Project 14 — The Song That Rides on a Light Beam

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

Project 14 — The Song That Rides on a Light Beam

The one idea: light can carry something. A song can ride on a beam across the room — and anything that blocks the beam steals the song.

This is the bridge projects’ capstone, and it’s two old friends pointed at each other: the Project 6 tone generator on one side, the electronic eye from Project 7 on the other — this time without the plastic slot, staring across open air.

What you need

Transmitter (one breadboard):

• The Project 6 555 oscillator, with its pitch knob if you have it
• A bright LED where the buzzer was — clear-lens high-brightness is best, but any LED works at short range — with its ~470 Ω resistor

Receiver (a second breadboard, or the far end of the same one — but a separate battery, so the “nothing connects them” point lands):

• The phototransistor left over from the sensing-arc parts order
• A piezo disc (or passive buzzer)
• A drinking straw or rolled-paper tube (the receiver’s “telescope”)
• Optional louder version: 1 NPN + two 10 kΩ

Build it — stage 1: the whisper version (3 parts!)

The minimal receiver is beautiful: battery + → phototransistor collector; emitter → piezo disc → battery −. That’s all.

On the transmitter, wire 555 pin 3 → 470 Ω → LED → GND. Power it up: the LED looks… steadily, boringly on. (Hold that thought.)

Now aim the LED into the phototransistor from a few centimetres and put an ear to the piezo: the tone is there. No wire between the two circuits — the song is arriving on the light.

• Slide a hand into the beam: mute. Remove it: music.
• Turn the pitch knob at the transmitter — the receiver changes pitch. The child at the receiver “hears the knob” from across the room.
• Slip the straw over the phototransistor and the beam works at a metre or two, even with the room lights on. Darken the room and go further.
• Mirror: bounce the beam around a corner. Paper blocks it; a clear glass of water doesn’t; a hand glows red but mutes the song.

Build it — stage 2: out loud

The piezo whisper is intimate, but for room-filling volume add the amplifier — which is exactly Project 7’s stage-1 wiring:

1. + → 10 kΩ → phototransistor collector; emitter → GND. The junction is the signal node, swinging at the audio rate.
2. Node → 10 kΩ → NPN base; emitter → GND; passive buzzer from + to the NPN’s collector.

Same rules as Project 7: if it’s silent, swap the phototransistor’s legs.

What to say to the child

“Look at the little lamp — just shining, right? It’s tricking you. It’s actually blinking on and off thousands of times every second — way too fast for your eyes, they just blur it into ‘on’. But our robot eye from the treasure alarm never blurs. It sees every single blink, and blink-blink-blink that fast — that’s exactly the buzzer song. The song is riding on the light! Put your hand in the beam… you caught the song. Where did it go?”

For you — the physics

• Persistence of vision, quantified: human flicker fusion gives out around 50–90 Hz; the LED is switching at hundreds to thousands of Hz. The same physical signal is “steady light” to the eye and “audio” to the detector — a nice, honest demonstration that bandwidth is a property of the receiver.
• This is why the fast eye mattered. The speed hierarchy becomes a product demo: an LDR (~10 Hz of bandwidth) in the receiver would output silence — it would average the blinking into DC. The phototransistor’s ~µs response makes kHz audio trivial. (Fibre-optic receivers, doing this at tens of GHz, use photodiodes — the next rung of the same ladder.)
• Why the piezo receiver ignores the room lights: a piezo disc is a capacitor — it passes only changes. Steady sunlight through the phototransistor is DC and does nothing; the kHz wiggle is what makes sound. The minimal receiver has a built-in high-pass filter by physics, not by design.
• What this is and isn’t, honestly: we’re doing baseband on–off keying — the light itself switches at the audio frequency. Real systems (IR remotes at 38 kHz, Li-Fi, fibre) modulate a faster carrier and filter for it, which is how they shrug off ambient light; Project 7’s explainer touched this. The concept the child takes away — information rides on light — is exactly right either way.
• The lineage is glorious: Alexander Graham Bell’s photophone (1880) sent voice 200 m on a sunbeam bounced off a voice-vibrated mirror, onto a selenium photoresistor. Bell thought it was a greater invention than his telephone. He was arguably right — its descendants carry this web page.

Try next

• Listen to the house. Unplug the transmitter, keep the loud receiver, and point the straw at things: cheap LED bulbs hum at 100 Hz, incandescents barely whisper, screens and some lamps sing weird tunes, a TV remote pressed at it chirps in bursts (that’s the 38 kHz carrier’s envelope). Light is full of songs nobody listens to; the child now owns the only ear in the house that can.
• Spy channel: swap the visible LED for the IR LED from the parts box — the beam disappears (except to a phone camera) but the music still arrives.
• Two stations + two pushbuttons = a light-beam telegraph between rooms.
• Beam-break, reborn: at this point the child may independently notice that Project 7’s alarm was this project with a boring transmitter. That moment is worth the whole parts order.

→ The series continues with a new force entirely: Project 15 — the electromagnet crane.

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