Beep-it is an optical theremin. It outputs a square wave whose pitch is controlled by the amount of light striking a photoresistor. You control the pitch by casting shadows over the light sensor, or by pointing it towards/away from a light source. Flashing lights induce an interesting oscillating effect. A single momentary button turns Beep-it on or off.

There is an 1/4" output jack for connecting to audio equipment like amplifiers, guitar pedals, recording, etc.

Beep-it! from Michael Una on Vimeo.