perform as an active crossover. Now we’ll describe how to prepare the case, mount the modules, wire it up and use it. O nce you have assembled & tested the three Digital Preamplifier PCBs, it is time to prepare the case so we can fit everything into it and wire the boards up. The first job is to drill and cut the holes in the rear panel so that you can get the main PCB into position, as many of its connectors go through that panel. We have provided a drilling guide for this panel in Fig.17. This is a view of the rear panel from the outside, rotated 90° and shown at half scale (50%) so it will fit in the magazine. Ensure you make the measurements from the outside of the panel. We applied a layer of masking tape over the whole rear panel and used a fine felt-tip pen to make a very precise transcription of the cutting and drilling guide. Another option is to print Fig.17 at exactly twice its original size (200% scale), line it up and stick it onto the 82 Silicon Chip panel, then mark the hole positions with a punch or pilot drill. Since the panel artwork at 200% size won’t fit on an A4 sheet of paper, if you don’t have an A3 printer, you could print it across a couple of sheets with some overlap so you can line them up. Centre punch the holes, as you want the drilled holes to be located as accurately as possible. Having marked the hole positions, it’s a good idea to line up the PCB as a sanity check, to make sure they are all correct before drilling. You don’t want to end up with holes in the wrong places! We started drilling with a 1.5mm pilot drill bit to get the initial holes exactly centred, then increased the hole sizes in a series of steps to ensure the centres were accurately located. If you go straight from punching to using a large bit, it will have a tendency to wander initially, so even if you punched the exact centre, the large hole may be offset. Slowly increasing Australia's electronics magazine the size with a series of larger bits mostly eliminates that error. It is important that the screw retention holes for the dual RCA connectors are in the right spots; by comparison, the larger 11mm holes really just need to clear the metal ground connection on the RCA socket, and they can be filed a little larger if needs be. Also make sure that you get the placement on the rear panel at the specified height. This has been selected to ensure clearance between the screw holes in the bottom panel. If the board is mounted too low, you will have possible interference with the PCB. As you test-fit the board, check these clearances on the back of the PCB. As specified, there is about 3mm clearance between the pins on the bottom of the PCB and threaded inserts on the bottom of the case. The IEC and two other square holes are easily made by drilling many small siliconchip.com.au holes and filing these to shape. The panel is soft aluminium, so it won’t take a lot of filing to get right. Check the fuse holder and IEC socket fit as you file, as it is easy to take more off than you want. We countersunk the RCA screw holes using an 8mm drill bit to reduce the protrusion of these screws near the RCA sockets. Once you’ve made all these holes, check that the connectors on the main PCB will fit through those holes without binding. Front panel preparation Now is also a good time to drill and cut the holes in the front panel, as per Fig.18. This is a view from the outside of the front panel. The procedure is much the same, although it should be quite a bit easier this time, as there are just six holes to drill, plus one rectangular cut-out to make. Like with the rear panel, we covered the whole front panel with masking tape and marked the hole locations before drilling. The presence of masking tape has a benefit of reducing the chance of scratching the panel while working it. The front panel drilling is complicated by the need to mount the LCD screen. We have specified a 3D-printed bezel for the LCD that we have used on a couple of projects now. This is in an STL file named “Altronics Z7018 LCD Bezel v8.stl”, which you can download from siliconchip.au/Shop/11/2917 Our cut-out matches this, but if you want to take a different approach to mounting the LCD, you need to consider the actual LCD and how it mounts. The 3D-printed bezel also ensures electrical isolation of the LCD from the chassis. One challenge we faced is that the space between the extrusions on the inside of the front panel leaves only 36mm. This is not a lot of space for a display, which limited our choice. You can use the ‘drill and file’ method for making odd-shaped holes; it is tedious, but the results are generally good. In this case, we used our Dremel rotary cutting tool with a thin metal cutoff disc. We found that being careful with our siliconchip.com.au rotary tool, we were able to cut the vast majority of the LCD hole neatly, and only had to file a little in the corners to get them just right. Remember that the bezel will cover this cutout, so if there is a little roughness around the edges, that will be hidden by the bezel. Mounting the main PCB The locations of the holes that need to be drilled in the base panel are shown in Fig.19; ignore the grid of blue-outlined holes on the left-hand side for now and just make the black outlined holes. These include four for mounting the main PCB (the four holes marked “A” that run down the middle from the top). However, we recommend that once you have drilled the rear panel, you dry-fit the main PCB to it and mark the locations on the base for the four mounting holes on the opposite edge of the PCB. This will ensure that they are in the right positions, so that the board is not stressed once installed. To mount the Digital Preamplifier PCB to the rear panel, jiggle the RCA sockets into the holes drilled for them and secure using a couple of 6mm-long, 4GA self-tapping screws (we used Jiffy box screws from Altronics). Now see where the holes line up with the base of the chassis. We stuck masking tape to the chassis and used this to mark where to drill the mounting holes. Once drilled, secure the PCB to the base using tapped spacers, 6mm M3 machine screws and shakeproof washers. With this in place, you have completed the trickiest part of the assembly process. Power supply assembly Mount the power supply board also using tapped spacers, machine screws and shakeproof washers. It is attached via four mounting holes in a roughly rectangular pattern on the right-hand side of Fig.19. Orientate it so the heatsinks are roughly centred in the case, with the terminal blocks near the main PCB. This requires four 10mm standoffs, 6mm panhead machine screws and crinkle washers. If in doubt, refer to Australia's electronics magazine ◀ Figs.17 & 18: the rear panel drilling details are on the lefthand side of this diagram. This is shown from the outside of the case; note the orientation labels and that this is at 50% scale. Try to position the holes as accurately as you can (there is some leeway, but not a lot). The holes required in the front panel are presented in the righthand side of this diagram. The edges of the large rectangular cut-out will be covered by the 3D-printed LCD bezel. Fig.19: the case bottom panel drilling details. All the smaller holes are 3.5mm in diameter. While we have provided their locations, the four holes for the PCB standoffs for the edge of the main board should be marked and drilled with the board fitted to the rear panel. The blue-outlined holes in the left are the ventilation pattern for the lid, not the base, and are shown rotated 180° compared to the rest to avoid interfering with PCB mounting holes. Fig.21. This board is Earthed through the chassis via an exposed pad under one of the mounting screws. This connection is made through one of the spacers that attaches the Power Supply board to the case and Earths the board through exposed metal under the screw head. Therefore, you should scrape away the paint under the screw head that attaches this spacer to the case to ensure it is Earthed properly. After mounting the Power Supply board, use a multimeter to measure the resistance from the power supply PCB ground to chassis Earth and ensure the reading is under 1W. Before going any further, you will need to make the safety panel from a sheet of 1mm-thick aluminium, Presspahn or similar. This panel will be placed between the mains section and the low-­voltage section. We made ours from a 46 × 225mm piece of 1mm-thick aluminium. The intent of this part is to ensure physical separation of these sections in the case. Cut it to the dimensions shown in Fig.20 and drill the five holes. Next, fold the two tabs up by 90°. Start by scoring along the edge; if it’s made of Presspahn, it should be easy to fold using a ruler. For aluminium, you can clamp the tabs between two pieces of straight timber tightly in a vice, with the bend line right at the top, then carefully bend the part projecting out over until it hits the top of the timber. The folding is most easily done with a little encouragement from a hammer, using a piece of timber against the aluminium to avoid dents. We intentionally cut out a section of the 10mm lip to avoid it running under the transformer. Ensure you cut this out. Australia's electronics magazine siliconchip.com.au 84 Silicon Chip The main PCB is only attached to the bottom of the case along one edge. At the other end, it attaches to the rear panel via numerous screws into the RCA socket housings. This version is using the DSP board and you can see some of the ribbon cables connecting the LCD screen and front panel controls. We used 3mm rivnuts on this section to make assembly easier (they’re basically internally threaded rivets). If you don’t have a rivnut tool, you will need to use nuts, bolts and lock washers to install this. Insert a 9.5mm hole grommet in the large hole to ensure there are no sharp edges that might cut the insulation of the transformer wires running through to the power supply PCB. You will note that we painted ours satin black; this is purely for aesthetics. Make sure this panel is installed prior to the transformer. The mains wiring is made with 7.5A mains-rated medium-duty wire, referring to the wiring diagram, Fig.21: 1. Mount the front panel to the case and install and tighten the power switch, ready for wiring. 2. Also mount and install the IEC connector and fuse holder to the rear panel. 