This hand-painted optical theremin uses two photo optic sensors to emulate the sound and playing technique of a traditional theremin.  One light sensor controls the pitch and the other controls the volume, both by detecting how much light is reaching the sensor (try with a strobe light!).   One of the knobs sets the master volume and the other sets the pitch threshold.

The case is made from an altoids tin and the plastic box from a chromatic tuner.