LED Transciever

I remembered the fact that LEDs (well all semiconductors) are sensitive to light after browsing through Forrest M. Mims' book Timer, Op Amp, and Optoelectronic Circuits & Projects again. I had some time and connected a white LED (which had resistor soldered to it) I had lying around from a previous project to an Arduino. The LED's cathode went to ground and the anode went into an ADC channel. I printed out the readings to the serial output and graphed them with some sample code for Processing I found online.

I was surprised by how effective the LED was at detecting changes in light. I was in a normally lit room, with the light bulb about five feet from the LED. I waved my hand back forth in the beginning of this graph and one more time near the end. When I tested with just my finger going over the LED quickly, I could easily see small dips in the graph.

On thing I did notice was that the LED had a "memory effect." While it could respond to fairly fast changes in light, if I kept my hand over the LED, it took a second for the voltage to drop to a steady level:

I will test the sensitivity with a bigger contrast between lit and covered and see how fast the LED can respond. I also don't know how much noise there is in this system. I'll need a way to not trigger on noise and detect a small dip in the average voltage. 

For now, these results are very interesting and I might make a two-way transmitter with only one LED on each side.