Level 2 · Sound · project 11 of 12 · 25 min · ●●○○○
Silent Alarm
Three panic buttons wired in parallel, all inside an armed lane — any one of them fires the buzzer, and a small LED tells you the alarm is watching.
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
- 8× Jumper wires a few extras — three parallel button paths take more wire than usual L1
- 1× Slide switch the arm switch — hidden under a desk or in a drawer L1
- 3× Push button (tactile) the panic buttons — one at each door, or all next to each other L1
- 1× Active buzzer (3–5 V)
- 1× Red LED (5 mm) the "armed" indicator — glows whenever the alarm is watching L1
- 1× 220 Ω resistor LED safety resistor L1
you'll learn: Circuits are loops · Switches · Series & parallel · Ohm's law · LED polarity
Circuit map
The circuit you'll build, running on the Workbench right now.
simulating…
If it doesn't work
The boring ways this circuit actually breaks — check these first before you unwire everything.
- Buzzer sounds without the slide armed. The arm slide is bypassed. It must sit in series with the shared + rail feeding all three buttons.
- None of the three buttons trigger. Battery pack switch, then confirm the slide is closed, then that all three buttons feed the SAME output rail (they're in parallel).
- One button dead, the rest fine. That button's wires are on the same diagonal. Move one to the opposite corner.
- Armed LED never lights. Flip the LED (long leg toward +), check the 220 Ω, then confirm the LED lane taps + AFTER the arm slide.
- Armed LED lights even with the slide off. The LED lane taps + directly instead of downstream of the arm. Rewire so it gets power AFTER the arm slide.
What you're building
An arm-then-panic alarm with three panic buttons instead of one. Slide the arm switch off and the whole rig is dead: press buttons all you like, nothing happens. Slide the arm switch on and the little red indicator glows — the alarm is listening. Push any one of the three panic buttons and the buzzer wails.
Arm & Ring did this with one ring button. Two-Door Signal proved that parallel switches make an OR gate — any one of them is enough. This project glues those two ideas into one alarm: the arm slide is the AND (must be closed for anything to happen), and the three panic buttons in parallel are the OR (any one fires it).
The big idea: arm (AND) times panic (OR)
Every "safety catch, then trigger" system in the world is this exact shape. The arm switch has to be closed and at least one trigger button has to be pressed. Break either half and the alarm stays quiet.
- Arm off: the armed rail can't reach the battery's plus side. Every button below is dead — pressing all three at once does nothing. Total current: 0 mA.
- Armed, no button pressed: the LED lane (armed rail → 220 Ω → LED → ground) closes its loop and the indicator glows. The panic lane sits waiting — the three push buttons are all open, so no current runs through them. Total current: ≈ 11 mA.
- Armed AND any button pressed: the pressed button closes its lane, giving current a path from armed rail through that button through the buzzer to ground. The buzzer hits ≈ 30 mA; the LED keeps its 11 mA; the battery supplies both lanes: ≈ 41 mA total.
- Armed AND two buttons pressed: identical to one button pressed. Two parallel closed switches share the same 30 mA of buzzer current. Extra paths don't make the buzzer any louder — the buzzer is the fixed load, and the buttons are just meaningless short bits of wire once closed.
Do the math (armed, no panic — the "ready" state)
Only the LED lane carries current. It's exactly First Light again:
- Battery push: 4.5 V
- Red LED forward drop: 2.0 V
- Voltage left for the 220 Ω resistor: 4.5 − 2.0 = 2.5 V
- Current through the LED lane: I = 2.5 ÷ 220 ≈ 0.0114 A = 11 mA
The panic lane draws 0 mA — all three buttons open means no path from the armed rail to the buzzer's plus side. A red LED at 11 mA is comfortably bright and the rig can sit armed for days on fresh AAs without a wobble.
Do the math (armed AND one panic — the "help!" state)
The LED lane doesn't change — it doesn't care what the panic lane is doing. Adding a second lane in parallel doesn't rob current from the first (you saw this in Do Not Disturb at Level 1 and Twin Bells last project). Each lane sees the full 4.5 V and does its own math.
- LED lane: unchanged, still 11 mA.
- Panic lane: any one closed push button gives current a path from the armed rail through that button to the buzzer's plus side, then through the buzzer (an active buzzer that acts like a ≈ 150 Ω load — it's rated for about 30 mA at 5 V) to ground. I = 4.5 ÷ 150 = 30 mA.
- Battery total: 11 + 30 = 41 mA.
Well inside what fresh AAs deliver.
Why "any one" is enough
Look at the panic lane as three little wires trying to connect the same two rows: the armed rail on top and the buzzer's plus side just below. Each push button, when pressed, becomes a nearly-zero-Ω wire between those two rows. Any one closed button is enough to complete that connection. Pressing two doesn't halve the buzzer current or double it — the buzzer is still just 4.5 V across a ~150 Ω load, still 30 mA.
That's the same rule you learned in Two-Door Signal: the load decides the current, not the number of parallel paths. Once the loop is complete, adding more redundant closed paths changes nothing.
Where "arm + any of many triggers" shows up in the world
- Bank alarms. A key or code arms the system. Then any window switch, door switch, motion sensor or staff-under-the-counter panic button trips the same siren. Dozens of triggers in parallel, one alarm.
- Fire alarm pull stations. A whole building has one arming state (the panel is powered and enabled), and every corridor pull station is one more parallel trigger.
