🪤 Trip Wire Alarm

What you're building

Stretch a thin wire across a doorway, tie both ends to a little box on the floor, and arm it. As long as the wire is intact, the box is silent. Snap the wire — a foot walking through, a door opening, a sibling sneaking in — and the buzzer wails until you either reconnect the wire or unplug the battery.

Alarms in real houses work exactly this way: a normally-closed sensor (door contact, window magnet, wire loop) holds the alarm silent, and "loss of connection" is the trigger. That feels backwards at first. A connected wire usually makes something happen. Here you'll see why engineers build alarms this way, and how that changes safety-critical circuits.

The big idea: normally-closed is safer than normally-open

Compare this to the Door Buzzer. That circuit was normally open: no button press, no beep. If the button's wire quietly fell off in the night, the doorbell just wouldn't ring — 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 goes off. Silence means "everything is fine and the sensor is connected". A scream 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 the same trick. Failure sounds the alarm.

Meet the pull-up resistor

Here's the new pattern that makes this work. In Automatic Night Light and Sunrise Alarm, a voltage divider set the transistor's base voltage based on how bright the room was. This time we do it with just one resistor plus a switch to ground.

• The 10 kΩ resistor connects battery + to the transistor's base leg. Engineers call this a pull-up because it pulls the base voltage up toward the battery.
• The trip wire (represented by the slide switch during testing) connects the same base leg to battery −. When the wire is intact, it drags the base down to 0 V — the pull-up loses the tug-of-war because the wire is a much lower resistance path.

Do the math

• Trip wire intact (switch closed): the wire is essentially a direct path to −. Base voltage sits at essentially 0 V, far below the transistor's 0.7 V wake-up line. Transistor off. Buzzer draws about 0 mA (a microamp of leakage at most). Silent.
• Trip wire cut (switch open): now the only path to the base is through the 10 kΩ pull-up. Current pours into the base until the base sits at its usual 0.7 V wake-up voltage. Ohm's law tells us how much: I_base = (4.5 V − 0.7 V) ÷ 10 kΩ = 0.38 mA.
• That 0.38 mA into the base amplifies, at roughly β = 200, into a potential 76 mA at the collector. But the buzzer only wants 30 mA — so the transistor saturates: it turns fully on, its collector-to-emitter drop shrinks to about 0.2 V (V_CE(sat)), and the buzzer sees 4.5 − 0.2 = 4.3 V. Straight from Ohm's law: I_buzzer = 4.3 ÷ 150 = 28.7 mA. Loud.
• Battery draw when armed and quiet: about 0.45 mA — that's the 4.5 V ÷ 10 kΩ flowing down the pull-up and out through the intact wire. The buzzer's current is essentially zero. At 0.45 mA a set of AAs lasts months armed, and you only spend real battery when it fires.

That low standby current — about 450 µA, not zero — is a real perk of the normally-closed pattern. The Cookie Jar Alarm's voltage divider draws a similar trickle in the dark; this one draws its 450 µA while armed, then about 29 mA when it goes off.

Build it

1. Power rails: battery + to one row on the breadboard, battery − to another. Same as always.
2. The pull-up: run a 10 kΩ resistor from the + rail down to a fresh middle row — call this the base node.
3. The trip wire (slide switch for now): put the slide switch between the base node and the − rail. When the switch is closed, the trip wire is intact; when open, the trip wire is cut.
4. The transistor: flat face toward you, legs E-B-C from left to right. Base node → Base (middle leg). Emitter (left) to the − rail.
5. The output lane: battery + → buzzer + leg; buzzer − leg to Collector (right leg). No resistor needed on this lane — the buzzer limits itself, same rule as the Door Buzzer.
6. Arm it: close the slide switch. Silence. Now slowly slide it open — the moment the contacts break, BZZZT. Close it again and the alarm goes silent instantly.

Deploy the trip wire

Once the breadboard version works, retire the slide switch and use a real trip wire:

• Take your longest piece of thin, uninsulated wire (a strand of copper from a scrap piece of stranded jumper works). This is your trip wire.
• Tie one end of the trip wire to a new jumper running to the base node; tie the other end to a jumper running to the − rail. Now you've replaced the slide switch with the trip wire itself.
• Stretch the trip wire across a doorway at ankle height, taped to the door frames on both sides. Any solid pull snaps it — no soldering required to arm the next round, just tie a new piece.

Where this pattern goes next

The pull-up is a Level 3 concept but it never goes away. At Level 7 (Code), every digital input pin on an Arduino uses an internal pull-up resistor exactly like this one — you turn on a built-in 20 kΩ pull-up with a single line of code, and any switch to ground becomes a readable input. The line you'll write is pinMode(2, INPUT_PULLUP);, and now you understand what it's actually building for you.