3. Solder a length of green/yellow-­ striped mains-rated wire to a solder lug, then attach the other end to the Earth terminal of the IEC mains input connector. Scrape away any paint on the case around the Earth lug mounting hole, then attach the solder lug with a 16mm M3 machine screw, hex nut and crinkle washers. Tighten it and verify that there is a low resistance from the mains input Earth pin and the chassis’s exposed metal work. 4. Using blue wire, run a Neutral connection from the IEC connector terminal marked “N” through to one of the upper terminals on the DPDT mains switch (not the common terminal). Insulate both these connections to ensure there is no possibility of accidentally touching any live parts (Neutral and Active can sometimes be swapped in house wiring or extension cords). siliconchip.com.au 5. Using brown wire, connect the “A” terminal of the IEC connector to the end connector of the fuse holder. Again, insulate this well. We added a short length of 13mm diameter heatshrink tubing, which we ultimately used to cover the whole rear of the fuse holder. 6. Continue with brown wire from the other terminal of the fuse holder to the switch terminal next to the one you’ve already connected, at the top on the other side. Again, insulate this well. 7. Connect the blue wire from the transformer to the middle terminal of the switch below the existing neutral wire. Similarly, connect the brown wire from the transformer to the terminal below the existing brown wire connection. Insulate both well. 8. Add insulation to the unused terminals on the switch ‘just in case’. Wiring up the power supply Next, loosely mount the transformer, making sure to include two rubber washers underneath it (with one between the transformer and the dish that holds it in place). Sleeve the four secondary wires together with heatshrink tubing after cutting them to a length that will allow them to comfortably reach the inputs of the power supply PCB, without too much slack. Ensure that the two wires that go to the middle two “GND” positions of CON1/CON4 on the power supply board are from opposite ends of the two windings (ie, start/finish and not start/start or finish/finish). You can verify this once the unit is powered up by measuring the AC voltage between the outermost two terminals of those blocks. You should measure close to 24V AC. If you get close to 0V AC, they are not phased correctly, or there is a bad connection. Fig.20: the safety panel shown at actual size. This can be made from aluminium, Presspahn or another thick insulating material. Cut the rectangle to size, remove the notch, drill the holes, then score and bend the two tabs up by 90°. Australia's electronics magazine December 2025  85 N Australia's electronics magazine 470mF 4148 1 + + ~ KBL404 BR1 ~ 1 1 1 470mF 2200mF + 2200mF + F2 1A F1 1A 470mF 10mF 10mF 10mF 100nF 470mF 100nF 100nF 47mF 100kW CON13 47mH 100nF 100kW 100kW J1 J2 + 470mF 470mF + 220mF 4.7kW 10kW J3 10kW 4.7kW 1 miniDSP MCHStreamer + 100nF 100nF 2200mF 100nF 100nF 100nF 470mF 2200mF 100nF 4.7kW 100nF 100nF 10nF 100nF + 10mF 470mF 100nF 100nF 2200mF 2200mF 100mF 10mF 47mH 100mF 47mH 100mF 4004 10mF 4004 220W 10mF 100nF + 10kW 10kW 1mF + + COIL 10nF Microcontroller 10mF 100nF 100mF 100mF 220mF 10kW 10kW 4148 100pF 100kW 100pF 22mF 100pF 4148 COIL 100kW 100nF 100pF 4148 10kW 10kW 10kW 10kW 470pF 22mF 47mF COIL 100kW 100nF DOWN 22nF S2 2025-03-24 5.6nF CLIP 100nF 100nF 150pF 10kW 1kW 100pF 22mF GND UP 22nF S2 + 100mF 100nF CON1 TGM Was Here Mar 2024 1 180W 180W 2.7nF 100nF 22nF RE1 22nF 2.7nF 100nF 2.7nF DAC Ch4 1 100nF 27nF + 200W 1 ITSOP4136 TP1 B A CK S1 22nF 2.7nF IRD1 22nF IR R X CON2 100nF Q14.4 4148 2.7nF 100nF + 200W 100nF 10W 200W 10W 200W 180W 180W 100nF 8.2nF 2.7nF 100nF 27nF + 100mF 2.7nF DAC Ch1 1 2.7nF 100nF 27nF 2.7nF COIL CON8.4 OUT1 100W 100nF RLY6.4 8.2nF 100mF + 100nF 8.2nF 8.2nF 8.2nF 8.2nF 100nF 27nF 2.7nF DAC Ch2 1 100nF CON8.3 OUT2 100mF 10W 200W 10W 200W 180W 180W 27nF + COIL RLY6.3 2.7nF 100nF 2.7nF 100nF 2.7nF 100mF + 200W 100nF 8.2nF 8.2nF Q14.3 4148 8.2nF 100nF 2.7nF 8.2nF 27nF 100mF 10W 200W 10W 200W 180W 180W 100nF Mar 2025 Digital Preamp V2.3a TGM Was Here 2025 100nF COIL RLY6.2 DAC Ch3 2.7nF 100nF 27nF + 100mF 100nF 8.2nF 8.2nF Q14.2 4148 8.2nF 100nF 8.2nF 100mF 10W 200W 10W 2.7nF 100nF COIL RLY6.1 200W 27nF + FOR PCM1794A 200W TO 270W 8.2nF TO 2.7nF 180W TO 0W 8.2nF 8.2nF Q14.1 BC547 4148 FOR PCM1794A 220W TO 560W OMIT 27nF Digital Preamp Controls v1.1 DSP CORE 100nF + + 100mF FOR PCM1794A 2.7nF TO 2.2nF 820W TO 750W 100mF 47mF 10mF BAT85 100mF 100nF 100mF + 100nF ADC 47mF 47mF 47mF 10W 10W 10W 10W 100W 100W 47mF CON8.2 OUT3 5 Note: as with any anodised aluminium, the rack enclosure will not necessarily have the rear, side, front and top panels Earthed due to the anodising providing an insulating layer between the panels. Each panel should be checked for electrical continuity to the bottom Earthed plate. That also applies to the internal safety panel if it is made from metal. Use separate Earth wiring between panels that don’t become Earthed when assembled using Earth wire and crimp eyelets and ensuring the anodising is scraped off at the mounting positions. LCD MODULE 10mF 1.5kW 100nF 10mF 10mF 100nF 100nF 1.5kW 23 34 220W DVDD3.3 12 1 10mF 10mF 4148 COIL 100pF 22mF 22mF INPUT SWITCHING 100pF 100kW 680W 100nF 220W 4004 PIC32MX270F256D-50I/PT GND 47mH 4148 COIL 75W 75W DIGITAL I/O 4.7kW 10kW Power + Supply 4.7kW 4148 4.7kW 4148 4148 + 4.7kW 4004 CON14 5819 22mF 1 2 3 4.7kW 10kW + 22mF 4.7kW 22mF 100kW 4.7kW BAT85 BAT85 BAT85 BAT85 100nF + 100kW + 4004 4004 47kW + + E + A + T1 12V+12V 30VA + 4004 BAT85 100kW 4.7kW 100kW 4.7kW 5.6W 91W 91W BAT85 100pF 100kW 1kW 680W BAT85 BAT85 BAT85 BAT85 680W 680W 100nF 680W 470pF 470pF 470pF CON7 1kW 91W 91W 10kW 1kW 47kW 100nF 47kW BAT85 47kW 100W 4.7kW 10kW 100W 47kW 180W 200W 220W 100nF 22nF 820W 220W 10kW 1kW 10kW 220W 220W 820W 820W 10kW CON8.1 OUT4 10kW CON6 MONITOR OUT 10kW 180W 47kW 100W 4.7kW BC547 220W 820W 820W 220W 220W 100W 47kW 220W 100W 47kW 180W 180W 200W 220W 180W 180W 200W 820W 820W 220W 47kW 100W 4.7kW BC547 220W 820W 820W 220W 220W 820W 820W 220W 47kW 100W 4.7kW BC547 220W 820W 820W 47kW 180W 180W 200W 200W 220W Silicon Chip 820W 86 820W 820W 3 Fig.21: this shows all the low-voltage and mains wiring, which are separated by the safety panel. Make sure you follow the mains wiring details carefully and insulate all exposed terminals, except for the Earth terminals, which may be left uninsulated. Don’t forget to tie the wires down with cable ties. siliconchip.com.au Above: a neat trick to getting the PCB front mounting holes in exactly the right spot on the base of the case is to mount the rear panel to the case, and put some masking tape gently under the standoffs. Push them down and they will magically mark the exact locations to drill. Right: a close-up of the mains wiring. Between the insulation & separating panel, none of the wires can come loose and contact any of the low-voltage circuitry. Now use zip ties to secure all the mains wiring to the base plate using the holes provided, as shown in Fig.21. Ensure there is only a little slack in all the mains wires. Mounting the control board The control PCB is secured to the front panel using the rotary controller’s threaded bush and the supplied nut. The three pushbutton switches need to be jiggled to get their shanks to fit into the holes we drilled. If any were soldered in slightly askew, you will need to adjust them. Having these a snug fit ensures the front panel controls are all solid and steady in use. Once it is mounted, install the washer and tighten the rotary encoder shaft screw. As noted earlier, adding epoxy or silicone sealant around the pushbuttons on the inside of the case will provide increased tolerance to rough treatment. Fit the LCD bezel into the front panel and, once it is neat, glue it in place with a few dabs of neutral-cure silicone sealant. Allow this to set, ready to install the LCD once it has its cable plugged in. Finishing the wiring Next, we need to connect the +5V DC and ±10V DC outputs of the power supply board to the main Digital Preamp board. Use lightweight hookup wire, and select colours that will avoid you making errors in the connection. The connections required are shown in Fig.21, although the exact routing shown is not necessarily ideal; refer to the photos to see how we ran the wires. We ran heatshrink over the groups of wires to ensure things remained tidy, and ran the wires under the Digital The final power supply PCB uses two terminal blocks for the transformer connection, making connecting the AC inputs easier. Also note that in the final boards, we have moved the DC input on the Digital Preamp board in from the edge of the board, so making connections to it is easier. siliconchip.com.au Australia's electronics magazine December 2025  87 Preamp PCB. It is a tight fit to get the wires up and out between the safety screen and the Digital preamplifier PCB – there is a 5mm gap, which is just wide enough to fish them out. In the final design, we have shifted the power connectors inboard from their original position on the Digital Preamplifier board to give you a little more room to get the wires into the screw terminals. We ran the ribbon cables under the power supply PCB as this makes things neat. Fold the ribbon cable so that the connector mates up to the control board and LCD neatly. Ensure that the red stripe of both wires connects to pin 1 at both ends. Fig.21 shows an alphanumeric LCD with a SIL header soldered to an adaptor board to allow the IDC connector to plug in. That is a valid way to connect it, but you will see from our photos that we used an LCD screen with a DIL header that the IDC connector plugged into directly. Whichever way you do it, make sure the wiring is correct such that the GNDs of the two boards are joined. Now attach rubber feet to the bottom of the case to stop the screws in the chassis scratching up your workbench. Lid preparation If the Digital Preamplifier is to be used in a very warm environment, it is a good idea to augment the ventilation in the case lid over the power supply heatsinks. The Altronics H0625 heatsinks specified are dissipating in the region of 2.5W each. They have a thermal resistance to ambient of 10°C/W, which in free air would mean a 25°C rise in temperature above ambient. However, the case compromises this, because the internal temperature will rise above the ambient temperature of the surrounding air. Many of our tests were carried out in a hot room (35°C) and the Digital Preamplifier