Shown above is the updated schematic of the DIY Photometer from Yang et.al.
The prototype deviates from this schematic in the following areas:
- the blue LED is connected to a 1K resistor in series with the the 1K potentiometer.
- the blue LED GPIO pin is digital 8
- The green LED GPIO pin is digital 9
- The light detector output is on GPIO pin analog 3
Comments. The 1k resistor reduces the blue LED intensity and prevents saturation of the detector. The one I put in is huge – but I have these on hand. A 1/4 watt resistor would fit in better. I moved the LED GPIO pins to test pulse width modulation (pwm) as a light intensity dimming mechanism. I tested on pin 9 which is pwm enabled – this didn’t work well, hence the 1k resistor. pwm could be tested further …seems like it should have worked.