Level 2 · Sound · project 3 of 12 · 15 min · ●●○○○
Dial-a-Buzz
Dial-a-Brightness for sound — one knob rides the buzzer between full alarm and library silence, with no button in the way.
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
- 4× Jumper wires L1
- 1× Slide switch the master mute — off means silent no matter the knob L1
- 1× 10 kΩ potentiometer the volume knob — wired as a rheostat, same as Dial-a-Brightness
- 1× Active buzzer (3–5 V) + leg toward the battery; no 220 Ω needed — the buzzer self-limits at ~150 Ω
you'll learn: Ohm's law · Series & parallel · Switches
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.
- Knob doesn't change the buzz. Pot wired to the two outer legs (fixed 10 kΩ). Use the middle (wiper) + ONE outer leg — that's the variable part.
- Buzzer silent at every knob position. Slide switch open, buzzer polarity wrong, or wiper feeding a dead breadboard row. Flip the switch, then flip the buzzer, then trace the wiper leg to a live row.
- Full-loud at every knob position. You bridged the two outer legs of the pot with a jumper — the pot is being bypassed. Pull that jumper.
- Weak thin sound, no matter the knob. You added a resistor in series with the buzzer 'just to be safe'. Take it out — the buzzer already self-limits at 150 Ω.
- Slide switch changes nothing. Middle leg + ONE outer, same as First Light. The other outer leg is dead.
What you're building
Dial-a-Brightness for your ears. One slide switch to arm the alarm, one knob to set how loud it beeps, and one buzzer that goes all the way from full-siren to library-silent as you twist. No button to hold, no LED splitting the story — the pot and the buzzer are the whole loop.
The big idea (from Dial-a-Brightness): the knob IS a resistor
The potentiometer is a resistor with a knob that slides its resistance smoothly from about 0 Ω to its full size — 10 kΩ for yours. In Dial-a-Brightness you put the pot in series with an LED, and every twist changed the current, and every current change changed the brightness. Here the load is different — a buzzer — but the wiring trick and the math are the same.
The active buzzer, from its datasheet check in Door Buzzer, behaves like a ~150 Ω load. So the whole loop is:
loop resistance = pot + 150 Ω
current = 4.5 V ÷ loop resistance
Do the math
- Knob at the low end (pot ≈ 0 Ω): current = 4.5 ÷ 150 ≈ 30 mA. Full-loud beep — same 30 mA as Door Buzzer. Cover your ears.
- Knob a tenth of the way (pot ≈ 1 kΩ): current = 4.5 ÷ 1150 ≈ 3.9 mA. Active buzzers need a few milliamps of drive current to make a proper tone — 3.9 mA is right at the edge, and what you'll hear is a scratchy whisper.
- Knob in the middle (pot ≈ 5 kΩ): current = 4.5 ÷ 5150 ≈ 0.87 mA. Silent. A milliamp isn't enough for the buzzer's internal oscillator to swing hard enough for you to hear.
- Knob at the high end (pot ≈ 10 kΩ): current ≈ 0.44 mA. Deeply silent — at arm's length you can't hear anything at all.
The knob is now a volume knob. Somewhere between "1 kΩ" and "0 Ω" you'll find the sweet spot where the buzzer just barely wakes up — that's your "polite doorbell" position.
Why no safety resistor this time
Dial-a-Brightness had a 220 Ω sitting next to the pot so the LED couldn't pop when the knob reached zero. Buzzers don't need one — the active buzzer has ~150 Ω of internal resistance built in, and its own tiny driver chip. Even at "pot = 0 Ω" the current tops out at 30 mA, which is exactly what the datasheet says the buzzer is designed to draw. If you added a 220 Ω anyway the buzzer would sound thinner and quieter but it wouldn't hurt anything.
Rule of thumb: parts that have a datasheet current on their front page (LEDs at 20 mA, buzzers at 30 mA, motors at 150 mA) tell you whether they need a helper or not. If the part does its own current limiting inside, you can wire it straight to the battery. If it doesn't, you're the safety resistor.
Build it
- Power rails: battery + → top row, battery − → bottom row. Same as always.
- Slide switch in the feed: top row → slide switch → volume row. Flipping this switch open cuts everything downstream.
- The knob: volume row → one outer leg of the pot, wiper (middle leg) → merge row. The third leg dangles. (Same 2-of-3 wiring you used in Dial-a-Brightness.)
- Buzzer to ground: merge row → buzzer + leg; buzzer − leg → battery −.
- Flip the slide switch on with the knob at min: full-volume beep. Twist toward the max: the beep starves down to a whisper, then to nothing. Twist back: it wakes up.
Where a "single-loop volume knob" shows up
- Bedside white-noise machines with a dial: nothing fancier inside — a small oscillator, a knob, a speaker.
- Cheap egg timers and wind-up alarms: one buzzer, one dial, no microcontroller in sight.
- Bike bells with a "quiet mode" slider: same physics, plastic case.
The step up from here is Whisper Alarm, which splits the same knob across a buzzer lane and a separate LED lane — so the knob only touches the sound, and the light stays constant. Try Dial-a-Buzz first, then that.
🧠 Your challenge
No single right answer. That's the point.
- Find the wake-up point. Turn the knob slowly from max toward min until the buzzer just starts to sing. What resistance is that, roughly? (Use the loop equation: at your wake-up current — call it the buzzer's minimum of ~2 mA — solve 4.5 ÷ R_total = 0.002 for R_total, then subtract the buzzer's 150 Ω.)
- Add the LED back. Wire a red LED with its own 220 Ω in parallel with the buzzer. Does the knob still control the sound? Does it affect the LED? (You just discovered why Whisper Alarm's pot lives inside the buzzer lane — in this parallel version the pot IS on the shared feed, and both loads dim together. That's a master volume, not a per-lane one.)
- Swap the pot for the fixed 1 kΩ. Wire the buzzer through a 1 kΩ and predict the current before you power up. (4.5 ÷ (1000 + 150) ≈ 3.9 mA — the same scratchy whisper as "knob at 10 %".) Now you know what "knob at 10 %" sounds like without needing the knob at all.
- Two-position mute. Move the slide switch so it shorts the pot instead of cutting the loop — the switch closed = pot bypassed = full 30 mA, switch open = pot in circuit = whatever the knob says. A two-position "loud vs polite" toggle. Sketch the wiring on paper before you build.
For grown-ups: safety notes
- Battery-safe all the way through — 4.5 V never rises past its arm's length shock-free number, and the loop tops out at 30 mA even at "loudest".
- The active buzzer is loud at close range with the knob at min. When you first power up, hold it away from your ears — a 30 mA beep at 3 cm is a startle-loud, especially for a nearby pet or a younger sibling.
- Nothing in this loop gets hot. If the buzzer or pot ever feels warm, disconnect the battery — a warm pot usually means a solder-tail leg is bridging two rows on the breadboard.
- If you leave the circuit built and the slide switch open, it draws literal zero — safe to leave overnight. If you leave it armed with the knob turned all the way to max, it's still silent and pulls under a milliamp; the AAs won't notice for weeks.
Checked against
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