This 4 Oscillator Opto-Theremin is built into a standard Nintendo Gameboy using a Quad 4093 Schmitt Trigger IC, 4 1uf electrolytic caps, 4 Photo-Resistors in the 140k Ohm Range, and 4 Diodes. Check out the rather small video on the link below to see it in action and listen to the effect it produces. Pretty cool build with lots of details on each step.

4 Oscillator Opto-Theremin build into a Gameboy