case was measured to be 45°C. This results in the LM317/LM337 devices sitting at around 67°C. This is well in specification for them, but higher than we would prefer. Using a larger heatsink for these devices didn’t help a lot. The right thing to do is to get the heat out of the case by increasing the ventilation over these heatsinks. We did this by marking and drilling an array of 5.5mm holes, which does not take that long to do. 88 Silicon Chip The process we used was to first mark the locations (see Fig.19), centre punch them all and drill 2mm pilot holes, then the 5.5mm holes, finishing with an 8mm drill bit to deburr the holes. A coat of satin black paint on the lid tidies this up nicely. Labels You can now install the labels for the rear and front panels. We thought about getting the front and real panels engraved, but the logistics and cost put us off. So we came up with some simple labels that we 3D printed. The approach was to print a 1.5mm-thick base layer using black filament, then we have extruded the text for our labels to be 0.8mm higher than this base. When printing these, we wait until the base layers are printed, then pause the printer and change the filament to a contrasting colour. One preamp got gold labelling, and another red. We then used a couple of tiny dabs (a dab is less than a drop) of superglue to affix the labels to the panels. We used a run of masking tape to define the mounting line so all are affixed level and even. We think this is not a bad way to label the Digital Preamplifier. You might come up with another method. Perhaps you’re keen to dust off the old Dymo and give it some of those oh-so-stylish green labels! Using the Digital Preamplifier The Preamplifier works just like any preamplifier, except it has many more features. The first time you use it, make Once your unit has been fully assembled, it should look something like this. You don’t need the extra bits of aluminium we added to the heatsinks. Double-check all the mains wiring and insulation before powering it up. very sure that you set your crossovers to appropriate bands for your drivers, and that you turn the volume down to a sensible level before you power on any connected amplifiers. If you don’t, you might be very surprised by some loud noises, and could damage your expensive speakers. The best way to set up the crossover will come down to what test equipment you have and how you are using it. Some of our active systems are only a subwoofer channel added to a good pair of speakers. It is entirely possible to set up such systems pretty well by ear. On the other hand, if you want to time align your speakers and implement a full multi-channel crossover, you will need some measurement equipment. The first thing to set up is the time alignment of your speakers; after that, the crossovers will behave much as you would expect. We have set limits on the volume control that allow you to turn the volume up to 11 (in fact, the maximum gain is 12dB). Be aware that turning the volume up on this preamplifier will generate no noise unless your input is noisy – so treat that control with respect. The good news is: no more pots that get scratchy over time! Remember that pushing the control knob in saves your settings; this includes the selected input and current volume. So the next time you turn it on everything will load up and the volume will be where you left it. Past this, we hope you enjoy your truly digital preamplifier. Here’s a quick run through the user interface. In the idle/normal state: ● The rotary encoder simply changes the volume. ● Channel selection is via the buttons to the left. ● The ‘back’ button to the right changes the interface into the Function Select state. ● Push on the main encoder saves the current state to EEPROM. In the Function Select state: ● The rotary encoder allows selection of: > Volume > Channel Setup > EQ Setup > Save > Load ● Pushing the encoder selects that function. ● Pressing ‘back’ exits. The Channel setup menu: ● Allows selection of channel 1-4. ● For each channel in turn, you can make live adjustments of: > Low-frequency crossover slope (6, 12, 24 or 48dB per octave) > Low-frequency crossover point: 5Hz to 15kHz > High-frequency crossover slope (6, 12, 24 or 48dB per octave) > High frequency crossover point: 5Hz to 15kHz > Channel attenuation: 0-20dB in 1dB steps > Channel delay: in 1.7mm steps (rounded on the display) > Mono selection: for channel 1, allowing you to mono a subwoofer output ● Invert selection: allowing you to invert the audio output on individual channels The Equaliser Setup: ● Allows selection of Common EQ1-3 and Channel 1-4 Equalisers 1-3 each (15 in total) ● For each of these: > EQ: off or parametric > Centre Frequency: adjustable > Q: from 0.1 to 10 > Gain: -10dB to +10dB (range can be increased by modifying software but this should be enough) Load and save loads or saves the selected set of parameters to EEPROM. Conclusion This is the first fully digital preamplifier we’ve published, and one of the most complex circuits we have described. While it will take some time to assemble, it isn’t an especially difficult job overall, and we think the SC result will be well worth it! December 2025  89 SERVICEMAN’S LOG The Bad Old Days I’ve been a little out of sorts these past few months, what with our winter and general cold and miserable weather. I’m also not getting any younger, and as with all machines, mechanical or biological, they tend to inevitably break down over time. I am no different. The old trope, “at least we have our health”, sometimes doesn’t hold a lot of water. I’m not on my last legs or anything, not by a long chalk (hopefully!), but dealing with any kind of out-of-the-ordinary health status is exhausting. Anyway, enough of the excuses and doom and gloom. I wrote recently about fixing stuff remotely. This has become a much more viable option than travelling all over the city for me these days, especially for smaller jobs that really don’t warrant the time or the journey. The problem is that when it comes to payment, the customer suddenly resists paying for the repair, more so than when I go to them. The fact is that as I am a one-man band these days, no longer enjoying the luxury of two vans and four techs working for the company. All that was lost in the aftermath of the ‘quakes here. I had to radically downsize and shed my staff, 90 Silicon Chip Dave Thompson Items Covered This Month • Remote work and remote pay • Repairing shop signage • Converting torches to wireless charging • Tinkering with a guitar amplifier 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 which I really hated doing; I kept them on as long as I could, but the writing was on the wall. I went from an average of 65 calls a day to zero overnight, extending for the six months after the bigger ‘quakes fizzled out. It was a tough time for everyone. Anyway, driving somewhere takes up time I could be in the workshop, and it is more than reasonable to charge extra for going out onsite, just as most call-out type service people do. Some charge by the distance from base, or time on the road, or just wrap it into the final fee. Even with pre-empting any possible surprises by making it clear what the costs will likely be, some people balk once I’m there and have done the job. It’s a tough ask to argue my case when standing in someone else’s living room. This is a fact of life for many service staff. I’ll bet many of the people reading this have been in exactly the same situation. Sometimes, it can be very intimidating, especially if there are others hanging about who feel they need to get involved and stand up for the original caller/customer. I’ve walked away from more than a few jobs, for various reasons, and taken the financial hit, but all that has taught me over the years to be much more careful and selective about the places I go and the jobs I take on. The truth is, I don’t go to many call-outs now anyway, triaging all potential problems and deciding if the repair is better carried out in my workshop. That said, some of my long-time clients are now in their dotage and can no longer travel, so I will usually make the exception to go out to see them, mainly because I know that they are good people who are prepared to pay and are well aware of the added costs a call-out entails. They are usually very grateful that I go out, and I am happy to support them after sometimes 25 years of their custom. Of course, during the pandemic, all this had to stop due to lockdown laws, requiring a different approach. Often, I would walk people through potentially simple problems over the phone, and they would send an electronic payment for my time. This approaching worked for a few common problems, but for the bigger ones; if the computer doesn’t boot at all, or Australia's electronics magazine siliconchip.com.au loops on startup (for example), it needs to be on the bench for me to be able to fully get to the bottom of things. This usually means I have to go and get it, especially if the client can’t make it in easily; otherwise, they bring it to me. So, all well and good for computer work. Some repairs can be done remotely, some can’t, and we stumbled through those dark days. But what about other things, like appliances or music gear? I mentioned helping someone fix an electric roller blind recently, but during pandemic times (and on either side of it), I have helped a few people out using social media and apps like Teams and Zoom. Obviously, it is more difficult just doing it with a chatonly type app, as we have to rely on descriptions and a lot more communication than someone simply playing their camera over the device, catching the model name and number and other details that might give me a clue on how to repair it (if that is even viable – many appliances these days are not designed to be easily repaired). Plus, there are laws in some countries that don’t allow people to repair their own appliances and instead need a licensed repair person to do it. I have seen some cowboy repairs over the years, but such laws mean that repairable appliances will end up being dumped because licensed repair can be incredibly expensive these days – just the callout fee can be more than the cost of a replacement appliance! (You have to wonder who would know if you opened something up and fixed it yourself – assuming you didn’t get electrocuted or burn the house down in the process. Many replacement parts are available from overseas, of