Project 9 — The Speedometer You Can Hear

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

Project 9 — The Speedometer You Can Hear

The one idea: a spinning wheel pokes the invisible eye many times a second — each poke is a click, fast clicks blur into a tone, and the pitch is the speed. You will hear the motor speed up and slow down as a rising and falling note.

Project 6 taught “fast on/off = a tone, faster = higher.” There the 555 chip did the switching. Today the motor does it, by spinning a slotted disc through the invisible eye from Project 7 — and suddenly you own a laboratory instrument: this is exactly how a bike computer, a printer, and a computer mouse wheel measure rotation. The dignified name is rotary encoder. Yours is cardboard.

What you need

• The motor from Project 3, on its own 2×AA or 3×AA supply (a shared battery sags when the motor loads it and the electronics gets cranky — two separate packs, joined only at GND if you add the Arduino later)
• A slotted optointerrupter + the eye circuit parts from Project 7 (220 Ω, 10 kΩ ×2, NPN transistor)
• A passive buzzer or bare piezo — we want the raw clicks; an active buzzer would sing its own note and ruin everything
• An LED + 470 Ω (a visible twin for the buzzer)
• Stiff cardboard, scissors, tape, blu-tack

Build it — stage 1: the chopper disc

1. Cut a cardboard disc, ~6–8 cm across. Find the centre honestly (trace something round, fold-crease twice — the creases cross at the centre).
2. Cut 8 slots around the rim: each ~5 mm wide and ~10 mm deep, evenly spaced. Even-ish is fine; the ear won’t audit. (A marker-drawn “pizza with 8 bites taken out of the crust” is the right mental picture for the child.)
3. Push the disc onto the motor shaft through a snug centre hole (or a blob of hot glue / a taped-on bottle cap as a hub). It should spin without wobbling into things.
4. Mount motor and optointerrupter (blu-tack is fine) so the rim runs through the slot: cardboard blocks the beam, each cut-out slot lets it flash through.

Build it — stage 2: the ear

This is literally Project 7 stage 1, unchanged — beam blocked = transistor on — except the buzzer is now passive, so instead of one long BZZZT you get a click at every transition:

1. + → 220 Ω → opto LED → GND.
2. + → 10 kΩ → phototransistor collector; emitter → GND. The junction is the signal node.
3. Node → 10 kΩ → NPN base; emitter → GND; passive buzzer from + to the NPN collector. Add the LED + 470 Ω in parallel with the buzzer.

Turn the disc slowly by hand first: click… click… click… — one per slot, and the LED blinks along. The child should own this moment: the eye sees the beam appear and vanish, and each change is one click.

Now power the motor. The clicks vanish — into a tone. That tone is the speedometer.

Play with it (this is the actual lesson)

• Finger brake: press a fingertip gently against the spinning shaft (or hub — not the disc edge). The note slides down as you press, up as you release. You are hearing friction.
• Fewer batteries: run the motor on 1.5 V instead of 3 V. Lower note. More volts, higher note — Project 3’s “more push = faster” now has a sound.
• Watch the LED while the pitch climbs: blink, blink, flicker, then — apparently steady light. Your ear still hears a tone long after your eye gives up. (That surrender point, ~50 flashes a second, is why movies at 24 frames need tricks, and why old TVs flickered at the edge of vision.)

Measure it — the party trick for the parent

The pitch doesn’t just indicate the speed; it equals it, via

with $N = 8$ slots. Open any guitar-tuner or spectrum app on your phone, hold it near the buzzer, and read $f$. Then:

A little motor humming at $f = 400$ Hz is doing 3 000 RPM — fifty revolutions every second, measured with cardboard, a phone, and one transistor. Log pitch vs. battery count, or pitch vs. finger pressure, and you’re doing real experimental physics.

What to say to the child

“Remember the chip that tapped the buzzer super fast to make a note? Today the motor is the tapper. Every slot in your wheel lets the secret light wink at the eye — wink, click, wink, click. Spin it slow with your finger: you can hear every single wink. Now let the motor spin it… hear that? The clicks got so fast they melted into a note! Higher note = spinning faster. Press your finger on it, gently… hear the note fall? You just heard how fast something is spinning. Cars know their speed exactly this way — a little wheel with slots, winking at an eye.”

For you — the physics

• An encoder is a frequency transducer: angular velocity ω in, pulse train at $f = N\omega/2\pi$ out. Frequency is the easiest physical quantity to measure precisely (just count), which is why so many sensors convert their quantity to a frequency first. Your bike computer: one magnet = 1-slot encoder. A mouse scroll wheel: this exact slotted-disc-in-an-opto, miniaturised (two eyes, offset — see below).
• Why the LDR could never: at 3 000 RPM and 8 slots the beam toggles every ~2.5 ms. The phototransistor’s ~10 µs response follows easily; an LDR’s ~100 ms would smear hundreds of slots into a constant blur. Project 7’s “junction beats bulk” speed claim, now audible.
• Waveform: the duty cycle is set by the slot-to-spoke width ratio, and a piezo driven by sharp edges radiates mostly at those transitions — the buzzy, ticky timbre. The pitch your ear names is the pulse repetition rate, solid and unambiguous.
• Persistence of vision vs. audio: the eye integrates over ~20–50 ms; the ear resolves pitch up to 20 kHz. The LED-vs-buzzer race the child just watched is a clean demonstration that different senses sample the world at wildly different rates.

Try it with the Arduino — a real tachometer

Feed the signal node (before the NPN) into pin 2 — common GND with the sensor supply — and count pulses:

const byte SLOTS = 8;
volatile unsigned long pulses = 0;
void tick() { pulses++; }

void setup() {
  pinMode(2, INPUT);
  attachInterrupt(digitalPinToInterrupt(2), tick, FALLING);
  Serial.begin(9600);
}

void loop() {
  pulses = 0;
  delay(500);                        // count for half a second
  Serial.println(pulses * 120UL / SLOTS);   // RPM
}

Numbers update twice a second; print to a 7-segment/LCD if you have one wired. Cross-check the Arduino’s RPM against the tuner-app method — two independent instruments agreeing is the most scientist-feeling moment in this series.

Try next

• Direction sensing: real mice use two eyes, offset by half a slot — which one blinks first tells you the spin direction (quadrature). A second optointerrupter and an oscilloscope-curious parent can reproduce it.
• Anemometer: paper-cup pinwheel on the disc — wind speed as pitch.
• The same “pulses per event” idea, one at a time instead of thousands per second, is Project 11’s counter.

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