Project 12 — The Light Theremin

Text written by Claude Fable 5 (claude-fable-5).

Project 12 — The Light Theremin

The one idea: here is a resistor that feels light. Put it where the pitch knob was, and the circuit plays whatever your hand does above it.

This was teased at the end of Project 6 as a one-line “try next”; it deserves a whole sitting, because it is the single highest wow-per-minute build in the series — and the first of three bridge projects (12–14) that mix the core arc’s circuits with the sensing arc’s ideas.

What you need

• The Project 6 tone generator, rebuilt on the breadboard (rebuilding it is the warm-up — the child has done it before, let them lead)
• One LDR — light-dependent resistor, the round part with the squiggle on its face (GL5528 or similar; the one part in this project you may not own)
• A flashlight, or a phone torch
• 4×AA (6 V) or a 9 V — whatever Project 6 ran on

Build it

One move: pull out R2 (or the potentiometer) and put the LDR in its place — from pin 7 to pins 6/2. The LDR has no polarity; either way round works.

Power it up. The circuit sings — and the pitch now belongs to the room’s light:

• Wave a hand over the LDR — the pitch swoops down under the shadow and glides back up as your hand leaves.
• Cup both hands over it — the tone falls and falls… and somewhere in the dark it stops being a tone and becomes individual clicks. (Remember Project 6’s warm-up, tapping the wire? You’ve slowed the tireless tapper back down to human speed.)
• Shine the flashlight point-blank — it shrieks up high.
• Play it like an instrument: hand height = pitch. That’s a theremin.

What to say to the child

“This little part is a resistor — like the ones in every circuit we’ve built — but it’s a strange one: it feels light. Lots of light makes it easy for electricity to get through; shadow makes it hard. And you know what the knob did to the pitch… so now the light is the knob. Your hand isn’t touching anything — the circuit is watching it. Can you play a song with your hand in the air?”

For you — the physics

• Why the pitch tracks light: the 555 astable’s frequency is $f \approx \dfrac{1.44}{(R_1 + 2R_2)\,C}$, and the LDR is $R_2$. A GL5528 runs ~5–20 kΩ in room light, ~100 kΩ under a hand, and climbs toward 1 MΩ in darkness — a factor of ~100 in resistance, hence roughly two decades of frequency: a few hundred Hz in the open, single clicks per second in cupped darkness. The child can hear a resistance–light curve.
• Why the notes glide (and why that’s the LDR’s fingerprint): the LDR is CdS bulk photoconductivity with slow trap states — its resistance takes tens to hundreds of milliseconds to follow a light change. In the sensing arc that sluggishness was a defect; here it’s the instrument’s charm: the pitch slides between values (portamento) instead of stepping. A phototransistor in the same spot would snap between two pitches and sound like an alarm. Same story, full detail: the optoelectronics explainer.
• A sensor is just a component that lets the world in. Every circuit so far was controlled by fingers on switches and knobs. This is the series’ quietest big idea: replace a fixed part with a world-dependent part and the circuit acquires a sense. Projects 13 and 14 pull the same trick with skin and with a light beam.

Try next

• Duet: build a second 555 with its own LDR and buzzer — two children, four hands, honest chaos.
• Darkness alarm: swap the LDR into R1’s position instead and think through with the child what changes (and see the pitch table invert).
• Calibrated hand: find the hand heights for two notes an octave apart — the height ratio is a resistance ratio you can actually reason about.

→ Next bridge project: Project 13 — You Are a Wire, where the circuit runs through the child.

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