course.) Anyway, I was once communicating via email with a guy who lived overseas, and he was having a problem with a dishwasher. I had written about a similar concern with ours (two actually) in my columns a while back, and he must have read one or the other, or been told I had written about it. He asked for advice on what to do and was prepared to pay for my time. As always, I said we’d see what we were dealing with and talk about money when the time came. He was on WhatsApp, so I sent my number, and he video called me soon after. We went through the pleasantries and siliconchip.com.au then the ‘show me the appliance’ scenario. He was very thorough and gave a good tour of it, including pictures of the brand and model numbers on the label inside the door frame. The brand was unknown to me and was likely some in-house, re-branded unit made for one of the big box stores over there (this turned out to be the case). Regardless, most of these dishwashers work pretty much the same way. This six-year-old one would start, fill with water, but wouldn’t go any further. It just sat there humming slightly (an unusual noise to the owner), and thus not completing the cycle. He had manually scooped as much of the water out as he could, but it was no solution. He also couldn’t find the user manual, so he emailed me, asking if I could offer any advice. There are really only a few things that can make this scenario happen. Water pressure fills the unit, sensors detect the level and stop the filling. Then the wash cycle starts, and the now-heated water should circulate through the sprayers and pre-rinse the dishes before the pellet or powder drops into the chamber. A wax motor built into the powder drawer heats up and opens the door at the right time, dumping detergent into the water (which wasn’t happening). And of course, it never got to emptying the grey water to prepare for the next rinse cycle, so it just sat there. There were no error codes to go by, as the control panel was very simple, with just a settings knob and a start/stop button – no display other than a couple of LED ‘idiot lights’. It seemed apparent to me right away that the pump was either fouled or had failed, although the fact that it hummed might indicate that the pump was trying to move but couldn’t for some reason. It could also be a pump controller valve not operating properly, if it were the type of machine to have such a valve. I advised him to unplug the dishwasher or switch it off at the wall, which he did. I then suggested we go through some basic checks, such as the filter and the pump impeller (if it was even visible). Australia's electronics magazine December 2025  91 Most filters come out easily in dishwashers, as they are intended to be taken out for regular cleaning. He said he’d never done it. He played his phone camera over the floor on the inside of the unit, and I could see the filter assembly there, with the centre almost like a hollow drain plug with a hard plastic surround, and a metal skirt around that with tiny holes in it. I suggested he look closely at it to see if there were any arrows embossed into the plastic to indicate how it comes out. He found one arrow and twisted the centre hub that way to release the bayonet-style thread that held it in. He then lifted the whole piece from the floor. I could see right away it was choked with hair and food debris – they had two dogs – and that certainly would not be helping things. I asked him to clean the whole thing thoroughly and call me back when he was ready to put the cleaned filter back in. This he duly did, showing off his handiwork. I also asked him to make sure the dishwasher hadn’t moved in its cavity. Sometimes (rarely), they can move off any floor mounting system (if they have one), and crush or kink the outlet hose, again causing water not to flow. It’s unlikely and wouldn’t explain the not-washing symptom; however, I have seen installers just sit them on the ground, and if people shut the door too hard, they can bump the thing back by a few millimetres at a time. So, while we were at it, it warranted checking. He grabbed the sides and pulled toward him, but it seemed to be fixed firmly to the ground. At least that ruled one potential problem out. I then asked him to shine his phone torch down the filter cavity and see what he could see. He reported that he couldn’t see much at all, as there was still some water lying in it, so I got him to feel around in there with his fingers, just to make sure there wasn’t anything physically fouling the pump inlet. There was nothing as far as he could tell. That was about as far as I could go with it on a walkthrough. I got him to reassemble the filter, ensuring the barrel was twisted back into place to lock it, power it back on and try to run the quickest cycle he could. I said to call back when it got to the point of it washing or draining. 20 minutes later he called and said he could hear some water splashing around but nothing draining, and it was taking longer than usual. At this point, I took a guess that the pump was getting tired, and after a quick search, found they were available on a wide array of online repair sites. The problem was, this guy wasn’t confident enough to tear it down and tackle the job himself, so he said he’d call a guy in. That was the end of my involvement. I told him not to worry about paying me. A few days later, I got an email from him telling me it took the service guy about an hour to pull the machine out, strip it down and swap out the pump, and now it was working fine. He again offered to at least give me something for my time, so he sent a small ‘donation’ via PayPal, and we were both happy. Another very remote fix was for a solid-state guitar amplifier owned by a former bandmate of mine who now lives in the south of England. This guy is reasonably capable, and on social media, he asked me if I could walk him through fixing what he considered a reasonably simple problem with his solid-state, 50W combo amplifier. A combo is so-called as it has one or more built-in guitar speakers, with the amplifier mounted above it, all in a 92 Silicon Chip single case. That way, the whole thing is self-contained and usually smaller and theoretically easier to carry around to gigs than a separate speaker cabinet and amp ‘head’ unit. The problem was that one of the input jacks, a 6.5mm mono type, had fallen into the guts of the amp, and likely onto a PCB behind and beneath the folded metal fascia. He said it had been loose for a while, but had now fallen in, and wondered if it was a job he could sort himself. When it happened, he was wise enough to unplug it and not switch it on again. That’s good practice; the last thing we would want is the magic smoke escaping. Though not a tube amp, there is still serious voltage and current running around inside there. He also gathered a nut and washer from the socket that had fallen to the floor in front of the amp. I told him to look at the top of the case. Obviously, there’s a handle or two, but I am looking for the amp tray mounting screws, which are often on the top. He said he could see four screws, one close to each corner, all with cupped washers underneath. I advised him to undo each one and remove it, but just make sure with the last one that the amp wouldn’t just fall away into the box (most are supported by timber battens, and the amp tray slides out the back like a drawer, but it still pays to be careful). I also asked him to remove the speaker/output wiring if it was connected via a plug. If it was hard-wired, there should be enough cable to get the tray out far enough. He got the screws out and slid the tray back until he could see the socket. There was also an inside washer, which he retrieved. He positioned the washer on the socket and put the whole thing back into the fascia hole. I suggested he hold the socket in with one hand while fitting the washer and nut to the outside and tightening it the same way. He had a crescent spanner (shifter) that opened enough to fit it, so carefully cranked it tight, being cautious not to overdo it. He slid the tray back in, aligned the screw holes and buttoned it back up. He fired it up, tested it and all was well. Another donation sent, and job done! Flashing LED shop sign repair Some time ago, a friend gave me a flashing LED shop sign. The bottom row of blue LEDs no longer lit up, so they’d replaced the sign with a different one. They would have tossed it in the bin if they hadn’t known me. I said I would have a look at it and see what was wrong with it. The LEDs used in this sign are the ‘straw hat’ type, which I did not have on hand. They are similar to regular domed LEDs, but the lens is about half as tall. The sign consists of a plastic frame with a front panel made from MDF. The LEDs are just poked through the MDF, Australia's electronics magazine siliconchip.com.au the leads bent sideways and soldered to the leads of the adjacent LEDs. This is actually a commercially made sign from in China, so I was surprised by this construction method. I got my multimeter out and tested each LED in the bottom row, and I found that three LEDs were open circuit. I found blue straw hat LEDs available from China via eBay. I knew it would take a few weeks for the order to arrive, but I wasn’t in any hurry, so I ordered a packet. Unfortunately, they didn’t arrive for more than a month, so I asked the seller to resend them. The re-sent order arrived in two weeks. The original order arrived 59 days after it was first sent. I didn’t need the extra LEDs, but I didn’t want the seller to lose on the deal, as they had been kind enough to re-send the order for me when it hadn’t arrived, so I placed a second order and told them they didn’t need to actually send another lot of LEDs. After replacing the three dead LEDs, I powered the sign up and I found that the bottom row of LEDs only lit faintly; not at full brightness like the other blue LEDs in the top row. So there must still be something wrong with one or more of the remaining original LEDs. I decided to replace the other 11 LEDs in the bottom row, as I thought that would fix it and the LEDs were not expensive. Anyway, I had way more than I needed with the two lots arriving. After replacing the remaining LEDs and taking care to orientate them correctly, I powered the sign up and it finally worked correctly. The OPEN LEDs stay lit all the time, while the red, green and blue LEDs at the top and bottom of OPEN flash