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.
Videos and other circuit bent projects at www.okhousecat.com