Simple optical theremin

Here’s a simple theremin-like device designed by Sacred and presented by Jameco. Unlike other theremins which vary the sound based on the user’s proximity to stationary antennae, this project uses two phototransistors and a 555 timer. The theory of operation is the variation of the sound based upon the amount of IR light reflected by the user’s hand. The article contains the schematic and BOM (conveniently keyed to Jameco part numbers, of course.)

Instructables via Jameco.