alternately. The sign is powered by a small circuit on a PCB with resistors in series with the LED chains. I could not see what was on the circuit board, as it was stuck to the MDF with what looks like hot-melt glue, and I didn’t want to disturb it. Now that the sign was working correctly, I replaced the back MDF panel, and it was ready to use again. I don’t really have a use for this sign, but it was interesting to repair it and get it working again. At least I saved the sign from landfill, even if I don’t need it. Perhaps my friend will want it back eventually. Bruce Pierson, Dundathu, Qld. Two torch conversions I will describe two torch conversions, the first being a Click-brand nightlight/torch purchased from Bunnings, with the second being an Eveready Dolphin unit. 1. Wireless Qi charging for the Click torch Externally, the Click nightlight appears to be well made with a durable white plastic case. It is normally held and charged in a base unit that plugs into a power point. The movement-activated nightlight illuminates the front panel and it can also be used as a portable torch using LEDs at the top. A button on the side switches through the various modes. The battery is charged via a wireless transmitter and receiver. After about 12 months, the charging failed. An internet search found numerous reports of wireless transmitter failures with this model. Opening the case of the base module showed burnt out components exactly as per the internet reports. The circuit is mains-driven via a capacitor to drop the voltage. The internals do not appear to have the insulation levels or clearances expected for a 230V AC appliance. Attempts to repair the circuit brought no success. I then wondered whether it would be possible to retrofit a Qi wireless charger of the type that is commonly used to wirelessly recharge mobile phones. 5W mini Qi receivers are available inexpensively online. They comprise a rectangular receiver coil about 20 × 25 × 1mm attached to a small circuit board with Qi receiver circuitry and a 3.7V Li-ion battery charge control chip. I ordered three units for $15 including postage, and they arrived after eight days. Fitting one was easy after removing the original receiver coil and mounting plastic. I taped the 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. siliconchip.com.au Australia's electronics magazine December 2025  93 ▶ Above: the wireless charging setup for the Click torch. Right: the Eveready Dolphin torch Li-ion conversion. ▶ Qi coil underneath the torch circuit board exactly in the middle of the unit, and fixed it with hot glue. The small circuit board was glued in the position of the original coil and the Bat + and Bat – battery charge terminals. The circuit board has red and green LEDs to indicate charging and end of charge, respectively. I drilled two 3mm holes partway through the torch cover immediately above the LED positions so that these indicators would be visible when the unit was placed on the charger. When reassembled, I placed the unit centrally over a Qi charger plate and the charger LED indicated a successful connection. On the torch, I was greeted with the red LED indicating charging. After a short while, the red LED turned green to indicate end of charge. The conversion has worked perfectly with no more mains power risks. 2. Eveready Dolphin Torch Li-ion battery conversion, Qi wireless charging & LED bulb My classic old Dolphin torch used a 6V dry cell lantern battery and an incandescent bulb. It had been unused for some time because the battery was flat. I thought I would upgrade its performance with a rechargeable battery, a bright white LED bulb and wireless charging. Because I already have several 18650 Li-ion cells removed from a defunct laptop battery pack with reasonable capacity and a spare 5W mini Qi receiver module, I estimated I could upgrade the torch for about 1/3 the price of a new 6V lantern battery. I only needed to purchase a P13.5S white LED bulb, which is a direct drop-in replacement for the old incandescent bulb. They are available with several voltage and power ratings. I chose the 3V ½W rating, connecting it in series with a 15W ½W resistor to limit the current from the battery. The resulting LED power is less than 1/4W but its brightness is exceptional. With this arrangement, it draws about 80mA from the battery, which should give over 12 hours of battery life. Prior to installation, I extended the wires between the charge receiver coil and circuit board to make it easier to locate the circuit board in a position where the charge LEDs would be visible through the plastic casing. Unfortunately, my Dolphin casing is red colour, so the green LED would look very dim through the case. To solve this, I drilled a 5mm hole so the LEDs would be visible. I fitted the battery to an 18650 single-cell holder and used two nylon cable ties to strap it to the bulb holder assembly. Next, I used a strip of PVC electrical tape to position the 94 Silicon Chip charge receiver coil on the inside bottom of the torch base, close to the torch’s centre of gravity and with the coil facing downwards. I secured it with hot-melt glue at each corner. I then the hot-glued the circuit board alongside the torch on/off switch in a position where the green LED aligned with the previously drilled hole, and covered the hole with a dob of hot glue to preserve the waterproofing. The remainder of the wiring was simple. Both torches are now fully functional, with the added convenience of wireless charging. Phillip Webb, Hope Valley, SA. A rock amplifier I have contributed several stories to this column about repairing powered speakers. This time it is something different, a guitar amplifier. Many guitar amplifiers are nothing more than a powered speaker with a built-in preamplifier to suit a guitar, but not the one I will describe here. This amplifier has a guitar input feeding a conventional preamp stage to get the signal to a suitable level, but it is then fed into a dual analog-to-digital converter (ADC). It then feeds two digital signal processor (DSP) chips, which can alter the tone or add effects such as delay and modulation. If the processing is different on each channel, the resulting outputs will form a stereo signal. The DSP chips are controlled by a microcontroller that adjusts their parameters. These parameters can be saved by the micro into a patch, which can be recalled at the push of a button. This model can store 32 different patches or Australia's electronics magazine siliconchip.com.au total setups of all the panel controls. The DSP also allows the amplifier to emulate the sound of 12 classic guitar amplifiers. The outputs of the DSPs are fed to two digital-to-analog converters (DACs) to turn the signals back into analog audio. From there, the audio goes to two sets of send/return jack sockets on the back panel. These allow the user to break into the signal chain to add more processing if desired. These turned out to be very helpful with the repair. The returns from these jacks then go to two 60W power amplifiers, each driving their own speakers. These power amplifiers are very unconventional. The manufacturer calls them “Valve Reactor Power Amplifiers”. Each channel contains one ECC83 twin-triode valve. There are many solid-state guitar amplifiers that have one or more valves in the preamp, but this is different. It has always been my opinion that the desired harmonically rich valve sound only comes from a valve push-pull amplifier with an output transformer. Single-ended valve amplifiers have a different sound. To this end, this amplifier has a valve running in pushpull into an output transformer with a power output of about 1W. Following the output transformers is a master volume control that allows the user to have the valve stage running into distortion, but with a lower ultimate sound level. From there, the signal is boosted by the two 60W solid-state amplifiers. Another feature of the Valve Reactor Stage is an extra cathode resistor, which can be switched in by a CMOS switch, allowing the user to run the valves in Class-A or Class-AB to mimic different classic amplifiers. The fault with this amplifier was “it just doesn’t sound right”. Further inspection revealed one output channel was not working, which made all the stereo effects sound odd. The send and return jacks allowed me to verify that the preamp and DSP sections were working correctly, but one of the power amps was not. There is another pair of jacks labelled Line Out (left and right), which are connected to the output transformers. The left Line Out jack had no output, so the fault was in the Valve Reactor Stage. This did not please me because it was difficult to get to in the chassis. I was able to probe the pins of the valves and determined one was getting a signal and the other was not. In a valve amplifier, the output valves are driven by a phase inverter stage, which normally consists of another dual-triode valve. This stage provides two signals with opposing phases to drive each output valve. In this amplifier, this function is carried out by two FETs. I was able to find two resistors that connect the FET drain terminals to HT. The junction of these feeds the audio via capacitors to the grids of the output valve. The voltages on both these junctions were very low, just a few volts, where I was expecting something greater than 100V. Two resistors form a divider from HT to ground to bias both FETs, and the voltage at their junction seemed wrong. Measuring both resistors showed that one was open-circuit, and the reason was obvious; it was covered in glue. This will limit its ability to dissipate heat, and was also turning the leads green with corrosion. A new resistor (R111) had the amplifier singing once again. It can be seen in the photo to the right of the two large electrolytic capacitors. SC Paul Mallon, Christchurch, New Zealand. siliconchip.com.au Australia's electronics magazine December 2025  95 Vintage Electronics The BC-221 US-Military Frequency Meter from 1941 Domestic valve radios were calibrated at the factory and might be tweaked occasionally during service. But military radios operate in rough conditions, out in the field, and need to tune accurately across thousands of channels. How were they kept in calibration? The BC-221 was the secret weapon. By Ian Batty W orking on VHF and UHF aircraft radios back in my RAAF days in the mid-1960s was simple. Everything operated at defined channel frequencies and was