The Challenger II is a an optical theremin named after the Space Shuttle Challenger. I built the Challenger II in 2008 along with the Challenger I, which was a similar optical theremin. Challenger I was given away to a local musician shortly after being built.
The Challenger II uses a 555 IC oscillator circuit. Pitch is determined by the amount of light that reaches the photoresistor mounted to the circuit board.
There is a small plastic guard that protects the circuit board and acts as a filter for the photoresistor.
The “OSC” LED is tied to the oscillators output, and blinks along with the frequency of the square wave output.
The only control aside from the photoresistor is the ON/OFF switch mounted at the top of the face of the unit. There is no volume control; the speaker is hard-wired into the output of the 555 IC.