Tonight’s 20 minute electronics project was to create a simple transmitter to send music using light. A trivial circuit modulates the current through an LED, and a different LED serves as an (inefficient, and not very good) light sensor. Normally you’d use a selenium photocell or the like, but I couldn’t find one in my junk box, and Radio Shack doesn’t have ’em anymore. But LEDs will generate a small current when exposed to light, so you can actually use them as a photodetector.
Addendum: The audio in the video above is pretty weak (the microphone on the iphone is located at the bottom of the phone, and isn’t ideal for recording low level audio). So, I went ahead and recorded a small sample using the voice memo application on the iPhone, and holding the microphone much closer to the speaker of the amplified speaker to give you a better idea of what the quality is. I also reduced the value of the input filter cap to just 4.7uF, which seemed a bit better, and also put in another 4.7uF cap in series with the sensor LED. I’m not sure that helped, but the levels and sensitivity seemed better. At the very least you should be able to hear the audio quality more clearly.
Addendum2: The guys over at the Make Blog had some more good information about using LEDs as light sensors.