Project 16 — The Switch Nobody Touches (Reed Switch Door Alarm)
Text written by Claude Fable 5 (claude-fable-5).
Project 16 — The Switch Nobody Touches (Reed Switch Door Alarm)
The one idea: a magnet can press a switch through solid things — no touching, no seeing, no wires to the magnet at all.
Project 15 made magnetism from electricity. Today the child uses a permanent magnet’s field as a finger — one that reaches through cardboard, wood, and skin — and builds the sensor that actually guards most real front doors.
⚠️ Magnet safety — the serious one. Small neodymium magnets are in the same danger class as coin cells: two swallowed magnets (or a magnet plus anything steel/a coin cell) attract through intestinal walls and mean emergency surgery. Count them out at the start of every session and back into a labelled box at the end, exactly like the coin-cell rule in Read Me First. Also: they leap at each other and pinch, they shatter if allowed to slam, and they’ll wipe hotel key cards. The reed switch’s glass body is fragile too — always bend its leads with two fingers holding the lead at the glass, never by pulling on the capsule.
What you need
- A reed switch — a tiny glass pill with a wire out each end and two overlapping metal tongues visible inside
- A small neodymium magnet (a 6–10 mm disc is plenty)
- LED + 330 Ω, the active buzzer, an NPN + two 10 kΩ
- Tape, and a door, drawer, or treasure box to guard
Warm-up — the magic trick
Wire the simplest loop of the whole series: battery + → reed switch → LED → 330 Ω → battery −. It’s dark: inside the glass, the two tongues almost-touch, with a hair’s gap.
Bring the magnet near. Click — light. Pull it away — dark. Now the part that makes six-year-olds gasp:
- Put the reed under a book. The magnet on top still works it.
- Reed taped under the table, magnet moved on the surface above: the light obeys a hand that touches nothing.
- Hide the reed in one fist, sweep the magnet past — the LED (wired out to the table) betrays which fist. Then let the child perform the trick on someone.
Build it — the door alarm
A real door alarm should be quiet while the door is closed and scream when it opens — that is, when the magnet leaves. So we invert, with the exact transistor trick from Project 7 (the reed simply takes the phototransistor’s place):
+6V ──┬── 10 kΩ ──┬─────────────────┐
│ │ │
│ [node]── 10 kΩ ── NPN base
│ │ NPN emitter ──┐
│ reed switch │
│ │ buzzer(−)── NPN collector
├───────────┼──────────────buzzer(+) │
GND ──┴───────────┴────────────────────────────┘- + → 10 kΩ → the node; node → reed switch → GND.
- Node → 10 kΩ → NPN base; emitter → GND; buzzer from + to collector.
- Tape the magnet to the door edge and the reed to the frame, facing it across the gap (long leads back to the breadboard).
Door closed: magnet holds the reed closed, the node is pulled LOW, transistor off — silence. Door opens: magnet leaves, reed springs open, the 10 kΩ pulls the node HIGH — BZZZT, for exactly as long as the door stands open.
Want it to keep screaming, thief-proof? Feed the node to the Project 7 555 one-shot instead of the transistor — wait: the 555 triggers on a falling edge, and our node rises on opening. Neat fix that reuses the trick above: put the reed in the top position (from + to the node) and the 10 kΩ below (node to GND) — now door-opens makes the node fall, which is exactly what pin 2 wants. Two positions of one resistor, two logics: this swap is the same move as the “two wiring polarities” table in the optoelectronics explainer.
What to say to the child
“The magnet’s invisible reach goes right through wood and glass and your hand — and inside this little glass pill are two springy metal tongues that feel it. Magnet near: they pull together and hold hands — click, the electricity can cross. Magnet gone: they let go. So we put the magnet on the door and the pill on the frame. While the door is shut they’re holding hands and everything is calm. Open the door and — they let go and the alarm shouts. Millions of real houses have exactly this hiding in their doors. Go look at a window sensor sometime — now you know its secret.”
For you — the physics
- How the tongues feel the field: the reeds are ferromagnetic Ni–Fe blades. The magnet’s axial field magnetizes them along their length, so at the overlap the two facing ends are opposite poles → attraction. When magnetic force beats the blades’ spring stiffness, they snap shut; contact force then increases (smaller gap → stronger pull), which is why the closure is crisp. Remove the field and the spring wins again.
- Hysteresis, mechanical edition: pull-in happens at one distance, drop-out at a noticeably larger one. The moving blade changes the magnetic circuit’s geometry, so the “on” state is easier to keep than to reach — built-in Schmitt-trigger behaviour, purely mechanical. It’s the same no-dithering idea as the 555’s ⅓/⅔ thresholds and the 4017’s clean edges in Project 11; nature keeps reinventing it because every threshold near noise needs it.
- Why a glass pill: the contacts live in sealed inert gas — no dust, no oxidation, switching happily for ~10⁹ operations. Reed switches were the workhorse of telephone exchanges for decades and still guard doors, laptops, and washing-machine lids (though Hall-effect sensors — the same idea with no moving parts — are taking over; the child will meet that pattern again with the relay-vs-transistor).
- Ratings footnote: reeds switch small currents only (~0.5 A). The buzzer through the transistor respects that automatically; don’t put a motor straight through one — that’s what Project 17’s relay is for, literally next session.
Try next
- Bike-wheel click counter: magnet on a spoke, reed on the fork — one click per lap. Feed those clicks to Project 11’s 4017 and you’ve rebuilt a bicycle computer’s front end — it’s Project 9 with magnetism instead of light.
- Secret cupboard light: invert the alarm (buzzer → LED, logic flipped) so opening the toy cupboard turns on a light inside.
- Field mapping: how far away does it click? Does it matter which way the magnet points? (Yes — axial vs. transverse approach is dramatic; the child is mapping field geometry with their ears.)
→ Next: Project 17 — the switch that presses itself.
← Back to Read Me First.