🔔 Twin Bells

What you're building

A doorbell that rings in two rooms at once. Kitchen bell over the sink, bedroom bell on the nightstand, one button at the front door firing both. And because bedtime is bedtime, a slide switch on the bedroom lane lets you silence just the bedroom bell without touching the kitchen one.

• Button up. Silent everywhere. Nothing drawing power.
• Button pressed, both lanes armed. Both buzzers ring at the same normal volume, at the same time. Battery has to supply enough current for both.
• Button pressed, bedroom lane muted. Kitchen buzzer rings alone. Bedroom buzzer stays quiet, no matter how long you hold the button.

The big idea: identical loads in parallel

You have already seen parallel lanes at Level 1 (the two LEDs in Do Not Disturb) and Level 2 (the buzzer + LED in Door Buzzer). What's new here is that both lanes carry the same kind of load — two active buzzers — instead of a mixed pair.

The rule for anything in parallel is short: every lane sees the same voltage across itself. So two buzzers wired in parallel each see the full 4.5 V, and each behaves exactly like a single buzzer on its own. Neither one steals from the other. The battery, however, has to supply the sum of the two currents.

That "each lane behaves like it's alone" fact is what makes the slide switch's job so clean. Drop a switch inside just the bedroom lane and the kitchen lane doesn't know the difference. Kill it and the kitchen keeps ringing exactly the same way. Independence — the same lesson from Whisper Alarm, applied at the output rather than the input.

Do the math (button pressed, both bells ringing)

Both lanes see 4.5 V across themselves. Each active buzzer looks like a ~150 Ω load at 4.5 V (that's the datasheet number: about 30 mA at 5 V, so R ≈ V ÷ I ≈ 150 Ω). Ohm's law twice:

• Kitchen buzzer: 4.5 V ÷ 150 Ω = 30 mA.
• Bedroom buzzer: 4.5 V ÷ 150 Ω = 30 mA.
• Battery total: 30 + 30 = 60 mA.

Twice the current, but each buzzer rings at its own normal volume. Fresh AAs handle 60 mA for many hours — a doorbell doesn't stay pressed long, so the actual drain per press is a blip. The "twice as loud" thing you might expect from twice the current does not happen: real loudness is logarithmic. Two buzzers of the same tone from the same spot are about 3 dB louder than one — a small perceived bump. The real benefit is that the two buzzers are in different rooms, so each room gets its own loud ring.

Do the math (button pressed, bedroom muted)

The slide switch is open. The bedroom lane is broken — no current can flow through it. The kitchen lane doesn't care, because it never depended on the bedroom lane for anything:

• Kitchen buzzer: still 4.5 V ÷ 150 Ω = 30 mA.
• Bedroom buzzer: 0 mA (loop open at the slide switch).
• Battery total: 30 mA — half the "both ringing" case.

The kitchen buzzer's volume, tone, and current draw are all exactly the same as before. The two lanes were never sharing; they were only sharing the same button. Cutting one lane leaves the other one untouched.

Do the math (button up)

The doorbell button is the master gate for both lanes. When it's open, there's no way for either lane to see the battery's plus side. Both buzzers are silent, both lanes carry 0 mA, and the battery supplies 0 mA total. The doorbell can sit built and idle for weeks without draining.

Build it

1. Power rails. Battery + to a top row on the breadboard, battery − to a bottom row.
2. Doorbell button straddling the centre groove, one leg on the + rail, its diagonal leg over to a "merge" row. Now the merge row is only live while the button is pressed.
3. Kitchen lane. Merge row → buzzer #1 + leg → buzzer #1 − leg → − rail. No resistor: the active buzzer limits its own current, exactly like in Door Buzzer.
4. Bedroom lane. Merge row → slide switch → buzzer #2 + leg → buzzer #2 − leg → − rail. Same lane shape, with the slide switch as a mute in series with just this buzzer.
5. Long wires. Swap the bedroom buzzer's short jumpers for your longest wires so it can sit on your nightstand while the kitchen buzzer stays in the breadboard. Tape wires along the wall — never across doorways.
6. Test. Slide on, press button: both bells. Slide off, press button: kitchen only. Release the button any time: silence.

Where "one control, many loads" shows up

• Church bells and school bells are usually one button on the wall of an office wired to a whole ring of speakers spread across the campus, all in parallel.
• Fire alarms in a building: one pull station triggers every horn on every floor at once, parallel wiring the whole way.
• String lights for the holidays sometimes use parallel bulbs so that a dead bulb doesn't kill the whole string — each bulb sees the same voltage no matter what its neighbours are doing.
• The horn on a car: one button on the steering wheel, but the actual horn assembly under the hood is often two horns side by side sounding different notes.