crystal-controlled. The crew needed only to switch to the appropriate channel, and the equipment would do the rest. This demanded one crystal unit per channel in both the receiver and transmitter; a total of 16 in the eight-­ channel American AN/ARC-3, and 44 in the British TR16440! Even by the 1960s, crystals were still expensive and labour-intensive to produce. During WWII, millions would have been needed for the many tens of thousands of military radios of all kinds. Up until the late 1940s, equipment as diverse as the United States’ Marines TBY Squad Radio (September 2020; siliconchip.au/Article/14580) and the British Wireless Set No. 38 MkIII used a calibration-frequency 96 Silicon Chip crystal oscillator to provide ‘markers’ at specific intervals over the operational band. The SCR-274 “Command” sets, working at HF, used a single crystal in each tuneable transmitter, which, in concert with a ‘Magic Eye’ tube, confirmed the calibration at one point in the operating band. This was far from ideal, though. Firstly, the HF band spans 3-30MHz, with the range of around 3-15MHz most commonly used. Allowing 1kHz channel spacing, that demands around 12,000 channels. The famous SCR536/ BC611 “handietalkie” used any one of 50 crystal-controlled channels in its allocated band of 3.6~6MHz, but had to be returned to a depot to change channels. So, general-purpose HF transmitters, receivers and transmitter-­ receivers, such as our Wireless Set No. 19, could only be continuously tuned. Few of these sets provided any internal Australia's electronics magazine calibration, so operators were in the position of ‘set and hope’. Sets were routinely brought back to company headquarters, field depots, or major repair centres, so it was possible to provide frequency calibration then. But we still have the problem of about 12,000 possible frequency allocations. The solution was to provide a highly accurate and stable signal generator/ receiver with a detailed calibration chart. This would give technicians a reference that was accurate to a few hundred hertz when calibrated against its own internal crystal oscillator. The SCR-211/BC-221 The BC-221 Frequency Meter (BC = Basic Component) was a part of the SCR-211 parent set (SCR = Set, Complete Radio). Interestingly, in this case, it was the only part of that set. Still, the Meter itself is the BC-221, not the SCR-211. The SCR/BC system was siliconchip.com.au ultimately replaced by the JETDS system in 1943, but the BC-221 was never allocated a JETDS designator. The BC-221 comprises a variable-­ frequency oscillator for setting reference frequencies, a heterodyne demodulator for measuring transmitter frequencies, and a crystal calibrator for setting either its own calibration, or that of external equipment. The Technical Manual for the initial -A, -B and -C types of the BC-221 is dated 1941, while the definitive TM 11-300 of 1944 lists 25 variants. The instrument was made in the tens of thousands, and was also made by countries other than the USA – Louis Muelstee’s Wireless For The Warrior website has a Russian example (www.wftw.nl/russian221.html). The BC-221 was used right down at field level, not just in depots, thanks to it being compact and battery-­powered. It was designed to be carried by radio operators, technicians and small unit signal sections, so they could keep their sets on-frequency without having to ship them back to a depot. The box is built like a brick, with shock-resistant mounts, big batteries, and the headphones-plug-as-power-­ switch trick to conserve them. By consulting the calibration book and zero-beating against the internal crystal, an operator in a forward area could check his transmitter or receiver on the spot. That said, depots and workshops did use them too, for aligning radios after major repairs, or as a reference when producing training or calibration manuals. In practice, company headquarters/field workshops would have at least one. Frontline signal units often carried one so their radios could be set and kept in tolerance. Higher-level maintenance depots had them in larger numbers for mass calibration. The BC-221 has two ranges, Low Band (125~250kHz) and High Band (2~4MHz). Each instrument was individually calibrated, with an extensive custom calibration book, bound inside the lid of the case inside the front cover. The Low Band has a calibration point every 100Hz, while the High Band has points at 1kHz spacings. Setting its output frequency to a precision of 100Hz (on the Low Band) or 1kHz (on the High Band) is done using a master dial drum (visible through a window) and a rotary vernier dial. siliconchip.com.au The manual also shows how to measure frequencies between the indicated values. Thus, within the instrument’s limits of calibration, it would be possible to tune a transmitter to 8.130MHz, midway between the chart listings of 8.128MHz and 8.132MHz. Although the oscillator is extremely frequency-stable, its Class-C operation makes its output rich in harmonics. Given its fundamental frequency ranges of 125~250kHz and 2~4MHz, the BC-221’s useful ranges extend from 125kHz to 2MHz (fundamental, 2nd, 4th and 8th harmonics) on the Low Band and 2MHz to 20MHz (fundamental, 2nd, 4th and 5th harmonics) on the High Band. The spectrum sweep in Scope 1 shows the harmonic output for a dial setting of 2MHz. Likewise, the internal reference crystal oscillator’s output is rich in harmonics, ensuring checkpoints beyond 20MHz. The expanded view of the frequency meter. This version used a timber cabinet, but there are also some that have an aluminium-alloy cabinet. Scope 1: both oscillators (the primary and crystal ones) have significant harmonics in their output, as shown here. This is not a flaw but in fact a useful feature, since you can use the harmonics to tune a radio to a multiple of the selected frequency. Why good radios are so important for militaries The battle of Port Arthur/Tsushima in 1905 saw the Japanese Fleet destroy or capture all eleven battleships of the opposing Russian Second Pacific Squadron. How was it possible for the Japanese, who had acquired their first warship less than 50 years previously, to defeat the Russians, with a naval fighting history reaching back to 1696? Both fleets were equipped with Morse code equipment, but the Japanese gear was locally designed, technically superior, more reliable, and used to much greater effect due to thorough operator training at the Yokosuka Training School. So radios can play a decisive role in armed conflict, but only if they are reliable and operated effectively. You need to know what channel or frequency to use to get that communication happening, hence the need for calibration. Australia's electronics magazine December 2025  97 Side views of the BC-211 frequency meter. There were as many as 25 different BC-211 models, each with slight variations to the circuit. Automatic calibration in the 1940s The method used to generate the custom calibration book for each set is most surprising, because it was done by computer – in the early 1940s! Engineers at the Philco Corporation Research Division, Engineering Department created an automatic calibration computer for this task using 126 valves. The BC-221 was calibrated by noting the dial reading for internal heterodyne beats and calculating how many cycles per dial division it was from the previous calibration point. The computer consisted of an automatic calibrator combined with an adding machine (semi-automatic), which recorded the calibration data at 327 points, interpolated between those points, and automatically printed the 3252 different frequency value numbers in each individual calibration book. The time to do all this was 6.5 hours; the actual printing of the values in the book took just 16 minutes. The time for an experienced human to do the work manually, in contrast, was averaged at 16 hours per frequency meter. A mechanical hand automatically turned the dial of the BC-221 to various predetermined settings while measurements were underway. The calculations were carried out It’s interesting to note the arrangement of the chassis (shown from the rear), with the separate boards and the valves placed at different angles. 98 Silicon Chip Australia's electronics magazine by automated complex adding machines of the type then used by banks and finance companies, and the results printed in the calibration book and also saved on paper tape. The adding machines were fitted with solenoids to depress the keys; the valves the computer used were mainly 0A4G cold-cathode thyratrons, similar in function to a modern bistable flip-flop. This description of the calibration was based on information from the webpage at https://jproc.ca/ve3fab/ bc221.html Operating principle Most modern devices that require precise high-frequency signals generate them using phase-locked loop (PLL) circuits. A master oscillator (MO) operates at a fixed frequency and feeds one input of a phase comparator. The phase comparator’s other input is fed by the output of a voltage-­ controlled, variable-frequency oscillator (VCO) via a frequency divider (usually a programmable one). The VCO’s frequency is controlled by the phase comparator’s DC output. If the phase comparator detects a difference between its inputs (VCO and divider), it will ‘steer’ the oscillator until its two frequency inputs are equal. In practice, the VCO will be forced to be in phase with the divider’s output. This is a servo system, similar to how a car’s cruise control can maintain the set speed, even when going uphill. In this case, the “servo” is providing an accurate, fixed frequency, using a reference and frequency divider. siliconchip.com.au The top view of the chassis, showing the master dial drum at the bottom. an external transmitter signal or the internal crystal reference. As you tune the BC-221, you hear a beat tone in the headphones. When the beat tone slows to zero, the VFO and the reference signal are at the same frequency. So, with the BC-221, you sense the difference between the BC-221’s frequency and that of the incoming signal. Instead of an automatic control loop as in the homodyne, the operator listens for the beat note and dials the oscillator until the difference can no longer be perceived. This manual zero-beating method was simple, reliable, and accurate enough that with the aid of the calibration book, operators could set or measure frequencies to within a few hundred hertz, more than sufficient for wartime