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Level 3 · Sense · project 14 of 17 · 30 min · ●●●○○

Trip-Wire Chorus

Trip Wire Alarm with a red LED added in parallel with the buzzer — one thin wire holds the whole chorus silent; snap it and both mouths fire.

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.

You'll make a chorus that fires when a wire snaps
You'll learn pull-up brain + two output mouths
Watch for shrink the pull-up 10x so both mouths get full drive

What you need

🔧 Try before wiring →

you'll learn: Transistor switching · Switches · Series & parallel · Ohm's law · LED polarity

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.

  • Only the LED lights when the wire is cut. The buzzer's + isn't wired to the shared collector rail. Both outputs must fork off the same jumper coming out of the transistor's collector.
  • Only the buzzer sounds. Flip the LED (long leg toward +) and confirm the 220 Ω is in the LED lane. The buzzer runs raw.
  • Both silent even with the wire cut. 2N2222 pins E-B-C (flat face toward you). Then confirm the 1 kΩ pull-up actually connects the base to +.
  • Both on all the time — even with the wire intact. The trip wire (slide switch) isn't reaching ground. Trace: base node → switch → − rail. If the switch is on the wrong two of its three legs, the wire never pulls the base low.
  • Buzzer weak, LED barely visible when it fires. You used a 10 kΩ pull-up instead of 1 kΩ. Ten kΩ was fine for Trip Wire Alarm's single mouth; two mouths need more base drive — swap it for the 1 kΩ.
  • Buzzer sounds but LED is very dim. You wired the LED in series with the buzzer instead of parallel. Each output gets its own lane; they share only the collector rail and −.
  • Alarm triggers on its own without cutting the wire. Long jumper standing in for the trip wire is loose. Push both ends firmly, or shorten it. A flaky trip wire makes for a flaky alarm.
  • AAs drain fast even when the alarm never fires. That's the trade-off — the 1 kΩ pull-up flows 4.5 mA continuously while armed. If you don't need both mouths, use Trip Wire Alarm's 10 kΩ pull-up instead (0.45 mA standby).

What you're building

Trip Wire Alarm with a flashing red light added alongside the scream. Stretch a thin wire across a doorway; while it holds, the box is silent and the lamp is dark. Snap the wire — a foot, a door, a sneaky sibling — and the buzzer wails and a red LED glows until you either reconnect the wire or unplug the battery. Loud and visible at once, so nobody can pretend they didn't notice.

Trip Wire Alarm was a single-mouth alarm on a pull-up brain (no divider — just one resistor from + to the base, with the trip wire pulling the base to ground). This project keeps that brain and adds a second mouth in parallel with the buzzer, the same "one brain, two mouths" trick you learned in Dusk Chorus and Warm Hand Chorus.

The brain × mouth grid, third brain edition

You already know the grid from Warm Hand Chorus and Dawn Chorus:

LED output Buzzer output Both in parallel
Light sensor (dark) Night Light Curfew Beeper Dusk Chorus
Light sensor (light) Sunrise Alarm Cookie Jar Alarm Dawn Chorus
Warm sensor Warm Hand Alarm Fever Beeper Warm Hand Chorus
Pull-up (wire-cut brain) (challenge) Trip Wire Alarm this project

Same recipe, brand-new brain. The pull-up doesn't measure the world with a sensor — it measures whether a wire is still connected. When the wire fails, the pull-up wins and the transistor turns on. Adding a second mouth in parallel changes nothing about when it fires, only what happens when it does.

The sensing half — from Trip Wire Alarm, one tweak

The brain is a pull-up — a single resistor from + to the transistor's base, plus the trip wire (represented by the slide switch) from the base to −.

  • Wire intact (switch closed): the wire is a near-short to ground. The base sits at essentially 0 V, well below the 0.7 V wake-up. Transistor off. Silent and dark.
  • Wire cut (switch open): the only path to the base is through the pull-up. Current pours into the base until the base clamps at its usual 0.7 V wake-up. Ohm's law: I_base = (4.5 − 0.7) ÷ pull-up.

The one change from Trip Wire Alarm: the pull-up is 1 kΩ, not 10 kΩ. Two mouths on one transistor draw ~5× the collector current of a single mouth, and the transistor can only pull as much collector current as its base drive × β allows. Shrinking the pull-up by 10× multiplies the available base current by 10×, which is plenty of headroom for both lanes together. "Why" is the interesting part — see below.

