🔈 Whisper Alarm

What you're building

A tune-able doorbell. Press the button and two things happen at once: a green LED lights and the buzzer beeps — same multi-output trick you learned in Door Buzzer. What's new is the volume knob: a potentiometer sitting inside the buzzer's lane. Twist it to full and the buzzer is a normal-loud alarm. Twist it the other way and the buzzer trails off to nothing — but the green LED stays exactly as bright as ever. The knob only affects the lane it's inside.

That "one lane, one knob" idea is what lets a real stereo have a separate bass knob, treble knob, and volume knob without them stepping on each other. You're about to build the world's smallest version of that.

The big idea: put the pot INSIDE a lane, not before the split

Look at the wiring in your head:

• Button → merge point → LED lane (220 Ω + LED) → back to −
• Button → merge point → buzzer lane (pot + buzzer) → back to −

Both lanes hang off the same merge point, so both see the same push from the battery (4.5 V minus a tiny drop across the button and the wires — call it 4.5 V for math). Now the important part: the pot is NOT on the shared feed — it's inside one specific lane. So it can choke that lane's current without touching the other.

Compare with what would happen if you moved the pot to between the button and the merge point: now every lane downstream of it sees less voltage, and the LED would dim right along with the buzzer. That's a master volume. It has its uses, but it's not what you want when you need the light bright and the sound quiet at 2 a.m.

(The challenges section walks you through building the master-volume version too, so you can feel the difference.)

Meet the potentiometer (again, if you built Dial-a-Brightness)

The pot is a resistor with a knob — three legs, but here we only use two of them, so it acts as a variable resistor (the fancy word is rheostat). Turn the knob one way, its resistance slides toward 0 Ω. Turn the other, up to 10 kΩ. Everything in between is smooth. In Dial-a-Brightness you used exactly this trick to fade an LED. Here you're using it to fade a buzzer.

Do the math (buzzer lane)

Same battery, same buzzer, same Ohm's law. The active buzzer, from its datasheet check in Door Buzzer, behaves like a ~150 Ω load at 4.5 V. Add the pot in series and you get one bigger speed bump:

• Knob at min (pot ≈ 0 Ω): buzzer current = 4.5 ÷ 150 ≈ 30 mA. Normal loud alarm — same beep you built in Door Buzzer.
• Knob at 1 kΩ: current = 4.5 ÷ (1000 + 150) ≈ 3.9 mA. Below what the active buzzer's internal circuit needs to make a proper tone — at most, you'll hear the scratchy whisper in the project's name.
• Knob at 10 kΩ (max): current = 4.5 ÷ (10 000 + 150) ≈ 0.44 mA. Silent — nothing to hear at arm's length. Active buzzers usually need at least a few milliamps to sing.

The pot has become a volume knob because it's slowly starving the buzzer's current below its "make sound" threshold.

Do the math (LED lane)

Now the surprise: the LED lane's numbers don't budge as you turn the pot.

• Voltage left for the resistor after the LED "eats" its 2.1 V (green): 4.5 − 2.1 = 2.4 V
• Current through the 220 Ω: 2.4 ÷ 220 ≈ 11 mA
• Same current, same brightness, at any knob position

The two lanes are in parallel, both hanging off the same near-4.5 V merge point. The pot only controls what happens inside the buzzer's lane. The LED's lane has its own private speed-bump (the 220 Ω) and doesn't know or care what the buzzer is doing.

Build it

1. Power rails: battery + to a top row, battery − to a bottom row. Same as always.
2. Button in the feed: battery + → push button → merge row. The merge row is where both output lanes will tap in.
3. Buzzer lane (with the volume knob): merge row → pot's middle leg (the wiper) → pot's outer leg → buzzer + leg → buzzer − leg → battery −. The pot's third leg dangles (same as Dial-a-Brightness).
4. LED lane (unchanged brightness): merge row → 220 Ω → LED long leg → LED short leg → battery −. This lane is fully independent of the pot.
5. Press the button with the knob at either extreme and confirm the behavior:
   • Knob toward min: loud buzz + green light. Perfect doorbell.
   • Knob toward max: silence + green light. Perfect silent doorbell. Great for a shared apartment at midnight.

Where "per-lane controls" show up

• Bike lights with a brightness dial: the dial is on the LED's lane only, not on the whole handlebar circuit.
• Volume vs brightness on your laptop: two separate knobs (well, key combos) controlling two separate lanes — they never fight.
• A mixing board at a concert: dozens of pots, each on its own channel, each affecting only that lane.

Once you get "the knob only touches the lane it's inside," you can start reading any circuit and predicting which knob does what without tracing the whole thing.