- Car horn buttons. The horn has one power rail (ignition ON = armed). The horn pad on the steering wheel and the alarm remote's panic button both connect to the same rail — either honks it.
- Household emergency stops on kitchen mixers, band saws, and garage doors. Multiple big red buttons around the machine, all wired to the same "any of us cuts the power" line.
Anywhere a designer wants one trigger anywhere to fire one alarm, they reach for this pattern.
Build it
- Power rails. Battery red (+) to a top row on the breadboard, battery black (−) to a bottom row.
- The arm switch. Slide switch's common leg on the + rail; the switch's outer leg on a new row we'll call the armed rail. When the slide is closed, the armed rail is at 4.5 V; when open, the armed rail floats.
- The armed indicator. From the armed rail: 220 Ω → red LED long leg → red LED short leg → − rail. Slide the arm switch on — the LED should light. Slide it off — it should go dark. That confirms the arm lane works before you add the panic lane.
- Three parallel panic buttons. Line up the three push buttons side by side. Each button's entry leg lands on the armed rail; each button's exit leg lands on the same shared row a little lower — call it the merge rail. Now any one closed button connects armed → merge.
- The buzzer. From the merge rail: buzzer + leg → buzzer − leg → − rail. The buzzer is polarised; the leg with the longer wire (or a tiny + mark on the case) is +. Backward and the buzzer stays silent.
- Try it. Slide off: press all three buttons — silent. Slide on: the LED lights, buzzer still silent. Press any one button — buzzer wails, LED unchanged. Release — silent, LED still on. Press two together — same wail, no louder.
What's really happening in the wires
The armed rail is one physical row of breadboard holes. Three buttons hang off it in parallel. Underneath, the merge rail is another row. The buttons are bridges — each button, when closed, is a bit of metal spanning the gap between armed rail and merge rail.
If you drew this in ladder-diagram style (like a real electrician would), it would look like this:
+ ──── ARM ──── ARMED RAIL ─┬─ 220Ω ─ LED ── −
├─ BTN A ─┐
├─ BTN B ─┼─ BUZZER ── −
└─ BTN C ─┘
Every "OR gate with an AND gate on top" you'll ever build in electronics — or in code, or in industrial safety wiring — is this picture.
🧠 Your challenge
No single right answer. That's the point.
- Add a fourth trigger. Rewire the slide switch's outer leg through your second slide switch (if you have one) or a spare jumper as a fourth parallel path. Now four inputs, still one buzzer. Does the buzzer sound any different? (No. Same 30 mA load, same 4.5 V push. The trigger count doesn't change the volume.)
- Silent test mode. Move the buzzer's + wire from the merge rail to a second slide switch (mute), and route that switch's other leg to the buzzer. Now you have arm → any of three triggers → mute → buzzer. Three switches in series-after the OR. What state must every switch be in for the buzzer to fire? (Arm closed, at least one panic pressed, mute closed. AND-of-three with an OR baked in.)
- Write the truth table. Fill it in for the base circuit: 2 states of the arm × 8 states of the three buttons (each open or closed) = 16 rows. Which rows fire the buzzer? (Half of them — every row with arm closed AND at least one button pressed.) Fewer than 16 unique behaviours, though — how many actually different states are there once you group "any one of the three is enough"?
- Swap the LED for a second buzzer. Move the LED and 220 Ω aside, and drop a second active buzzer into the armed lane. Now arming the alarm produces a continuous "watching" tone plus a louder panic tone when someone triggers. What's the total battery current in the armed-and-panic state? (30 + 30 = 60 mA — twice the drain, and it won't sound twice as loud because your ears perceive volume roughly logarithmically.)
- Turn OR into "exactly one". Can you rewire this so the buzzer fires only when exactly one button is pressed (not two, not three)? Try it on paper. (You'll find you can't — not with only the parts you own. That's called an XOR gate, and it needs transistors, which are Level 3. Every logic project you've built so far — AND, OR, NAND — can be done with switches. XOR can't.)
For grown-ups: safety notes
- Battery-safe throughout. The worst case (armed and one button pressed) draws about 41 mA — well inside what fresh AAs deliver for hours on end.
- If the LED lights but the buzzer never sounds when you press a button, walk the panic lane with your finger: armed rail → panic button → merge rail → buzzer + leg → buzzer − leg → − rail. Wherever your finger can't trace, that's the break. The classic trap is landing the button on the wrong pair of legs — most tactile buttons have four legs, but only two of them switch; rotate the button 90° on the breadboard and try again.
- If the buzzer sounds without the arm switch on, the panic lane is probably tied straight to the + rail instead of the armed rail. The arm switch must be between the + rail and the armed rail — walk the loop from + and confirm it goes through the slide switch before reaching any button.
- The active buzzer is loud at close range. Point it away from ears when you first press a panic button, and warn anyone nearby. If you're playing with this in a bedroom, keep a hand near the arm switch — the fastest way to silence it is to slide arm off.
- Never wire this to mains power. The whole site is battery-only for good reason; wall power is hundreds of times what these parts can survive.
- Multi-button lanes are the first project on the site where wiring a button the wrong way round is a real risk of the loop firing by accident. If a button seems to "trigger" even when you're not pressing it, the button legs are probably on a permanently connected pair — pull that button, turn it 90°, put it back.
Checked against
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