communications. Accuracy To change frequency, the divider is simply commanded to a different division ratio. The division ratio of the divider, through negative feedback, becomes the frequency multiplication factor. While “PLL” is a modern term, the principle goes back to 1924 as the homodyne (“same power”) system, in contrast to Armstrong’s heterodyne (“different power”). If we heterodyne an AM signal with a pure sinewave of the same frequency (eg, the output of a PLL locked to the carrier), we have a superhet with an intermediate frequency (IF) of 0Hz – all that would be left is the modulation! This type of circuit appeared as early as 1924 as the homodyne, and later as the synchrodyne in Electronics Australia, June 1975. The original homodyne used a weakly oscillating circuit that would ‘pull in’ to an incoming signal of sufficient strength and synchronise to it. Connect a pair of headphones to the valve’s anode lead and off you go. This circuit demonstrated the ‘lock-in’ principle but, since it used a filter to create the control voltage, it was an automatic frequency control (AFC) system, rather than a true PLL. Similar techniques would be widely used in FM tuners before the ready availability of integrated-­ circuit PLLs. The generic homodyne circuit in Fig.1 shows just one tuning component: C1 tunes the oscillator into its siliconchip.com.au ‘capture’ range, and the control voltage does the rest. As the homodyne/ synchrodyne converts directly at the incoming carrier frequency (‘direct conversion’), it offers a frequency response from essentially DC to some upper limit set only by the filter that follows the demodulator. In a homodyne or synchrodyne receiver, the local oscillator is automatically pulled into lock with the incoming signal when the frequencies are close. When that happens, the two are at the same frequency and the audible ‘beat note’ disappears. The BC-221 uses a related principle, but instead of locking, it relies on manual adjustments by the operator. The instrument’s variable-frequency oscillator (VFO) is mixed with either The instrument is highly precise; the following lists the maximum frequency error expected due to each source of imprecision: 1. Small shocks (caused by handling, and the thrust on the dial and pressure on the panel when using the equipment): 100 cycles maximum 2. The action of locking the dial: 30 cycles maximum 3. Warming up: 100 cycles maximum 4. Changing of load on the antenna post: 50 cycles maximum 5. A drop of 10% in battery voltage or a change of 5°C in the surrounding temperature: 325 cycles maximum 6. Error in calibration: 500 cycles maximum 7. Error in crystal frequency: 250 cycles maximum Fig.1: the general concept of a homodyne. Once tuned close to the input signal frequency, the oscillator will ‘lock on’ to it. The output of the mixer is therefore just the modulation signal, with the carrier removed entirely. December 2025  99 Total error: 1355 cycles maximum or 0.034% at 4000 kc. The manual notes that “Actual tests show that the maximum errors can be assumed no greater than 50% of the values given…” In practice, it’s also unlikely that all errors would sum in the same direction. The manual notes that the maximum errors will occur at a frequency of 4MHz and a temperature of -30°C (!). Robert Watson-Watt’s dictum, “Give them the third-best to get on with. Second best takes too long and the best never comes”. That approach provided the radar systems that won the Battle of Britain in 1940. But who doesn’t love going down the rabbit hole? Using an atomic clock reference (the carrier of 774 ABC Melbourne), it is accurate to well under 100 millihertz (0.1Hz). My HP 8656B signal generator came in 4Hz high, and Power supply all following figures are quoted relative The instrument was designed to corrected figures on the HP. for battery operation. Four series-­ On power-up after who-knowsconnected “Number 6” cells supplied how-many-years, with the recomthe 6V valve heaters, while six 22.5V mended supply voltage and after a BA-2 batteries in series supplied the few minutes of warming up, the crys135V HT. tal oscillator came in at 1,000,002Hz I found that -0.5V and +0.5V changes (1.000002MHz), a +2ppm error. Surin heater voltage gave a frequency error prisingly, during the performance of -33Hz and -2Hz, respectively, at measurement process, it settled to 2MHz, while a drop from 135V to 1,000,017Hz, an increase to +17ppm. 100V HT caused an increase of 146Hz Still, not bad after 70+ years. at 2MHz. To measure that tiny 2Hz error, I just It’s easy to get lost in the Jungle of set my BC-211 to exactly 1kHz above Excess Precision – it’s where you chase the reference frequency and sent the down some parameter to the point of resulting audio tone to my frequency undetectablity. I’m reminded of Sir counter. This indicated 983Hz and 998Hz. Subtracting my 1kHz offset gave the -17Hz/-2Hz results. I checked my counter’s calibration as part of the exercise, so I’m OK with claiming my 998Hz measurement. Circuit details Components in the circuit (Fig.2) are numbered in order (with sub-­ numbering when values are shared): the capacitors are #1 to #10-3, resistors are #20-1 to #26, with minor components filling the gaps to #36. The valves are not numbered. Confusingly, the -A circuit shows the variable oscillator with the screen grid enclosing the control grid, and shows a pentode with four grids! This is an erroneous symbology dating from the early days of circuit diagrams. In reality, the ‘77 is a conventional triple-­ grid pentode, with its first (control) grid connected to the band switch and main tuning capacitor. The -B circuit is correct. The VFO, a VT77/‘77 in the initial issue, or a VT116/6SJ7 in later versions, operates as a cathode-­coupled Hartley oscillator. The cathode con- Fig.2: the BC-221-C and -D circuit looks deceptively simple, but this instrument has been carefully designed to minimise drift so that after factory calibration, it will remain very accurate in field service. You just need to refer to the calibration booklet attached to the unit to tune it to just about any frequency. 100 Silicon Chip Australia's electronics magazine siliconchip.com.au Fig.3: this shows how accurately my unit tracked despite its age. Compare the green and dark blue curves to see the benefit of calibration. nects to a tap on the selected tuning coil, and feedback to the grid is taken from the top of the coil. Unusually, there’s no grid bias resistor, and the grid returns to DC ground; the circuit uses cathode bias via resistors #20-2/ #22-2 (5kW/10kW). The circuit is tuned by main capacitor #1 (150pF). There is an individual trimmer for the top end of each band (#3, #4), and a master Corrector trimmer #2 mounted on the front panel. The RF output is taken from the oscillator’s anode; this is electron coupling, ensuring that any changes in the output circuit do not affect the oscillator’s frequency stability. The oscillator output connects to the antenna terminal and also the grid of the heterodyne detector, a 6A7 pentagrid or a VT167/6K8 triode-hexode. The antenna connection allows the set to operate as a low-power signal generator for receiver calibration. As the oscillator is not modulated, exact receiver adjustment is easiest done by calibrating a tuneable transmitter to match a frequency set on the BC-211, then tuning the receiver to the calibrated transmitter. The BC-211 manual details this method in paragraph 11, on page 6 (a link to this is under the References cross-heading). The 6A7/VT167/6K8 converter operates in one of three modes: a. as a direct heterodyne comparator of an incoming transmitter signal and the local VFO b. as a 1MHz ‘marker’ generator for calibrating external equipment siliconchip.com.au c. as a 1MHz ‘marker’ generator for calibrating the BC-221 itself The converter’s anode feeds the audio amplifier, a VT76/’76 in the -A issue, or a VT116/6SJ7 afterwards. This amplifies the converter’s heterodyne output to drive external headphones. As a final nice touch, you have to plug the ‘phones in to apply power to the heater circuit. As you have to remove the headphone jack to close the case, battery life would be preserved during transport even if the power switch was left on. How good is it? It is outstanding, as you can see from Fig.3. For equipment that’s been unused for decades to come in with its calibrator only 17ppm high is outstanding. Its only limitation is the lack of a modulator for the VFO – this would have made it easier to use when lining up receivers. It’s capable of very high accuracy once calibrated, but, as my ‘as found’ results show, you can just check the chart for the frequency you want, dial up and off you go. The dial uses vernier graduations. This scheme allows accurate dial-­ setting to within one-tenth of a minor division without the burden of reading minuscule engravings. The handbook shows clearly how to do this. References TM 11-300 (1944 issue); which counts as the BC-211 (SCR-211) manual: www.qsl.net/zl1bpu/IONO/TM11300.pdf The original 1944 article on automated calibration: siliconchip.au/ link/ac94 (see pages 96~107). For a brief description: https://jproc. SC ca/ve3fab/bc221.html The underside view of the chassis. The cabling is tied with a continuous run of waxed string, known as “looming”. 