Level 2 · Sound · project 17 of 19 · 15 min · ●●○○○
Hidden Mute
A doorbell with a secret slide switch tucked inside just the buzzer lane: flip it and the ring button still lights the LED, but the sound stays off.
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
- 6× Jumper wires L1
- 1× Push button (tactile) the doorbell button at the front of the circuit L1
- 1× Slide switch the hidden mute — lives inside the buzzer lane only L1
- 1× Active buzzer (3–5 V)
- 1× Green LED (5 mm) the "someone rang" indicator — stays active even with the sound off L1
- 1× 220 Ω resistor LED safety resistor L1
you'll learn: Circuits are loops · Switches · Series & parallel · Ohm's law · LED polarity
Circuit map
The circuit you'll build, running on the Workbench right now — shown with the switch closed.
simulating…
If it doesn't work
The boring ways this circuit actually breaks — check these first before you unwire everything.
- Pressing the button fires nothing — buzzer silent and LED dark. Button isn't in the loop. Trace: + → push button common leg → merge point → the two lanes. If the button is wired across two always-connected legs (the ones on the same side of its plastic groove), it's doing nothing. Rotate the button 90° on the breadboard.
- LED lights but buzzer stays silent in both slide positions. Buzzer polarity. The + leg faces the button side of the slide switch. Flipped, the buzzer just sits silent even with the slide closed.
- Buzzer beeps but LED stays dark in both slide positions. LED is reversed, or the 220 Ω is in the wrong lane. Long leg of the LED toward +. Check that the 220 Ω is only in the LED lane — the buzzer lane needs no series resistor.
- The slide switch kills BOTH outputs when open. The slide landed between the button and the merge point, not between the merge and the buzzer. Move it one step closer to the buzzer so the LED lane branches off BEFORE the slide.
- Buzzer beeps even with the slide open. A loose jumper is bridging around the slide. Check that the only path from the merge to the buzzer's + leg runs through the slide — no bypass jumpers on the same row.
- Nothing works at all. Battery pack switch is off, or the main rails aren't reaching the breadboard. Confirm with a known-good *First Light* build on the same breadboard first.
What you're building
A doorbell with a mute switch that only touches one of its two outputs. The push button is the ring: press it and both the buzzer beeps and the green LED lights. The slide switch is the hidden mute: slide it one way and the ring is loud; slide it the other way and the ring is silent — but the LED still lights every time you press the button.
So the button says someone's at the door; the LED is the always-visible proof; the slide switch is the family courtesy of please don't wake the baby.
The big idea: a switch inside one lane
Up at Level 1, Do Not Disturb taught you that a switch placed on one of two parallel lanes only touches that lane: break the red lane and the green keeps shining; break the green and the red keeps shining. Here you're going to use that same trick with a louder output. The lane with the slide is the buzzer; the lane without the slide is the LED. Flip the slide and the sound stops without touching the light.
Walk the loop
- + rail → push button: everything downstream only sees power when the button is held.
- After the button the circuit splits into two parallel lanes
that both return to the − rail:
- Buzzer lane: → slide switch (the mute) → buzzer → −
- LED lane: → 220 Ω → green LED → −
The slide switch sits inside the buzzer lane, after the merge point. Opening the slide breaks the buzzer lane only — the LED lane is a different path home, unaffected.
Do the math (button pressed, mute closed — "ring loud")
Both lanes carry current in parallel, each with its own voltage story.
- Buzzer lane. Active buzzer looks like ~150 Ω. With the slide closed its contact adds practically nothing. Ohm's law: I_buzz = 4.5 ÷ 150 = 30 mA. Full-volume beep.
- LED lane. Battery 4.5 V − green LED Vf 2.1 V = 2.4 V left across the 220 Ω. I_LED = 2.4 ÷ 220 ≈ 11 mA. Normal green glow, same as every other green-LED project you've built.
- Battery total: 30 + 11 = 41 mA.
