Level 4 · Motion · project 8 of 8 · 25 min · ●●●○○
Trip Wire Fan
Trip Wire Alarm's brain wearing a motor. The wire is intact — the fan is silent. Snap it — the fan spins as the alarm.
new here? Start with First Light — one battery, one resistor, one LED. It teaches the loop, LED direction, and why the 220 Ω resistor matters.
What you need
- 1× 3×AA battery pack (4.5 V) L1
- 1× Solderless breadboard L1
- 7× Jumper wires one long jumper doubles as the trip wire itself across the doorway L1
- 1× Slide switch stands in for the trip wire while you prototype (closed = wire intact, open = wire cut) L1
- 1× 10 kΩ resistor pull-up — holds the base up whenever the trip wire is open L1
- 1× NPN transistor (2N2222) flat face toward you: legs are E-B-C L3
- 1× DC motor (3 V) the same little hobby motor from First Spin — a paper flag on the shaft makes the "alarm" visible
- 1× Diode (1N4001) flyback protection — cathode toward battery +, same as First Spin
you'll learn: Transistor switching · Switches · Ohm's law
Circuit map
The circuit you'll build, running on the Workbench right now — shown with the switch opened.
simulating…
If it doesn't work
The boring ways this circuit actually breaks — check these first before you unwire everything.
- Fan spins when the trip wire is intact. The slide switch isn't shorting the base to −. Tactile-style switches often bridge diagonally; slide switches bridge the two closest legs when the actuator sits over that half. Move a wire until closing the switch actually stops the fan.
- Fan silent even when the trip wire is cut. The 10 kΩ pull-up is missing or on the wrong side. It should sit between + and the base node — trace + → 10 kΩ → base → transistor B with a finger.
- Transistor gets hot even when the wire is intact. E-B-C pins scrambled. Flat face toward you, legs left-to-right are Emitter, Base, Collector. Backwards transistors dump heat instead of switching.
- Fan spins weakly and something smells warm. 1N4001 wired backwards — banded end (cathode) must face battery +. Reversed, it shorts the battery through itself the moment the transistor turns on.
- Fan doesn't quite start when the wire is cut. Static friction — a stopped motor needs a nudge to spin at 76 mA. Flick the blade once with your finger; the transistor's linear-region drive holds it going but doesn't always break stiction.
- Fan stops as soon as I close the wire again. That's correct — the base gets pulled back to −, the transistor turns off, the flyback diode dumps the coil's kick. For an alarm that *latches* on until reset, you'll need a Level 6 chip (the 555 in flip-flop mode).
What you're building
Stretch a thin wire across a doorway. Tie both ends to a little box on the floor. As long as the wire is intact, the box is silent and the battery barely notices. Snap the wire — a foot walking through, a door swinging open, a sibling sneaking in — and the fan spins as the alarm.
You already built this shape at Level 3 in Trip Wire Alarm. Only the mouth changed. The brain — a pull-up resistor + a normally-closed switch + a transistor — is exactly the one you learned there. Swap the buzzer for a motor + flyback diode and the same trip-wire circuit becomes a motion alarm: a spinning shaft you can see (and, if you tape a paper flag to it, hear thwapping against the desk).
The big idea: fail-safe means "failure sounds the alarm"
Compare this to Handheld Fan. That circuit is normally open: no button press, no motor. If the button's wire quietly fell off in the night, the fan just wouldn't spin — and you'd never know until you tried it.
This project is normally closed: the trip wire holds the alarm silent. If anything at all goes wrong — the wire is cut, a solder joint fails, the battery gets bumped and jiggles the loop apart — the alarm fires. Silence means "the wire is genuinely still there". A spinning motor means "either someone crossed the wire OR something is broken; either way, come look".
Every burglar alarm, refrigerator door switch, and safety interlock on every microwave oven uses this same trick. Failure sounds the alarm.
Reuse from Level 3 (the "brain")
- 10 kΩ pull-up from the + rail to the base node. When nothing else pulls the node down, the pull-up quietly tries to drag the base up to +.
- Slide switch as the trip wire between the base node and the − rail. Closed = wire intact = base shorted to − = transistor off. Open = wire cut = pull-up wins = transistor on.
- 2N2222 transistor turns on when its base sits above the ~0.7 V wake-up line.
New at Level 4 (the "mouth")
- DC motor (~30 Ω coil).
