Silicon ChipColour recognition using LEDs and an LDR - October 2021 SILICON CHIP
  1. Outer Front Cover
  2. Contents
  3. Publisher's Letter: The chip shortage is now a component shortage
  4. Mailbag
  5. Feature: Detecting Gravitational Waves by Dr David Maddddison
  6. Product Showcase
  7. Project: Tele-com – an intercom using analog phones by Greig Sheridan & Ross Herbert
  8. Project: Two- or Three-Way Stereo Active Crossover – Part 1 by Phil Prosser
  9. Feature: El Cheapo Modules: 3.8GHz Digital Attenuator by Jim Rowe
  10. Project: SMD Test Tweezers by Tim Blythman
  11. Review: PicoScope 6426E USB Oscilloscope by Tim Blythman
  12. Project: Touchscreen Digital Preamp with Tone Control – Part 2 by Nicholas Vinen & Tim Blythman
  13. Review: Solder Master ESM-50WL Cordless Iron by Tim Blythman
  14. Serviceman's Log: Life on the 'bleeding edge' by Dave Thompson
  15. Feature: UT-P 2016 MEMS Woofer by Allan Linton-Smith
  16. Vintage Radio: Reinartz 4-valve reaction radio by Fred Lever
  17. Circuit Notebook: Colour recognition using LEDs and an LDR by Les Kerr
  18. Circuit Notebook: Battery charger with WiFi interface by Phillip Webb
  19. PartShop
  20. Ask Silicon Chip
  21. Market Centre
  22. Advertising Index
  23. Notes & Errata: Tapped Horn Subwoofer, September 2021; Touchscreen Digital Preamplifier, September 2021; Battery Manager, August 2021; Bush VTR103 AM/FM radio, August 2021; Silicon Labs FM/AM/SW Digital Radio, July 2021; 7-Band Stereo Equaliser, April 2020; CLASSiC DAC, February-May 2013
  24. Outer Back Cover

This is only a preview of the October 2021 issue of Silicon Chip.

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Items relevant to "Tele-com – an intercom using analog phones":
  • The Tele-com Intercom PCB (aka OzPLAR) [12110211] (AUD $30.00)
  • The Tele-com Intercom PCB pattern (PDF download) [12110211] (Free)
  • Tele-com front & rear panel 3D printing files (Panel Artwork, Free)
Items relevant to "Two- or Three-Way Stereo Active Crossover – Part 1":
  • Two- or Three-Way Stereo Active Crossover PCB [01109211] (AUD $15.00)
  • Two- or Three-Way Active Crossover PCB patterns (PDF download) [01109211] (Free)
Articles in this series:
  • Two- or Three-Way Stereo Active Crossover – Part 1 (October 2021)
  • Two- or Three-Way Stereo Active Crossover – Part 2 (November 2021)
Articles in this series:
  • El Cheapo Modules From Asia - Part 1 (October 2016)
  • El Cheapo Modules From Asia - Part 2 (December 2016)
  • El Cheapo Modules From Asia - Part 3 (January 2017)
  • El Cheapo Modules from Asia - Part 4 (February 2017)
  • El Cheapo Modules, Part 5: LCD module with I²C (March 2017)
  • El Cheapo Modules, Part 6: Direct Digital Synthesiser (April 2017)
  • El Cheapo Modules, Part 7: LED Matrix displays (June 2017)
  • El Cheapo Modules: Li-ion & LiPo Chargers (August 2017)
  • El Cheapo modules Part 9: AD9850 DDS module (September 2017)
  • El Cheapo Modules Part 10: GPS receivers (October 2017)
  • El Cheapo Modules 11: Pressure/Temperature Sensors (December 2017)
  • El Cheapo Modules 12: 2.4GHz Wireless Data Modules (January 2018)
  • El Cheapo Modules 13: sensing motion and moisture (February 2018)
  • El Cheapo Modules 14: Logarithmic RF Detector (March 2018)
  • El Cheapo Modules 16: 35-4400MHz frequency generator (May 2018)
  • El Cheapo Modules 17: 4GHz digital attenuator (June 2018)
  • El Cheapo: 500MHz frequency counter and preamp (July 2018)
  • El Cheapo modules Part 19 – Arduino NFC Shield (September 2018)
  • El cheapo modules, part 20: two tiny compass modules (November 2018)
  • El cheapo modules, part 21: stamp-sized audio player (December 2018)
  • El Cheapo Modules 22: Stepper Motor Drivers (February 2019)
  • El Cheapo Modules 23: Galvanic Skin Response (March 2019)
  • El Cheapo Modules: Class D amplifier modules (May 2019)
  • El Cheapo Modules: Long Range (LoRa) Transceivers (June 2019)
  • El Cheapo Modules: AD584 Precision Voltage References (July 2019)
  • Three I-O Expanders to give you more control! (November 2019)
  • El Cheapo modules: “Intelligent” 8x8 RGB LED Matrix (January 2020)
  • El Cheapo modules: 8-channel USB Logic Analyser (February 2020)
  • New w-i-d-e-b-a-n-d RTL-SDR modules (May 2020)
  • New w-i-d-e-b-a-n-d RTL-SDR modules, Part 2 (June 2020)
  • El Cheapo Modules: Mini Digital Volt/Amp Panel Meters (December 2020)
  • El Cheapo Modules: Mini Digital AC Panel Meters (January 2021)
  • El Cheapo Modules: LCR-T4 Digital Multi-Tester (February 2021)
  • El Cheapo Modules: USB-PD chargers (July 2021)
  • El Cheapo Modules: USB-PD Triggers (August 2021)
  • El Cheapo Modules: 3.8GHz Digital Attenuator (October 2021)
  • El Cheapo Modules: 6GHz Digital Attenuator (November 2021)
  • El Cheapo Modules: 35MHz-4.4GHz Signal Generator (December 2021)
  • El Cheapo Modules: LTDZ Spectrum Analyser (January 2022)
  • Low-noise HF-UHF Amplifiers (February 2022)
  • A Gesture Recognition Module (March 2022)
  • Air Quality Sensors (May 2022)
  • MOS Air Quality Sensors (June 2022)
Items relevant to "SMD Test Tweezers":
  • SMD Test Tweezers PCB set [04106211+04106212] (AUD $10.00)
  • PIC12F1572-I/SN [0410621A.HEX] or PIC12LF1572-I/SN [0410621C.HEX] programmed for the SMD Test Tweezers (Programmed Microcontroller, AUD $10.00)
  • 64x32 white OLED screen (0.49-inch, 1.25cm diagonal) (Component, AUD $10.00)
  • Improved SMD Test Tweezers kit (Component, AUD $35.00)
  • Firmware for the SMD Test Tweezers [0410621A.HEX] (Software, Free)
  • SMD Test Tweezers PCB patterns (PDF download) [04106211/2] (Free)
Items relevant to "Touchscreen Digital Preamp with Tone Control – Part 2":
  • 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)
Articles in this series:
  • A high-performance MEMS tweeter that’s just 6.7 x 4.7mm! (May 2020)
  • UT-P 2016 MEMS Woofer (October 2021)
Items relevant to "Colour recognition using LEDs and an LDR":
  • Software, drawings & Gerber files for Colour Recognition using LEDs and an LDR (Free)
Items relevant to "Battery charger with WiFi interface":
  • Arduino sketches and user manual for the WiFi Battery Charger (Software, Free)