The output half — buzzer AND LED, side by side

Once the transistor is on hard, its collector-to-emitter link becomes a near-short (the classic V_CE(sat) ≈ 0.2 V drop). Both output lanes hang off the top rail on one side and off the collector on the other. Each lane sees 4.5 − 0.2 = 4.3 V across itself and does its own Ohm's-law math:

  • Buzzer lane: the active buzzer behaves like a ~150 Ω load at 4.3 V, so I_buzzer = 4.3 ÷ 150 ≈ 29 mA. Loud, same as Trip Wire Alarm. No resistor on this lane — the buzzer self-limits.
  • LED lane: red LED "eats" 2.0 V, so 4.3 − 2.0 = 2.3 V sits across the 220 Ω. I_LED = 2.3 ÷ 220 ≈ 10.5 mA. A hair under the 11 mA of First Light (the transistor stole 0.2 V), still comfortably bright.

The two lanes are independent. Adding the LED lane does not quiet the buzzer, and adding the buzzer lane does not dim the LED. Each lane sees a full 4.3 V push and each does its own math.

Why the pull-up got smaller

Every 2N2222 has a hidden speed limit called β (beta) — roughly 200 for this part. It says: the collector can only carry β times as much current as the base does. Base gets 0.1 mA → collector can carry at most 20 mA. Base gets 1 mA → collector can carry 200 mA. Push harder on the base and the collector opens up.

Trip Wire Alarm's pull-up was 10 kΩ, which gave the transistor about 0.38 mA of base current when the wire was cut. β·I_b ≈ 76 mA — enough for a single 29 mA buzzer with a comfortable ~2.6× overdrive.

Add an LED lane and the collector demand jumps to ~40 mA (29 mA buzzer + 10.5 mA LED). Suddenly 76 mA of β-limited drive gives only ~1.9× overdrive. The transistor would still just barely saturate — but V_CE(sat) would creep up from 0.2 V to maybe 0.4 V, both lanes would see less voltage, and both would run visibly weaker.

Fix: shrink the pull-up from 10 kΩ to 1 kΩ. Same brain, same trip-wire story, but the base now sees ~10× as much current — about 3.8 mA with the wire cut. β·I_b jumps to ~760 mA, way past the 40 mA demand. The transistor slams deep into saturation and both mouths run at full strength.

The trade-off: standby current

There's a cost. The pull-up flows current all the time the alarm is armed, straight through the intact trip wire to ground. With the old 10 kΩ pull-up:

I_standby (armed) = 4.5 V ÷ 10 kΩ = 0.45 mA

With the new 1 kΩ pull-up:

I_standby (armed) = 4.5 V ÷ 1 kΩ = 4.5 mA

Ten times higher. Trip Wire Alarm could stay armed for months on a set of AAs; Trip-Wire Chorus uses that same 4.5 mA burn per hour whether or not it fires. Fresh AAs (~2000 mAh) still give you weeks of armed standby, but not months. That's the trade you're paying for the second mouth on this brain.

Do the math (all together)

Battery total when armed and silent: just the pull-up trickle, ≈ 4.5 mA.

Battery total when the wire is cut and it's screaming:

  • Buzzer lane: ~29 mA
  • LED lane: ~10.5 mA
  • Pull-up (no longer pinned to ground): ~3.8 mA
  • Sum: ~43 mA

The 2N2222 handles it comfortably: β·I_b ≈ 760 mA of available Ic, 40 mA asked for. Fully saturated, V_CE(sat) ≈ 0.2 V, both mouths at full strength.

Build it

If your Trip Wire Alarm is already breadboarded, this is a three-minute upgrade — swap the pull-up (10 kΩ → 1 kΩ) and add a lane (220 Ω + LED in parallel with the buzzer).

Fresh build:

  1. Power rails: battery + to a top row, battery − to a bottom row.
  2. The pull-up: run a 1 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. Closed = wire intact; open = wire 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 buzzer lane: battery + → buzzer + leg; buzzer − leg → transistor Collector (right leg). No resistor on this lane.
  6. The LED lane in parallel with the buzzer — this is the new step:
    • From battery + rail: → 220 Ω → LED long leg.
    • LED short leg → same transistor Collector as the buzzer's negative leg. Both lanes share the collector row.
  7. Arm it: close the slide switch. Silence, dark. Now slowly open it — the moment the contacts break, BZZZT and the red LED snaps on together.

The key wiring step is that both lanes come out of the collector row. If you accidentally wire the LED to the emitter row instead, it will light whenever the battery is connected (bypassing the transistor entirely) — a common first-try mistake. Trace the LED lane: + rail → 220 Ω → LED → collector → transistor's guts → − rail. That's the loop.