- 1N4001 flyback diode across the motor, cathode (banded end) toward battery +. Reverse-biased and invisible while the motor runs; catches the coil's inductive kick the moment the transistor turns off.
Do the math
Trip wire intact (switch closed):
- The switch pins the base to − through ~0 Ω. Base at 0 V.
- Transistor off. Motor idle. Battery current through the coil: microamps of leakage.
- The pull-up still sees a full 4.5 V across it: I = 4.5 V ÷ 10 kΩ = 0.45 mA flowing continuously from + through the 10 kΩ, through the closed trip wire, back to −. That's the armed drain.
- Two fresh AA cells (~2500 mAh) at 0.45 mA last about 5,500 hours ≈ 7 months armed. Real house-alarm loops draw about this much for exactly this reason.
Trip wire cut (switch open):
- No path from the base node to −.
- Pull-up drags the base up until it clamps at ~0.7 V (transistor's base junction acting as a diode).
- I_base = (4.5 − 0.7) V ÷ 10 kΩ = 0.38 mA — modest.
- β × I_base ≈ 200 × 0.38 = 76 mA — the collector's maximum.
- The motor's natural current at 4.5 V is (4.5 − V_CE) ÷ 30 Ω. If the transistor were fully saturated, V_CE ≈ 0.2 V and the motor would pull ~143 mA. But 76 mA < 143 mA, so the transistor can't quite saturate the motor. It stays in its linear region and passes exactly what the base commanded.
- Motor at ~76 mA. V_R_motor ≈ 76 mA × 30 Ω = 2.3 V. V_CE ≈ 4.5 − 2.3 = 2.2 V. Transistor dissipates I_C × V_CE ≈ 170 mW — warm to the touch, well inside the 2N2222's 625 mW ceiling.
- Motor sees ~2.3 V — roughly half of full-speed voltage. Fan runs at about half speed. Slower than Handheld Fan, but unmistakably alive.
Why not saturate the motor? You could shrink the pull-up to 1 kΩ. That gives the base 3.8 mA and reaches full saturation → 143 mA motor. But then the armed drain climbs to 4.5 mA, and the AAs die in three weeks. The 10 kΩ pull-up is the same value Trip Wire Alarm used, for the same reason: a trip wire needs to sit armed for months without draining the battery. Trading fan speed for battery life is the right call for a burglar alarm.
Compare to Trip Wire Alarm
Same left half, different right half:
| Trip Wire Alarm | Trip Wire Fan (this) | |
|---|---|---|
| Pull-up | 10 kΩ | 10 kΩ |
| Trigger | slide switch (NC) | slide switch (NC) |
| Transistor | 2N2222 | 2N2222 |
| Output lane | active buzzer | motor + 1N4001 |
| Current when firing | ~29 mA (saturated) | ~76 mA (linear) |
| Battery drain armed | ~0.45 mA | ~0.45 mA |
| Alarm signal | audible beep | spinning fan / flapping flag |
The buzzer saturates the transistor because 76 mA of collector cap is way more than a 29 mA buzzer needs. The motor doesn't saturate because its 143 mA appetite outruns the collector cap. Same brain; different-appetite mouths land the transistor in different operating regions. That's a real insight — every big industrial motor driver has to reason about "am I going to actually saturate this transistor with the base drive I've got?"
Build it
If your Trip Wire Alarm is still on the breadboard, this is a one-minute upgrade: pull the buzzer out and drop a motor + flyback diode in its place.
Fresh build:
- Power rails: battery + to a top row, battery − to a bottom row.
- Pull-up: battery + → 10 kΩ → base node (call it row B).
- Trip wire: base node → slide switch → battery −. Closed = wire intact.
- Base link: row B → transistor Base (middle leg; flat face toward you, legs E-B-C left-to-right).
- Transistor: Emitter → − rail; Collector → fresh row A.
- Motor lane: motor between the + rail and row A.
- Flyback diode across the motor: cathode (banded end) on the + rail side, anode on row A. Same placement as First Spin and every L4 project.
- Slide switch closed → fan silent. Slide switch open → fan spins at about half speed. You'll hear the difference from Handheld Fan: this fan isn't being pushed to its limit, so it sounds calmer.
