📣 Pocket Beeper

What you're building

The tiniest possible sound project — the First Light of Level 2. One battery, one push button, one active buzzer, three jumper wires. Press the button and the buzzer sings; let go and it stops. That's it. No resistor. No LED. No knob.

It looks trivial next to the two-lane doorbells that come after, and that's the point: this is the atom. Every buzzer project on the site — Doorbell, Panic Doorbell, Twin Bells, Silent Alarm — is this loop with more switches, more lanes, or a knob stitched onto one of the wires. Get this one solid and every one of those makes sense as "the Pocket Beeper plus one thing".

The big idea: an active buzzer is its own resistor

First Light needed a 220 Ω speed bump because an LED, connected straight to 4.5 V, would try to gulp so much current that it would pop. An active buzzer doesn't have that problem. Inside its little black can lives a tiny oscillator circuit — really just a piezo disc plus a bit of electronics — and that circuit sets its own current draw. At 4.5 V it settles at about 30 mA, and no matter how long you press, it can't run away.

If you want to prove it to yourself with Ohm's law, treat the buzzer like a resistor:

buzzer's effective resistance = voltage ÷ current = 4.5 V ÷ 0.030 A = 150 Ω

That is the number you'll see quoted for "the buzzer" everywhere on this site. It isn't a real resistor inside — it's the behaviour of the internal oscillator when fed 4.5 V. And it means: you don't need to add a resistor. The buzzer is already the safety.

Do the math

• Loop while pressed: battery + → push button → buzzer → battery −.
• Voltage: 4.5 V from the 3×AA pack.
• Loop resistance: ~150 Ω (the buzzer, since the button and wires are almost 0 Ω).
• Current: I = V ÷ R = 4.5 ÷ 150 = 0.030 A = 30 mA.

That is a normal, loud, close-to-your-ear "beep". A fresh pack of AAs holds about 2500 mAh, so continuous pressing would give roughly 2500 ÷ 30 ≈ 83 hours of noise. Between presses the loop is broken at the button, so the draw is zero — leave this on your desk for months and the batteries won't notice.

Build it

1. Power rails. Battery red (+) wire → top row of the breadboard. Battery black (−) wire → bottom row.
2. Push button. Plant it so it bridges two rows. Wire one leg to the top + row.
3. Buzzer. Its long leg (the one by the small + mark on the top sticker) goes to the other leg of the button. Its short leg goes to the bottom − rail.
4. Press. You should hear a fixed-pitch ~2.3 kHz beep the whole time your thumb is down. Let go — silence.

If it doesn't beep the first time, the two most likely reasons are the buzzer being backwards (silent, and eventually damaged, so fix this first) or a leg not clicking all the way into the breadboard.

Where a "button-plus-buzzer" shows up

Almost every piece of consumer plastic that beeps once when you press something is a variation of this loop:

• Doorbells at the front of houses — one button outside, one buzzer inside, wired through the door frame.
• Kitchen "hurry-up" clickers and oven-timer done beeps.
• Personal safety alarms on keychains — press the button, it screams. The one that comes on a house key is exactly this circuit with a slightly louder piezo.
• Elevator "call" buttons signal a controller, but the "ding" next to your ear when you press one is a Pocket Beeper under a brass cover.

The step up is Doorbell, which is this loop with a red LED (plus its 220 Ω safety resistor) added in parallel with the buzzer — one button, one beep, one "someone's at the door" light.