101 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 Digital Preamp crossover capabilities I was surprised and excited to see the article on the Digital Preamplifier in the October 2025 issue (siliconchip. au/Series/449). I have read it several times and am looking forward to the next articles. Of what DSP and crossovers is it capable? My current system uses four-way active speakers with a miniDSP Flex8 and 48dB/octave crossovers at 100Hz, 800Hz and 4kHz. Is there any baffle step compensation, and frequency and gain presets? (F. C., Kelso, Tas) ● Phil Prosser responds: it has four stereo outputs and the digital crossovers can be configured for 6dB/ octave, 12dB/octave, 24dB/octave or 48dB/octave. Each has a programmable frequency from 5Hz to 20kHz, a programmable delay on each output in 1.7mm increments and the option for phase inversion on each channel. It can also convert stereo to mono for the subwoofer output. Each channel has programmable attenuation from 0dB to 20dB in 0.5dB increments. There is a three-band parametric EQ that applies to all channels (on the input prior to crossover). The centre frequencies can vary from 5Hz to 20kHz, Q from 0.1 to 10, with a gain range of at least ±10dB. There’s also a separate three-channel parametric EQ on each channel. The device’s sampling rate is 192kSa/s. The ADC and DAC are all 24-bit devices, with all processing done as 32 or 64 bits. From what you describe, it should cover your needs. I have not chosen to put in FIR filtering; a user could do this if they wanted, but there would be a fair amount of programming involved. Baffle step correction is very dependent on the speaker you are tuning. I usually deal with that using the parametric EQ if it is even distinguishable in the measurements. The EQ calculations in the software have a shelving EQ capability, so adding a baffle step correction 102 Silicon Chip feature would ‘just’ require expanding the user interface and configuration storage. Alternative Li-ion cell for USB Power Monitor I have been unable to find a supplier in New Zealand with an exact equivalent to the lithium-ion cell specified for the USB-C Power Monitor (August & September 2025; siliconchip.au/ Series/445). Stockists in Australia who will supply NZ customers cannot supply the cell to NZ because of safety regulations. A New Zealand stockist of lithium cells has a 3.7V 380mAh Li-Po rechargeable cell measuring 40 × 20 × 8mm. The dimensions of the cell specified for the project are 38 × 25 × 6mm. It appears that the thickness difference of an additional 2mm is the only possible obstacle against using this cell. Will such a cell fit in the USB-C Power Monitor? (J. W., Auckland, New Zealand) ● Based on our measurements, there should be just enough space for an 8mm-thick cell. There is 15mm available between the bottom of the case and the back of the PCB. If we allow 2mm for the components on the back of the PCB (in the space between the PIC and the LED) and 4mm for the second PCB, that leaves 9mm for the cell. There is about 44mm of horizontal space between the PIC and the LED, so the longer length of the cell is not a problem. Setting up WiFi on the Big Clock I have built the BIG LED Clock from the January 2025 issue (siliconchip.au/ Article/17603) and would like some help with getting the clock to work. I don’t know what to do about the WiFi credentials at the top of the sketch. I can’t see any reference to the WiFi. I assume the Arduino can connect to Australia's electronics magazine my mobile phone, tablet or PC. (J. P., Port Augusta, SA) ● The credentials are set at lines 2 and 3 of the code, which initially read: #define SSID “****” #define PASS “****” The SSID is the network name, so you should change the **** to match the name of your network, keeping the quote marks. Similarly, PASS is the network’s password. For example, if your WiFi network is called John’s WiFi and the password is 12345678, you would change these to: #define SSID “John’s WiFi” #define PASS “12345678” Then upload the modified sketch and it should automatically connect to the WiFi network. SMD Test Tweezers no longer power up Probably three or more years ago, I purchased two of the original SMD Test Tweezers kits (October 2021; siliconchip.au/Article/15057). They worked great at the time, and were a useful adjunct to my workbench. I hadn’t used them for quite a while (possibly 8-12 months). I went to use one today, only to find it dead. Of course, the CR2032 cell was flat, so I replaced it, but it still wouldn’t power on. I then got out the second one and replaced its cell with the same result. I have checked that power is getting to the circuit on both boards, but I am unable to get them to work. The screens do not light up at all and nothing is displayed on them. I checked the voltage across the probe tips, which reads close to the cell voltage and goes to zero when shorted. Have you heard of this happening before? Is this likely to be the PIC chip and, if so, if I purchased a couple of the newer, pre-programmed version PIC 16F15214 chips, would I just be able to replace the old ones with them and 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 PCB PRODUCTION 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 USB-C Power Monitor August-September 2025 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 PMD WAY offers (almost) everything for the electronics enthusiast – with full warranty, technical support and free delivery worldwide. Visit pmdway.com to get started. Short-Form Kit SC7489: $60 siliconchip.au/Series/445 This kit includes all non-optional parts, except the case, lithium-ion cell and glue. It does include the FFC (flat flexible cable) PCB. OATLEY ELECTRONICS HAS CEASED NORMAL OPERATION. We have a huge number of components including LED modules, LED assemblies, valves, small solar panels, small stepper motors etc to sell at great prices. For more info or an inspection (near Woy Woy, NSW Central Coast) call Branko – 0428 600 036 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 December 2025  103 Advertising Index Altronics.....................37-40, 59, 69 Blackmagic Design....................... 5 Dave Thompson........................ 103 Emona Instruments.................. IBC Hare & Forbes............................... 9 Jaycar............................. IFC, 11-14 Keith Rippon Kit Assembly....... 103 LD Electronics........................... 103 LEDsales................................... 103 Microchip Technology.............OBC Mouser Electronics....................... 3 Oatley Electronics..................... 103 OurPCB Australia.......................... 8 PCBWay......................................... 7 PMD Way................................... 103 SC Christmas Decorations......... 51 SC HiFi Headphone Amp............ 36 SC USB-C Power Monitor......... 103 Silicon Chip Back Issues........... 36 Silicon Chip PDFs on USB......... 78 Silicon Chip Shop...................... 15 The Loudspeaker Kit.com............ 6 Wagner Electronics..................... 95 Errata and on-sale date Digital Preamplifier part one, October 2025: in Fig.3 on page 34, the pins of IC7b and IC8b are swapped. The upper pin of each should be pin 5 (+) and the lower pin should be pin 6 (-). The connections of the other op amps in the circuit are correct. Serviceman’s Log, October 2025: the photo at lower left on page 89 is of the wrong computer; it is an Acer, not a Toshiba P750. Next Issue: the January 2026 issue is due on sale in newsagents by Monday, December 29th. Expect postal delivery of subscription copies in Australia between December 29th and January 14th. 104 Silicon Chip go from there, or do you think it may be something else? (E. W., Denistone, NSW) ● We have not heard of this situation before, where the Tweezers worked and then suddenly stopped. It could be the microcontroller, but we also suspect the OLED module, since they can be quite delicate. We have had reports of readers fitting the cell upside-down and thus flattening it, but if you are getting power to the circuit, that is not likely to be the case. One way to tell if it is the microcontroller or OLED at fault would be to see if there are signals getting to the SDA or SCL pins on the display. An oscilloscope should show activity several times per second. We have also used a piezo transducer to ‘listen in’ to data at times, so if you don’t have an oscilloscope, try connecting one temporarily between those pins and GND. If you detect some traffic, that means the PIC is probably OK and the OLED screen may be faulty. If there is no traffic, we would suspect the microcontroller. Another thing worth trying is to reset the micro by briefly shorting pins 1 and 3 of the ICSP header (CON1), taking MCLR to ground. That might force the microcontroller to re-initialise the OLED, in case the two are out of sync. Alternative switch for Signal Tracer The Audio Signal Injector & Tracer project (June 2015; siliconchip.au/ Article/8603) lists a DP4T slide switch from element14. This switch is no longer made. Can you suggest a replacement? (E. M., Capel, WA) ● Altronics Cat SX2040 is a suitable alternative (www.altronics.com.au/p/ sx2040-dp4t-pcb-mount-miniatureslide-switch). Fixing Playmaster amplifiers In the late 1990s, I bought kits for the Playmaster Series 4 preamplifier, Playmaster 300W Subwoofer Amplifier and three of the Jaycar Playmaster Pro Series 3 Power Amplifier from Jaycar in Clayton, Melbourne. I recently replaced the four electrolytic capacitors in the Pro Series 3 Power Amplifiers after noticing odd behaviour in the sound. The capacitors Australia's electronics magazine were leaking and needed replacement. I sourced replacement capacitors from Jaycar in North Lakes, Queensland. Yesterday, I had another setback. It appears the 300W Subwoofer had a catastrophic failure of the power amplifier Mosfets. Unfortunately, it took out the connected subwoofer as well. I have not yet disassembled the amplifier. Since I do not have the assembly instructions anymore, can you provide the assembly instructions and circuit diagrams? As I recall, the instructions had various voltage check test points documented in the instructions. Do you have or know of a source of replacement Mosfets? I have really enjoyed the setup in my office with this equipment and would like to continue use if possible. (S. W., North Brisbane, Qld) ● We don’t have access to Jaycar’s kit instructions, and after around 30 years, we doubt they would still have them. They would have been based on the articles published in Electronics Australia: December 1996 & January 1997 for the Series 4 preamplifier, April & May 1995 for the 300W Subwoofer and February & March 1994 for the Pro Series 3 Power Amplifier. Scans of these articles are available from our website for the preamp: siliconchip.au/Shop/15/3980 siliconchip.au/Shop/15/3981 And for the subwoofer: siliconchip.au/Shop/15/3244 siliconchip.au/Shop/15/3441 Lastly, the power amplifier: siliconchip.au/Shop/15/6640 siliconchip.au/Shop/15/6641 Replacement Mosfets are available from Jaycar (N-channel) and DigiKey (P-channel). There are other suppliers if you search for the parts online: www.jaycar.com.au/p/ZT2460 www.digikey.com.au/en/products/ detail/2SJ162-E/1244174 There appear to be three Mosfets for each half of the amplifier, so you will need three 2SK1058s and three 2SJ162s. Check the circuits for anything else you may need before ordering anything. Since you will have to back-order the 2SJ162s, we suggest you do that first. The estimated delivery date is early next year. The manufacturer will stop making 2SJ162s in March 2026. 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