🛎️ Two-Tone Doorbell

What you're building

One doorbell — buzzer + porch LED — with two call buttons that ring it in two different ways:

• Button A (LOUD). Buzzer rings at full volume and the porch LED lights. Perfect for guests who need you to notice.
• Button B (QUIET). The buzzer gets only a starved trickle — a faint rasp, a click, or on some buzzers nothing audible at all — and the porch LED stays off. Meant for a family member coming home at midnight who wants to signal without waking the house (and, as you'll see, a lesson in what an active buzzer does when you underfeed it).
• Press both at once. LOUD wins — same as pressing A alone.
• Nobody pressing. Silent, dark, drawing nothing from the battery.

This is Panic Doorbell's OR trick — either button triggers the alarm — plus Whisper Alarm's "starve one path with a series resistor" trick, combined into one project. The new idea: the two activation paths don't have to be identical. Each path can shape the current on its way to the shared load.

The big idea: shape the path, not just the load

In Panic Doorbell, both buttons wired straight through to the merge point, so the alarm behaved exactly the same no matter which button was pressed. In Whisper Alarm, a single button fired a pot that lived inside one output lane, so twisting the knob shaped that lane's current.

Here you're combining them: two buttons in parallel like Panic Doorbell, but the two paths carry different series resistances on the way to the merge. Path A is a bare wire (~0 Ω). Path B has a 1 kΩ resistor sitting in series with its switch. That resistor starves the current through B's path. In the simulator you'll see a small current and a dim buzzer icon. A real active buzzer is a tiny oscillator circuit rated for 3–5 V, and at only 0.6 V it may stop oscillating rather than get quietly quieter — you'll hear a rasp, a click, or nothing. That is the honest lesson of this build: a series resistor makes a lamp dimmer, but it does not make an active buzzer reliably quieter. (For a real quiet mode you'd want a passive piezo driven by a 555 — that arrives in Level 5.)

The buzzer doesn't know or care which path delivered its current, only how much arrives. The LED lane doesn't care either — but the LED needs a minimum voltage to conduct, and B's path can't supply it. Same shared load, two very different behaviors, decided entirely by what's in each activation path.

Do the math (LOUD button pressed)

With A closed and no resistor in the way, the merge point sits at the full battery voltage of 4.5 V. Both output lanes run their normal numbers, exactly like Door Buzzer:

• Buzzer lane: the active buzzer behaves like a ~150 Ω load at 4.5 V (from its datasheet). Ohm's law: I = 4.5 ÷ 150 = 30 mA. Loud enough to hear across a house.
• LED lane: green LED "eats" 2.1 V, so the 220 Ω resistor drops 4.5 − 2.1 = 2.4 V. I = 2.4 ÷ 220 ≈ 11 mA. Bright porch light, same as First Light.
• Battery total: 30 + 11 = 41 mA. Fresh AAs handle that for hours without a wobble.

Do the math (QUIET button pressed)

Now A is open and B is closed. The 1 kΩ resistor is in series with the merge point, so the merge voltage drops as current flows through it. Both lanes still hang off the merge — but they behave very differently.

Step 1 — assume the LED lane draws nothing (we'll check this in a second). Then all the current from the 1 kΩ has to go through the buzzer:

• Battery pushes into a series stack of 1 kΩ (path B resistor) + 150 Ω (buzzer). Total = 1150 Ω.
• Current: I = 4.5 ÷ 1150 ≈ 3.9 mA.
• Voltage across the buzzer: 3.9 mA × 150 Ω = 0.59 V. Way below the 3 V minimum the active buzzer needs to make a proper tone — at most, you'll hear the scratchy whisper behind the name "quiet".

Step 2 — check the LED assumption. The merge point sits at ~0.6 V. The green LED needs about 2.1 V across itself before it starts to conduct. 0.6 V is nowhere close. So yes — LED stays dark, drawing essentially zero current. The assumption holds.

Battery total in QUIET mode: ~3.9 mA. A tenth of the LOUD case, which is why the alarm feels "small" — it's literally drawing a tenth the electricity.

Do the math (both pressed at once)

The LOUD path is a bare wire from battery + to the merge. When A is closed, the merge is pinned at 4.5 V no matter what the QUIET path is doing. So the QUIET path sees 0 V across its own 1 kΩ (both ends are at 4.5 V, the loud path's end and the merge) — no current flows through it. LOUD wins, cleanly, every time. Buzzer 30 mA, LED 11 mA, same as A alone.

This is a general rule worth memorizing: the lower-resistance path in a parallel pair sets the voltage at the merge point. If two paths are competing to feed the same node, the one with less resistance in the way is going to dominate the voltage there.

Build it

1. Power rails: battery + to a top row on the breadboard, battery − to a bottom row.
2. Button A (LOUD): press it in so it straddles the centre groove (the "diagonal legs" trick from Morse Code Messenger). One leg to the + rail; its diagonal leg → the merge row.
3. Button B (QUIET): same trick, a couple of rows away. One leg to the + rail; its diagonal leg → one end of the 1 kΩ. The other end of the 1 kΩ → the same merge row as A. B now reaches the merge through the resistor.
4. The output lanes, both fed from the merge row:
   • Buzzer lane: merge row → buzzer + leg → buzzer − leg → − rail. No resistor on this lane — like Door Buzzer and Panic Doorbell, the active buzzer limits its own current.
   • LED lane: merge row → 220 Ω → green LED long leg → LED short leg → − rail.
5. Press A alone: loud buzzer + bright green LED. Full-volume ring.
6. Press B alone: barely-there whisper + no LED. Silent doorbell for late arrivals.
7. Press both: exactly the same as pressing A alone. Try it — you'll hear no difference.

Where "shape the path" shows up

• Two-speed fans on old table lamps: two buttons wire the fan motor through two different resistors — one path gives low speed, the other high speed.
• Bike lights with a "bright" and "eco" mode: each mode is a different path with a different current-limiting resistor. The LED is the same LED.
• Elevator floor buttons vs the emergency call button: pressing a floor button lights an indicator and starts the elevator quietly. Pressing the emergency button ALSO rings a loud alarm and connects a phone call. Same button interface, very different paths behind it.

Anytime the same output has to behave two different ways depending on which input triggers it, this "different resistance in each path" pattern is what's under the hood.