I think I wrote this: http://www.arduino.cc/playground/Learning/LEDSensor http://www.arduino.cc/playground/Learning/LEDSensor The key points are remembering th

I think I wrote this: http://www.arduino.cc/playground/Learning/LEDSensor http://www.arduino.cc/playground/Learning/LEDSensor The key points are remembering that there is capacitance on the pins of the microcontroller, and that while forcing an electron across from P to N emits a photon, absorbing a photon lets an electron back from N to P if it wants to go.