Michael Beyhs shows us how to use an LED as a sensor: How the code works: Configure all analog inputs to outputs and pull low to turn on all red LEDs. Turn one analog input pin into an actual Input and read the amount of light that is reflected into it. Turn that input back into an output and continue on to next. After all inputs have been read adjust the PWM of the blue LEDs according to the amount of light that was measured. loop forever… Via Hacked Gadgets.