Silicon ChipSimple tripwire alarm - September 2021 SILICON CHIP
  1. Outer Front Cover
  2. Contents
  3. Publisher's Letter: Upcoming price changes
  4. Mailbag
  5. Feature: Advanced Imaging - Part 2 by Dr David Maddison
  6. Feature: The Cromemco Dazzler by Dr Hugo Holden
  7. Project: Touchscreen Digital Preamp with Tone Control – Part 1 by Nicholas Vinen & Tim Blythman
  8. Review: IOT Cricket WiFi Module by Tim Blythman
  9. Project: Second Generation Colour Maximite 2 – Part 2 by Geoff Graham & Peter Mather
  10. Project: Tapped Horn Subwoofer by Phil Prosser
  11. Serviceman's Log: 'Playing' with fire by Dave Thompson
  12. Project: Micromite to a Smartphone via Bluetooth by Tom Hartley
  13. Review: the tinySA Spectrum Analyser by Allan Linton-Smith
  14. Circuit Notebook: Multiple RAM banks for the IR Remote Control Assistant by Robbie Adams
  15. Circuit Notebook: Solar garden light uses supercapacitor by Bera Somnath
  16. Circuit Notebook: 1-2-5 switching arrangements by Barry Moore
  17. Circuit Notebook: Simple tripwire alarm by Thomas Scarborough
  18. Circuit Notebook: Letterbox counter by Raj. K. Gorkhali
  19. PartShop
  20. Vintage Radio: Sanyo 8-P2 TV (1962) by Dr Hugo Holden
  21. Product Showcase
  22. Ask Silicon Chip
  23. Market Centre
  24. Advertising Index
  25. Notes & Errata: Programmable Hybrid Lab Supply with WiFi, May & June 2021; Hugh-Current Four Battery/Cell Balancer, March & April 2021; Speedo Corrector Mk.3, September 2013
  26. Outer Back Cover

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Articles in this series:
  • Advanced Medical & Biometric Imaging – Part 1 (August 2021)
  • Advanced Imaging - Part 2 (September 2021)
Items relevant to "Touchscreen Digital Preamp with Tone Control – Part 1":
  • Touchscreen Digital Preamp PCB [01103191] (AUD $12.50)
  • Touchscreen Digital Preamp ribbon cable/IR adaptor PCB [01103192] (AUD $2.50)
  • PIC32MX170F256B-50I/SP programmed for the Touchscreen Digital Preamp, 2.8in screen version [0110319A.hex] (Programmed Microcontroller, AUD $15.00)
  • PIC32MX170F256B-50I/SP programmed for the Touchscreen Digital Preamp, 3.5in screen version [0110319B.hex] (Programmed Microcontroller, AUD $15.00)
  • Pair of AD8403ARZ10 quad digital potentiometer ICs (Component, AUD $40.00)
  • Micromite LCD BackPack V3 complete kit (Component, AUD $75.00)
  • Micromite LCD BackPack V2 complete kit (Component, AUD $70.00)
  • Micromite LCD BackPack V1 complete kit (Component, AUD $65.00)
  • Firmware for the Touchscreen Digital Preamp (Software, Free)
  • Touchscreen Digital Preamp PCB patterns (PDF download) [01103191/2] (Free)
Articles in this series:
  • Touchscreen Digital Preamp with Tone Control – Part 1 (September 2021)
  • Touchscreen Digital Preamp with Tone Control – Part 2 (October 2021)
Items relevant to "Second Generation Colour Maximite 2 – Part 2":
  • Second-generation Colour Maximite 2 PCB [07108211] (AUD $15.00)
  • Colour Maximite 2 software and documentation (Free)
  • Second-generation Colour Maximite 2 PCB pattern (PDF download) [07108211] (Free)
Articles in this series:
  • Second Generation Colour Maximite 2 – Part 1 (August 2021)
  • Second Generation Colour Maximite 2 – Part 2 (September 2021)
Items relevant to "Tapped Horn Subwoofer":
  • Dimensions and sheet cutting diagrams for the Tapped Horn Subwoofer (Panel Artwork, Free)
Items relevant to "Micromite to a Smartphone via Bluetooth":
  • Micromite Bluetooth sample software (Free)
Items relevant to "Multiple RAM banks for the IR Remote Control Assistant":
  • Multiple RAM banks for the IR Remote Control Assistant PCB design (ZIP download) (PCB Pattern, Free)
Items relevant to "Sanyo 8-P2 TV (1962)":
  • Sanyo 8-P2 Diagrams (Software, Free)

Purchase a printed copy of this issue for $10.00.

