Project 18 — The Machine With Hiccups (Self-Interrupting Relay)

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

Project 18 — The Machine With Hiccups (Self-Interrupting Relay)

The one idea: wire a machine so that turning on switches itself off — and it can’t ever decide, so it buzzes.

This is the series’ best predict-first moment. Set up the relay from Project 17, lid off, and pose the riddle before touching a wire.

The riddle (ask it out loud)

“The coil pulls the arm down. Fine. But what if we send the coil’s own electricity through the arm’s resting contact — the one the arm breaks when it moves? Turn it on… the coil pulls the arm… which cuts the coil’s power… so the magnet dies… so the spring pulls the arm back… which reconnects the power… which pulls the arm… Now what?”

Let the child sit in the paradox. Most adults get stuck here too, and the answer is delicious: it does all of it, forever, very fast.

What you need

• The relay (lid off — this project is theatre, the mechanism must be visible), its 1N4007 removed for now — no diode in this build (see physics: the diode would muffle exactly the effect we’re playing with; at 6 V and by hand this is fine)
• 6 V pack, alligator clips
• The big electrolytic capacitor from Project 4
• LED + 330 Ω, and optionally a screw + metal jar lid

Build it

1. Battery + → relay COM.
2. Relay NC (the contact the resting arm touches) → one coil pin.
3. Other coil pin → battery −.

Connect the last clip: BZZZZZZ. The arm is a blur, making and breaking its own lifeline a few hundred times a second. The child predicted a paradox; the universe answered buzzer. Watch it, feel it, hold a paper scrap against the arm to hear it drum.

Turn the lights off. Where the arm meets the contact there’s a faint, flickering blue-orange pinprick of spark — the coil biting back at the switch on every single break (Project 17’s seatbelt lesson, now visible a few hundred times per second). Perfectly safe at 6 V, and one of the best things the series lets a child see.

Slow-motion mode

Clip the big electrolytic across the coil (stripe/− leg to the battery-− side). The buzz drops to a slow, deliberate tick … tick … tick — the arm visibly flapping in slow motion. The capacitor is a bucket (the child knows this): each time the arm breaks the contact, the coil keeps drinking from the bucket for a moment, so every cycle stretches. Swap capacitor sizes and the tempo changes — a metronome with a mechanical heart. Wire the LED + 330 Ω through the relay’s NO contact to its own battery tap and it flashes on every beat.

Bell mode (optional, five minutes)

Tape a screw to the moving arm as a hammer, hold a metal jar lid where it strikes: ding-ding-ding-ding. That is the electric doorbell, the school bell, the fire alarm of the last century — the child has built the actual mechanism, not a model of it.

What to say to the child

“You found the answer! It turns on, which turns it off, which turns it on — it’s a machine with hiccups. It can never finish deciding, and each hiccup is one buzz. People discovered this trick almost two hundred years ago, and it became the doorbell, the school bell, and the buzzer in old alarm clocks. And remember the black chip that tapped the buzzer for us, thousands of times a second? This is its grandfather. Same idea — a circuit that keeps changing its own mind — just made of springs and magnets instead.”

For you — the physics

• What sets the frequency: each half-cycle is armature travel time (~1–2 ms each way, set by spring constant and moving mass) plus the coil current’s rise to pull-in through $\tau = L/R$. Net: a few hundred Hz — audible as a coarse buzz, and mechanical resonance explains why it has one loud preferred note rather than the 555’s tunable whine.
• This is a relaxation oscillator — the same species as the 555 astable: an energy reservoir (spring + field, or the 555’s capacitor), a threshold (pull-in/drop-out, or ⅓/⅔ $V_{CC}$), and negative feedback with delay (the state works to undo itself, but not instantly). Every relaxation oscillator is a machine prevented from having a stable state. The explainer lines the two up column by column; hysteresis (Project 16’s lesson) is what makes both snap between states instead of settling at a smoky equilibrium in the middle.
• The spark is the flyback story, uncensored: every break interrupts ~85 mA through ~mH in microseconds; $L\,dI/dt$ drives the contact gap past breakdown and a micro-arc burns for the microseconds the field needs to die. This is why we removed the diode (it would freewheel the current and quench the show — and also slow drop-out, muffling the buzz), why century-of-service bells used tungsten contacts, and why real designs put RC snubbers across contacts. Erosion is real but glacial: this demo spends maybe 10⁴ of the contact’s ~10⁶-cycle arcing budget per minute — fine for play sessions, and “the spark slowly eats the metal” is itself an honest engineering lesson.
• The capacitor trick, honestly: C across the coil does two things — after break, the cap sustains coil current (slower drop-out), and at make, the discharged cap steals charging current (slower pull-in). Period stretches from mechanical-milliseconds toward $RC$-tens-of-milliseconds; with 1000 µF you can push it to a countable few Hz. You’ve continuously morphed a mechanical oscillator into an RC one — by Project 6’s formula, pitch falls as C grows, in both machines.
• Lineage for the storytelling: Wagner hammer / trembler (1830s) → electric bell → Model T ignition trembler coils → vibrator power supplies in tube-era car radios. And the active buzzer from Project 2? Inside is a transistor doing exactly this dance with no moving contact — the child has now seen the inside of that black box too.

Try next

• Tempo knob: a chain of different capacitors across the coil — plot (by ear) tick period vs. C with the child choosing “bigger bucket or smaller?”
• Drive something: the flashing-LED beat makes a fine metronome for practicing; the NO contact can just as well pulse the Project 11 counter — a clock, in the computer sense, made of spring steel.
• Compare the two buzzers side by side: this one and the 555 + passive buzzer from Project 6. Same job, seventy years of technology apart.

→ The arc’s finale: Project 19 — making electricity.

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