Do the math (button pressed, mute open — "ring silent")
The slide switch is open, so the buzzer lane is broken: zero current through it, zero sound. The LED lane doesn't notice the slide at all — it's a different circle through the parts, hanging off the same button.
- Buzzer lane: 0 mA. Silent.
- LED lane: unchanged at 11 mA. LED still lights at normal green brightness.
- Battery total: 11 mA — three times less draw than the ring-loud state. Mute mode is cheaper on the AAs and kinder on the neighbours.
Do the math (button up)
Doesn't matter what the slide is doing — the button is the master gate. Nothing can reach either lane.
- I_buzz = 0, I_LED = 0, battery total = 0 mA.
Where "mute one output" shows up
- Phones have a mute button that silences the ringer and haptics, but you can still see the screen light up. One switch, two outputs: cut the sound, keep the visual.
- Alarm clocks have a silence while vibrate mode. Same pattern.
- Oven timers often have a "visible beep only" setting — the LED display continues to blink even when the sound is off.
- Baby monitors usually have a parent-side mute that silences the speaker but keeps the "sound detected" light on so a parent reading in bed can see without hearing.
Every one is this circuit: a shared trigger, two parallel outputs, and a per-lane switch that only touches one of them.
🧠 Your challenge
No single right answer. That's the point.
- Mute the OTHER output. Move the slide switch into the LED lane instead of the buzzer lane. Now the ring button fires the buzzer every time, but the LED only lights when the slide is also closed. Which "mode" is this good for? (A bedside bell for someone with trouble sleeping — the sound is always on so it can wake you, but the light is suppressed so a flicker doesn't disturb a partner in the same room. The exact inverse of our main build.)
- Mute both from one switch — the wrong way. Try moving the slide switch between the push button and the merge point (so it's in series with the whole alarm, not just one lane). Both outputs now die together — but what did the slide switch replace? (The push button! You've accidentally built a 2-switch AND where both the slide and the button have to be closed. That's the Arm & Ring pattern. Not wrong, just a different circuit.)
- Two mutes, one per lane. Add a second slide switch so each output has its own mute. Four states: loud-and-lit, lit-only, loud-only, dark-and-silent. Can you draw the truth table before you build it? (Four slide positions × one button state = four observable outcomes. The LED and buzzer are now fully independent.)
- Can you prove the lanes really are independent? With the button held down and the slide open (buzzer silent, LED lit), measure the LED current. Then close the slide — the LED current should not change by even one tenth of a milliamp. (This is the heart of lane independence: parallel outputs each see the full 4.5 V between the plus of the merge point and the − rail, so neither cares what the other is drawing. The battery just supplies the sum.)
- Would a tactile button work as the mute? Swap the slide for the push button. Now the buzzer only beeps while two buttons are held at once. That's a Two-Hand Bell-style AND inside the buzzer lane, with the LED still single-handed. Which real-life product would want this?
For grown-ups: safety notes
- Battery-safe end to end. The worst case (ring loud) draws 41 mA on 4.5 V — a 185 mW load that your AA pack delivers for hours of on-time without warming up.
- The buzzer is loud at close range. The first time you build this and press the button, point the buzzer away from your face and warn anyone within arm's reach. 30 mA at 3 cm startles pets and nearby siblings reliably.
- Nothing in this loop gets warm at normal use. 220 Ω carrying 11 mA is 29 mW — a tenth of its 250 mW rating. If your resistor feels hot, the slide switch is probably in the wrong row and you're shorting part of the circuit. Pull the battery, trace the loop with a finger, and rewire.
- Buzzer polarity: the + leg (longer pin or marked +) must face the battery's + rail, which is the leg of the slide closer to the button. If the buzzer stays silent even with the slide closed and the button pressed, flip the buzzer and try again.
- Keep small parts and loose wires away from pets and toddlers. Never wire this to mains power — the whole site is battery-only for good reason.
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