An upright white box with a soft sheen, photographed face on. Along the top of the face, a small 3.5 mm jack at the left, a round photocell in the middle and a chrome toggle switch at the right. Below them, opto duet in pastel pink and blue letters, then a round chrome LED bezel in the middle of the face, and at the bottom a second photocell on the left and a black DC socket on the right.

Two light-coupled 555s · One of a kind

Opto Duet

Synth Available · 1 of 1

My Opto Duet, exactly as the guide draws it, in a box painted glow-in-the-dark with a UV LED in the middle of the face, so the box charges itself while it plays and keeps glowing after you switch it off. Two 555 theremins that only talk to each other through light, two photocells for your hands, and no knobs: the light in the room is the controls.

TypeTwo 555 oscillators coupled by light, on stripboard
BoardMy own layout, the free Opto Duet guide
ControlsTwo exposed photocells and a power toggle. No knobs; it is played with light
Output3.5 mm jack on the face, top left. No speaker inside
Enclosure125B, stood upright, painted glow-in-the-dark, the name in pastel lettering
Power9V DC, barrel jack on the face, bottom right
BuiltSeptember 2026
StatusOne of a kind. When it is gone it is gone

$60 free shipping Email to Buy

What it is

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

This is the vertical box from the demo on the guide page: the glow-in-the-dark one. The LED in the middle of the face is ultraviolet, which the glow paint drinks, so a few minutes of running leaves the whole box lit for a while after.

The controls

There are none, on purpose. A photocell top center and another bottom left, and the toggle top right is power. Cover a cell, uncover it, hold it near a lamp, play it in the dark by the light of its own LED.

The box

A 125B stood on end, painted glow-in-the-dark, everything on the face: output top left, power bottom right, the name in pastel lettering across the middle. Same upright format as the Devil's Triangle and the Dragonfly.

Demo

Demo video coming soon - it's on the bench being filmed.

Or build it yourself

The board inside is the free Opto Duet guide: the stripboard drawing, the parts list, the netlist, the file for Copper Bottom, and the vertical drill template that made this box.