🔔 Door Buzzer

What you're building

A doorbell for your room: button outside the door, buzzer inside — plus a green LED so you can see the ring even with headphones on. This is your first multi-output circuit: one press, two things happen.

The big idea: outputs in parallel

You already know parallel lanes from the Tabletop Traffic Light. Here the lanes carry different passengers: one lane is the buzzer, the other is the LED. Because both lanes hang off the same button, one press powers both at once. Notice what you're doing now — you're not learning new physics, you're combining pieces you already own. That never stops being true, all the way to Level 10.

Meet the buzzer

Your buzzer is an active buzzer: it has a tiny circuit inside that makes its own tone the moment it gets power — you can't change its pitch, only turn it on and off. (Its cousin the passive buzzer needs to be fed a wobbling signal to sing — that one becomes fun at Level 7, when you can program the wobble.) Like an LED it has a polarity: the + marked leg goes toward the battery's plus side. Unlike an LED it's happy on 4.5 V directly — no resistor needed on its lane; it only sips about 30 mA.

Build it

1. Button straddling the center groove, battery red (+) to one leg.
2. From the diagonal leg, split into two lanes:
   • Lane 1: straight to the buzzer's + leg; buzzer − to battery black.
   • Lane 2: through the 220 Ω to the LED long leg; short leg to battery black.
3. Press: BZZZT + green glow.
4. For door duty, swap the button's short jumpers for your longest wires so the button can live outside the door.

Why no resistor for the buzzer?

Fair question — drill it every time something new appears in a lane. The LED needed a resistor because it would gulp unlimited current. The active buzzer's internal circuit already limits what it drinks (~30 mA at this voltage). The rule isn't "everything gets a resistor"; the rule is know what each part needs before you power it. Reading the part's label or datasheet first — that's a real engineering habit.