From breadboard to real trip wire
The slide switch is a stand-in. Once the breadboard version works:
- A loop of thin thread taped across the doorway, with each end wound around a small metal pad or paper clip on the breadboard. A snapped thread breaks the connection → fan spins.
- Kitchen aluminium foil across the doorway, bridging two paper clips. Pushing through tears the foil → fan spins.
- A magnetic reed switch on the door frame: closed while the door is shut, opens as it swings.
- A pressure-mat switch under a rug: closed while nobody's standing on it; broken when weight lifts.
Every one of these is "a normally-closed sensor" replacing the slide switch. The circuit doesn't care what the sensor's made of — only whether it's currently short-circuit or open.
What this pattern opens up
- Freezer alarm: a reed switch on the door. Door open = motor spins on a warning fan.
- Bedroom "did anyone come in" sensor: thread across the door before you go out. Snapped = fan spun = someone was here.
- Chair monitor: a switch under a cushion held closed by weight. Person stands up = switch opens = fan spins as an "empty chair" alert.
Level 6 will let you latch the alarm on so it stays running after the wire is retied (a 555 in flip-flop mode). Level 7 replaces the wire itself with an infrared beam and an Arduino reading it. The mouth — the motor with a flyback diode — stays exactly this.
🧠 Your challenge
No single right answer. That's the point.
- Trip wire + horn. Add an active buzzer in parallel with the motor (buzzer's − leg on the collector, + leg on the + rail). Now cutting the wire spins the fan and beeps. What's the total collector current when firing? (The transistor is already at its 76 mA cap. The buzzer wants ~29 mA, the motor wants 143 mA — together 172 mA. The cap doesn't get any bigger, so the two loads share the 76 mA available. Both end up under-driven: fan slower still, buzzer quieter than Trip Wire Alarm. Real fix: shrink the pull-up.)
- Faster fan, shorter armed life. Swap the 10 kΩ pull-up for the 1 kΩ. Redo the math. What's the new base current? Motor current? Armed drain? (I_base = 3.8 mA → deep saturation → motor at 143 mA. Armed drain = 4.5 mA — AAs last three weeks instead of seven months. Whether the trade is worth it depends on what you're monitoring.)
- Two trip wires, one alarm. Wire a second slide switch in parallel with the first — both go from the base node to −. Either one closed = base held low = alarm silent. BOTH cut = alarm fires. When would you want this? (Two-of-two safety — the alarm only fires if both sensors agree "something's up", useful for a system prone to false alarms. The trade is that a single failing wire won't sound it.)
- AND-gate trip wires. Now put the two slide switches in series between the base node and −. Both must be closed (intact) to keep the alarm silent. Either wire cut → alarm fires. What's the failure story now? (Cutting the window wire OR the door wire spins the fan. That's the more common alarm topology — cover more zones with one motor.)
- Add a manual disarm. Put your push button in parallel with the slide switch. Pressing the button (while the wire is cut) shorts the base back to − — the fan stops as long as you hold it. Useful for silencing the alarm while you inspect. Draw the schematic first.
For grown-ups: safety notes
- Battery-safe throughout. 4.5 V and ~76 mA through your fingers is imperceptible.
- Diode polarity matters most. Cathode (banded end) toward battery +. Reversed, the diode shorts the battery through itself the instant the transistor turns on — it'll get hot fast. Disconnect and flip it if you smell anything warm.
- The transistor gets warm when firing. That's real physics, not a mistake — the transistor is in linear region (not saturated), dissipating ~170 mW as heat. Warm to the touch is normal. Hot enough to hurt within seconds isn't — that means E-B-C pins are scrambled. Flat face toward you, legs left-to-right are Emitter, Base, Collector.
- The 10 kΩ pull-up drains ~0.45 mA while the loop is armed. Fresh AAs last months in this state — the battery outliving a normal-use flashlight.
- The trip wire can be any conductor: thin thread wrapped around bared jumper ends, aluminium foil, a reed switch, a pressure mat. Don't use stripped mains wire; use only low-voltage hobby wire. Nothing in this project should ever touch the wall socket.
- Fingers, hair, and paper blades don't mix well. At half-speed the motor is still turning fast — tie hair back and keep loose sleeves clear.
- If you tape a paper flag to the motor shaft as the visible-alarm trick, make sure it's small and light so it doesn't unbalance the shaft or catch on nearby parts.
- Never wire this to mains. Every part is 4.5 V-safe.
Checked against
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