🚨 Panic Doorbell

What you're building

A single alarm — buzzer plus red LED — with two separate call buttons. Press either button and the alarm fires. Press both buttons and the alarm still fires the same way. Only when both are released does everything go quiet.

That "either one triggers it" logic is called OR, and it's the other half of the world of switches. In The Vault you built AND (two switches in series — the loop only closes if both are on). Here you're building its opposite: two switches in parallel — either one closing on its own is enough to complete the loop.

Put a button on the fridge for kids to summon a parent for a snack, another by the bed for a "come check on me" call — and both ring the same buzzer in the living room.

The big idea: switches in parallel

If two switches sit side by side — both connecting the same two spots — they give the loop two ways to close. The current only needs one path to be closed, so either switch alone is enough. Closing both just gives it two paths at once, which is fine — the buzzer doesn't care whether one lane is delivering the current or two.

Compare, side by side, with a diagram in your head:

• Series (AND, The Vault): + → [A] → [B] → buzzer → −. Break either switch and the loop is dead.
• Parallel (OR, this project): + → [either A or B closed] → buzzer → −, where A and B are two side-by-side wires. Only need one wire to have its switch closed.

Same two buttons. Wire them one way for AND, and another way for OR. The behavior flips.

Where OR shows up in the real world

• A car horn in most cars is wired OR from any horn button on the steering wheel — two, three, sometimes four contacts, any of which honks.
• Ceiling lights with two switches — the kitchen light you can flip from the doorway or from over the sink. (Real houses use a slightly fancier version called a "three-way switch", but the spirit is OR.)
• Every 911 phone in a building — dial from any handset and it connects the same emergency call.

Anywhere a system needs to be trigger-able from more than one place, OR is the wiring pattern.

Do the math

When either button is pressed, the loop through the buzzer is exactly the same as the Door Buzzer — one battery, one buzzer, one closed path. And the LED lane in parallel matches the Door Buzzer's headphones-on light. So the numbers you already know apply here too:

• Buzzer lane: the active buzzer behaves like a ~150 Ω load at 4.5 V (from its datasheet). Ohm's law: I = 4.5 ÷ 150 = 30 mA. Loud enough to hear from the next room.
• LED lane: red LED "eats" 2.0 V, so the 220 Ω resistor drops 4.5 − 2.0 = 2.5 V. I = 2.5 ÷ 220 ≈ 11 mA. Nice glow.
• Battery current when someone presses: the two lanes add in parallel — 30 + 11 = 41 mA. Fresh AAs handle that for hours without a wobble.
• Battery current when nobody presses: both loops are broken (both switches open) → the battery pushes exactly zero current. This alarm costs you nothing to leave armed overnight.
• Battery current when both buttons are pressed: the buzzer and LED currents don't double — the loop is already closed as hard as it can be closed, and the second switch just adds a redundant lane. Adding more switches in parallel can't increase the current through a load whose voltage is already set. Keep that rule for later.

Build it

1. Power rails: battery + to a top row on the breadboard, battery − to a bottom row. Nothing new here.
2. Button A: press it into the breadboard so it straddles the centre groove (that keeps its two "always-connected" legs on opposite sides — same trick as in Morse Code Messenger and The Vault).
3. Button B: same, a few rows over.
4. Wire them in parallel: the top-left leg of both buttons goes to the + rail with a short jumper each.
5. The alarm side: the bottom-right leg of both buttons goes to the same row on the breadboard — call this the alarm-fed row. This is the moment the two lanes merge into one output.
6. The two output lanes, both fed from the alarm-fed row, both returning to −:
   • Buzzer: alarm-fed row → buzzer + leg → buzzer − leg → − rail. No resistor needed on this lane — the active buzzer limits its own current, like in Door Buzzer.
   • LED + 220 Ω: alarm-fed row → 220 Ω → LED long leg → LED short leg → − rail.
7. Press A alone: buzzer + light. Release, press B alone: buzzer + light. Press both together: still buzzer + light — no louder, no dimmer, exactly the same.

The "long wire" upgrade

Once the breadboard version works, one of your two buttons can live somewhere else in the house. Cut two long lengths of any insulated wire (an old headphone cable, a stripped-out garden tie), and run them from the buttons' two legs out to wherever you want the second call button — the fridge door, the bathroom, the front porch. Wire length doesn't hurt the loop; the whole electrical world is built on this. (You've now done what a professional installer calls "extending a switch loop", which is a respectable sentence to drop on a grown-up.)

OR vs AND vs XOR — a preview

You now know two logic patterns from bare switches: AND (The Vault — both must be on) and OR (this project — either is enough). There's a third one, XOR, "exactly one, not both". You can't build it from just two plain switches; you need either a swap-style switch (called SPDT) or a chip. When you meet the XOR chip at Level 6, the first two patterns will already make sense.