The lamp theremin

Most theremins are about playing a melody. This one is about resolving a chord.

Two tones, a little out of tune, make a slow wah-wah-wah. Move your hands until the wobble stops. When it does, you’ve won. Nobody needs instructions: the sound tells you everything.

1. Two distance sensors read your hands. One is twice as sensitive as the other, so one hand tunes coarse and the other fine.
2. The pitch follows your hand, and the second tone is detuned by 1.2 %, which makes the beating.
3. The sweet spot drifts slowly, on a 45-second cycle, so it never stays solved.
4. Find it, and the tones slide into unison, a sub-bass kicks in and the bulb burns steady at full.
5. Then nine lamps around the room black out, flash and breathe again.

• A Raspberry Pi 4 with a TRIAC hat dims a 230 V incandescent bulb. At rest it flickers like an old machine hall. Out of tune, it flickers faster.
• The sound is generated in Python with NumPy: phase-continuous oscillators, so no clicks, plus a layer of electric crackle.
• Nine Shelly dimmers do the cascade. The switchboard flashes a warm WLED at the same time.
• The first sensors were open ultrasonic modules. Kids pressed the membranes in with their fingers, and they went deaf. They were swapped for waterproof ones.
• If a sensor breaks, it keeps running on one.