The Challenger II is an optical theremin, originally built in 2008. It is similar to a traditional theremin in that it is played without physical contact.
The Challenger II uses a 555 IC as an oscillator and a photocell as a resistor to generate a square wave. As the amount of light that hits the photocell increases, the frequency of the square wave increases. The circuit is based off of the schematic designed by Beavis Audio Research, with some modifications to make use of the photocell.
There is only one physical control on the Challenger II, a power switch located on the front panel. There are no controls for manipulation of tone, volume, or any other effects. There is no audio output on this instrument.
The 2″ speaker provides the audio output, and is attached to the unit. This creates an entirely self-contained musical instrument. The power source, oscillator, amplifier, and speaker are all onboard. The circuit board is held up with an Express for Men 1MX dress shirt collar stay punch-out.
The Challenger II showcases the flexibility of the 555 IC oscillator circuit, and demonstrates just one of the multitude of forms this style of synthesizer has taken since its creation.