Opto Duet CV - Optical Theremin

Two 555 theremins on one stripboard, coupled by nothing but light. The slow one flashes an LED sealed against a photocell, and that photocell is the timing resistor of the fast one, so the voice chirps at the rate the engine ticks. Two more photocells sit out on the face, one for each half. What your hands and the room do to them is the whole instrument.

New: a CV input for the pitch. It goes into the voice 555’s control pin through a 4.7K resistor, so an LFO, an envelope or a sequencer can push the pitch as well as the light can. More volts is lower. With nothing plugged in it plays exactly as it did before.

There are no knobs, on purpose. Cover a cell, uncover it, hold it near a lamp, or play it in the dark by the light of its own LED.

The box is a 125B stood on end and painted glow-in-the-dark, with the name in pastel lettering. The LED in the middle of the face is ultraviolet, and the glow paint drinks it up, so a few minutes of playing leaves the whole box glowing for a while after you switch it off.

• Two 555 oscillators coupled by light, on my own stripboard layout
• Two exposed photocells, a power toggle and a pitch CV input. No knobs
• 3.5 mm output on the face, top left. No speaker inside
• CV in bottom left, power bottom right
• 9V DC barrel jack
• Built September 2026, CV input added October 2026
• One of a kind

The board inside is my free Opto Duet guide at messopedals.com/diy/, if you’d rather build your own.