Purchase a printed copy of this issue for $10.00.

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. Colour recognition using LEDs and an LDR I decided to make a box that determines the colour of objects placed upon it. Rather than using a prebuilt module or phone app, I’m doing it from first principles, using three LEDs (red, green & blue) and a light-dependent resistor (LDR). I have found that both adults and children are very impressed and spend quite a bit of time playing with it. On the top, it has an LCD screen that shows the result, an on/off switch, a pushbutton for initiating a test and a cylinder that has the LDR and LEDs mounted in the bottom. The object to be measured is placed over the cylinder. When you press the pushbutton, the microcontroller inside switches each coloured LED on sequentially and measures the intensity of the reflected light using an LDR, a constant current source and the analog-todigital converter in the microcontroller. Circuit Ideas Wanted siliconchip.com.au It converts the LDR resistance to a voltage that it can measure using a constant current source. The percentage of each colour is calculated, along with the overall colour of the object. The result is displayed on the LCD screen. The results are quite reproducible, and it works for different shade intensities of each colour. Consider a purple object. It would reflect most of the blue or red light incident on it, but not green. So when the blue or red LED is lit, the LDR resistance is low, and when the green LED is lit, the LDR resistance is high. This is how the unit can determine the colour of the object. All the components are available offthe-shelf and are mounted on a simple PCB that plugs into the display. To cater for the considerable variation in parameters of the coloured LEDs and LDR, four potentiometers are used for the initial calibration, which only requires a DVM. Place a white object over the sensor and remove IC1 from its socket. Short socket pin 2 to ground, set VR2 mid-way and measure the voltage at socket pin 15. Then short socket pin 13 to ground instead of 2, and adjust VR3 to get the same voltage at pin 15. Repeat for socket pin 12 and VR4. If you can’t get the voltages equal, adjust VR2 one way or the other then repeat the above steps. Finally, replace the micro, press the test button and check the readings. Adjust VR1 to get them all above 200, and re-adjust VR2-VR4 (if necessary) to make the readings equal. The Gerber files for the PCB and CAD drawings for the box and LED/ LDR holder, along with the HEX file and BASIC source code for the PIC are available from siliconchip.com. au/Shop/6/5931 Les Kerr, Ashby, NSW. ($120) 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 October 2021  103