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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