1-2-5 switching arrangements Many instruments offer adjustment in period or frequency range in 1-2-5 steps across a single decade as a knob or switch is changed. It is usually followed by a 10-20-50 in the next decade and so on through subsequent decades. The 1-2-5 relationship makes sense as it approximates a geometrical progression across the decade with just three steps, with the step multipliers being 2x, 2.5x and then 2x. As the period for monostable and astable circuitry is determined as a product of some factor times a resistance, it would be helpful to have a simple way to create resistance values inversely proportional to these steps. This can be done efficiently with either a centre-off single-pole toggle switch or a 3-pin header with a single jumper/shorting block. With the switch set to centre off (or the link removed), the only component connected between IN and OUT is the 5kW resistance. With the switch in the up (or link 1-to-2) position, the 5kW resistance is in parallel with the 3.333kW resistance, giving 2kW. With the switch in the down position (or link 3-to-1), the 5kW is in parallel with 1.25kW for an equivalent value of 1kW. Sourcing accurate 5kW, 3.333kW and 1.25kW resistors is not easy. But if we use a total of 13 resistors of the same value, we can get theoretically 13 100kW resistors soldered to a toggle switch, giving 1-2-5 resistance steps. Simple tripwire alarm I wanted an alarm that was so simple that (almost) anyone could use it without instructions. The result is a simple yet versatile circuit that stopped an intrusion into my car in its first week of use. It activates a powerful siren (the load, between C and D) for 70 seconds if someone disturbs a wire, ie, if the circuit is broken between points A and B. This can be multi-strand wire with bared ends twisted together, making it easy to separate. Alternatively, a piece of string may be tensioned such that it separates the wire when flexed. With the tripwire intact, the charge Circuit Ideas Wanted 92 Silicon Chip across the 3300μF capacitor is limited to less than 1V by the current flowing through diode D1. This is insufficient to switch Mosfet Q1 on, and Mosfet Q2 is held off by the tripwire keeping its gate voltage low, so the siren is not powered. Once the circuit is broken between A and B, the gate of Mosfet Q2 is immediately pulled up by the 100kW resistor and the siren switches on. The 3300μF capacitor can then charge, and eventually the gate voltage of Mosfet Q1 rises high enough to switch it on, pulling the gate of Q2 low and silencing the siren after about 70 seconds. Connect points A and B again, and perfect values by arranging them in parallel sets. We can create the 5kW value with two 10kW resistors in parallel, the 3.333kW value by putting three 10kW resistors in parallel and the 1.25kW value by putting eight 10kW resistors in parallel. As 1% resistors are cheap, the only real disadvantage of this method is the space required. If a single-pole centre-off switch is used, all these resistors can be soldered to the appropriate terminals of the switch on a front panel, and this makes it a simple way to change the period (or frequency) by a factor of 1, 2 or 5 with just one switch. Fewer resistors can also be used if you’re willing to accept slight errors. For 3.333kW, you can use 3.9kW in parallel with a 22kW (an error of just 0.6%). For 1.25kW, we can use 1.5kW in parallel with two 15kW resistors, which is an exact match. You can also scale all the resistor values by the same amount, eg, use sets of 1kW or 100kW resistors instead of 10kW. The accompanying photo shows 13 100kW resistors soldered to a toggle switch as suggested above. Barry Moore, Minto, NSW. ($80) without a squeak, the circuit is ready for another round. In the interests of simplicity, I wanted an alarm without an on/off switch or reset switch. If desired, one may simply clip the circuit onto a 12V battery, and it is ready to go. Three possible scenarios are shown below the circuit diagram. In the first, the alarm is powered up, the tripwire broken and the siren sounds for the full 70 seconds, then times out. The second is identical except that the tripwire is reconnected before the timeout, and the siren is immediately silenced. In the third case, points A & B are never connected, and power to the circuit is simply switched on to power the siren for a fixed period. Got an interesting original circuit that you have cleverly devised? We will pay good money to feature it in Circuit Notebook. We can pay you by electronic funds transfer, cheque or direct to your PayPal account. Or you can use the funds to purchase anything from the SILICON CHIP Online Store, including PCBs and components, back issues, subscriptions or whatever. Email your circuit and descriptive text to editor<at>siliconchip.com.au Australia’s electronics magazine siliconchip.com.au Letterbox counter This circuit starts counting when someone inserts a letter in the letterbox at your home or office. It is designed to save you time from going to the letterbox to check if there are letters inside. The number of letters present in the box is indicated on a seven-segment display. It uses a white LED (LED1), an LDR, a 555 timer (IC1) in monostable mode, a 4033 seven-segment driver chip (IC2) and a few other components. LED1 and LDR1 together work as a sensor. The resistance of LDR1 changes in accordance with the intensity of incident light on it. When light from LED1 falls on LDR1, its resistance is low. So when the light beam is broken, the voltage at pin 2 of IC1 is low; otherwise, it is high. When a letter is inserted into the letterbox, it passes between LED1 and LDR1. This change in resistance provides a triggering pulse to pin 2 of IC1, generating a short-duration pulse at its output pin 3. This pulse acts as the clock input for the 4033 counter and display driver, IC2. The output pins of IC2 are connected to various segments a, b, c, d, e, f and g pins of the seven-segment display, with the common pin of the display connected to ground. Each segment has its own current-limiting resistor for consistent brightness. When a letter is delivered to the letterbox, LED2 momentarily glows, which indicates that a letter is received, and the displayed count increases by one. When the counter reaches nine, it resets to zero and the cycle repeats. Switch S1 is used to reset the counter when you fetch the letters. Raj. K. Gorkhali, Hetauda, Nepal. ($60) The gate threshold voltages of Mosfets Q1 and Q2 are fairly critical. To avoid complications, I chose identical transistors. Even so, component tolerances may vary. If the alarm does not decisively turn off, or if current consumption does not fall to about 0.25mA when A and B are closed, insert another diode in series with D1 to raise the voltage at Q1’s gate. Almost any 12V battery may be used as long as it supports the load. Q2 can handle loads up to 74W. But note that a heatsink will be required for heavier loads. For driving a standard piezo siren, it will require no heatsink, as they only draw a few watts. One could also switch on a 12V lamp if desired. This circuit will remain on standby for about eight months using a small 12V 1.4Ah gel battery. A battery pack of alkaline AA cells may be used for a similar period of service. Thomas Scarborough, Cape Town, South Africa. ($75) siliconchip.com.au Australia’s electronics magazine September 2021  93