🎵 Duet Doorbell

What you're building

A doorbell with two buzzers ringing at the same time from one button — but at two different volumes. Buzzer A sits directly across the battery when the button is pressed and beeps at its full 30 mA. Buzzer B sits behind a 220 Ω speed bump and beeps at only about 12 mA — audible, but softer.

Two beeps at the same instant. Different loudnesses on purpose. That's a duet: one player is the melody (loud), one is the accompaniment (soft). Real doorbells use exactly this trick when you want a room-to-room fade or a "small chime in the study, big chime in the entryway" effect.

The big idea: two output lanes fed by ONE button

Every project so far mixed inputs and outputs a particular way. In Twin Bells, one button fired two identical buzzers in parallel — both at 30 mA, so both equally loud. In Two-Tone Doorbell, TWO buttons fired a single buzzer, and only one of the buttons had a 220 Ω on its way in.

Duet Doorbell combines the two ideas: one button, two output lanes, and each lane has its own series resistance shaping its own current. The button is the shared "on" — both lanes wake up together — and the resistors decide how each lane sounds.

Wiring it in your head:

• Button → merge point. The button connects the battery + to a single "hot" node when pressed.
• Lane A (LOUD). From hot → buzzer A → − rail. Just the buzzer. Loop resistance ~150 Ω. Current 30 mA.
• Lane B (SOFT). From hot → 220 Ω → buzzer B → − rail. Loop resistance ~370 Ω. Current 12 mA.

Both lanes hang off the same hot node, so both see the full 4.5 V from the battery when the button is pressed. But each lane's current is set by what's in that lane — not by what the other lane is doing. That's the golden rule for parallel branches: they add at the merge, but each branch's own current is set by its own resistance.

Do the math (button pressed)

Active buzzer ≈ 150 Ω at 4.5 V (from Pocket Beeper).

Lane A (LOUD):

I_A = V ÷ R = 4.5 V ÷ 150 Ω = 0.030 A = 30 mA

Same as Pocket Beeper and every full-volume doorbell you've built.

Lane B (SOFT):

I_B = V ÷ R = 4.5 V ÷ (220 + 150) Ω = 4.5 ÷ 370 ≈ 0.012 A = 12 mA

Well above the buzzer's ~5 mA wake-up threshold — this one really does sound, just quieter. That's the Two-Tone Doorbell's QUIET path revisited: a 220 Ω in series produces an audible-but-softer beep. (A 1 kΩ would starve it below the wake-up line and give a whisper or silence.)

Total current from the battery:

I_total = I_A + I_B = 30 + 12 = 42 mA

That's Kirchhoff's current law again: what the battery pushes out equals the sum of what all the parallel lanes accept. Well within the pack's comfortable range — a 3×AA pack sources hundreds of mA without complaint.

Build it

1. Power rails. Battery + → top row. Battery − → bottom row.
2. Button in the feed. + rail → push button → hot node. Nothing fires until this button is pressed.
3. LOUD lane (Buzzer A). From the hot node → buzzer A's + leg → buzzer A's − leg → − rail. No resistor.
4. SOFT lane (Buzzer B). Also from the hot node → 220 Ω resistor → buzzer B's + leg → buzzer B's − leg → − rail. The 220 Ω is between the hot node and buzzer B, in series with buzzer B only.
5. Press. Both buzzers should ring at once — A loud, B softer. Let go — both stop.

Why the two buzzers make a "beat", not a clean chord

An active buzzer at 4.5 V sings at whatever pitch its internal oscillator picks — usually 2.3 kHz. Two of them, sitting a few centimetres apart, sing at almost the same pitch. Real oscillators don't match to the Hz — one might sit at 2.3 kHz, the other at 2.29 kHz.

Sound waves that close in pitch interfere: sometimes their peaks line up (loud), sometimes they cancel (quiet). The ear hears that as a slow wow-wow-wow underneath the beep — the same "beat" a piano tuner listens for when tuning two strings. You are hearing the pitch difference, in real time. Try holding the board so both buzzers face you and the beat will be obvious.

Squeeze both buzzers between two fingers and the beat can shift or speed up — piezos change pitch slightly under mechanical load. This is a very small physics lab hiding in a doorbell.

Where a "two-lane on one button" shows up

• Church doorbells with a big chime in the hall and a little chime in the kitchen, both on one button.
• Fire alarms that ring a loud outdoor bell and a softer indoor speaker off the same trigger.
• Wired game buzzers with a loud stage bell and a small crew-headset click on one press.

Whenever you want the same event to sound different in different places, this circuit is the shape underneath.