Deploy your chorus

Same story as Trip Wire Alarm:

  • Retire the slide switch and use a real trip wire: a strand of stripped copper from a scrap piece of stranded jumper, tied between two jumpers (one to the base node, one to −).
  • Stretch across a doorway at ankle height, taped to the door frames. Any pull snaps the wire — no soldering needed to arm the next round, just tie a new piece.
  • Because it now flashes as well as beeps, this works better than the original alarm in loud rooms or for anyone hard of hearing. The red LED is visible across a dark room even when the buzzer competes with a TV.

Where "one brain, many mouths" leads next

At Level 4 the mouth family grows to include a motor (with a protection diode). A three-mouth trip-wire chorus — buzzer + LED + a spinning warning flag — is a Level 4 rewrite of exactly this project. At Level 6 you'll add a 555 timer that turns the steady scream into a pulsing siren. At Level 7 the Arduino replaces the transistor and every lane becomes programmable. The pull-up brain doesn't go away; it just gets more sophisticated mouths.

🧠 Your challenge

No single right answer. That's the point.

  • Green trip light. Swap the red LED for a green one. Redo the LED lane math with the green LED's higher forward drop (2.1 V instead of 2.0 V). What's the new LED current? (Answer: 2.2 ÷ 220 ≈ 10 mA — a half-milliamp lower. The buzzer lane is unchanged, because the two lanes are independent.)
  • Old-standby, new-mouths. Put the 10 kΩ pull-up back in place of the 1 kΩ and keep the LED lane. Predict before powering it up: will the alarm still fire? Will it be as loud and bright as with the 1 kΩ? (It fires — the transistor still just barely saturates at β·I_b ≈ 76 mA vs 40 mA demand — but V_CE(sat) creeps up, so the buzzer runs a bit quieter and the LED a bit dimmer. Standby drops back to 0.45 mA, which is the payoff.)
  • Two-wire tamper. Add a second slide switch in series with the first (both between the base node and −). Now cutting either wire trips the alarm. Wrap the second one around the battery pack so unplugging the alarm also fires it. What does the base voltage do when only one of the two switches is open? (Answer: the same as when the wire is cut — because current has no path to ground through a broken chain of series switches. Either failure is enough.)
  • Latch it. Right now, reconnecting the wire silences the chorus instantly. Sketch (or describe) how you'd wire a second push button as a "reset" so the chorus keeps firing after a trip until you press reset. (Hint: you need "the trip flipped it on, and now it holds itself on" — a pattern called a latch. Level 6 will give you a proper chip for this; for now, just sketch it.)
  • Add an arm switch. Put your other slide switch in series with the + rail before the pull-up. Now you can disarm the whole chorus manually while you're testing. Battery draw when disarmed? (Zero — the whole circuit is disconnected.)

For grown-ups: safety notes

  • Battery-safe throughout — no shock risk at 4.5 V, even when the chorus is firing. Total draw of ~43 mA when it fires is well inside what fresh AAs deliver.
  • Active buzzers are loud at close range. Point the buzzer away from ears when you test — a 2 kHz screamer six inches from a face is a startle even when you know it's coming.
  • Trip wire placement: don't string a wire across a stairway, a hallway anyone walks in the dark, or anywhere a fall would hurt. The point is to detect the crossing, not to trip a person.
  • Never use fishing line, metal string, or anything that could cut someone as they walk through. Thin bare copper snaps cleanly under a foot's weight without cutting.
  • Standby burn: the 1 kΩ pull-up flows 4.5 mA continuously while the alarm is armed (10× more than Trip Wire Alarm). Fresh AAs (~2000 mAh) still give you weeks of standby, but not months — budget accordingly. If you need long-term armed standby more than you need loud + bright, use the 10 kΩ pull-up from Trip Wire Alarm instead.
  • If the LED lights but the buzzer stays silent, one of two things: the buzzer's + leg is on the wrong side (buzzers are polarised — the plus mark on top faces the battery +), or the buzzer lane goes to the emitter row instead of the collector row (in which case the transistor isn't in charge of it). Trace the buzzer lane end to end.
  • If the buzzer chimes but the LED never lights, check LED polarity (long leg toward +, short leg toward the collector). Reversed LEDs don't hurt anything at 4.5 V but they also don't light.
  • If the transistor ever feels warm to the touch, disconnect the battery and re-check E-B-C leg order. Backwards transistors are always the first suspect.
  • Never wire this to mains. Every part in this project is low-voltage; wall power would destroy the transistor, the LED, and the buzzer instantly, in that order.
  • Never build a real security alarm you'd rely on out of this circuit alone. This is a fantastic teaching alarm; real intruder alarms use supervised loops, tamper detection, and a fallback battery. This is the foundation they're all built on, not a substitute.

Checked against

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