Level 2 · Sound · project 18 of 19 · 15 min · ●●○○○
Power Pilot
A green pilot LED that glows the whole time the battery is plugged in, next to a red alarm LED and buzzer that only fire when the button is pressed.
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
- 7× Jumper wires L1
- 1× Push button (tactile) the alarm trigger — fires the red LED and the buzzer together L1
- 1× Active buzzer (3–5 V)
- 1× Green LED (5 mm) the pilot — glows whenever the battery is connected, no switch in its lane L1
- 1× Red LED (5 mm) the alarm — only lights when the button is held L1
- 2× 220 Ω resistor one safety resistor per LED, each in its own lane 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.
simulating…
If it doesn't work
The boring ways this circuit actually breaks — check these first before you unwire everything.
- Pilot LED stays dark even when the battery pack switch is on. Pilot LED polarity (long leg toward +) or its 220 Ω is dislodged. Pilot lane must be + → 220 Ω → LED long leg → short leg → −, with no switch anywhere.
- Pilot LED only lights when the button is held. The pilot lane is wired AFTER the button instead of OFF the raw + rail. Move the pilot lane's top end upstream of the push button so the button never affects it.
- Pressing the button dims the pilot visibly. The two lanes are sharing a 220 Ω by mistake. Each LED must have its OWN 220 Ω in its own lane. If one 220 Ω feeds both LEDs, you have the *One for All* fault from Level 1 — the current splits and both LEDs are dimmer than they should be.
- Alarm LED and buzzer never fire on press. Button isn't wired into their lanes. Trace: + → push button → merge → (alarm LED lane) and + → push button → merge → (buzzer lane). If the button straddles the always-connected legs, it's a dead-end.
- Alarm LED fires but buzzer stays silent. Buzzer polarity. + leg of the buzzer faces the button-side of the merge. Reversed = silent.
- Everything works but AAs die quickly. Expected — the pilot draws 11 mA 24/7 even in idle. Unplug the battery when the project is on the shelf. Alternatively, add a slide switch at the battery to turn off the whole thing when not in use.
What you're building
Every real appliance you own does this: a little green light says I'm plugged in even when the thing is doing nothing. Your coffee maker, your Wi-Fi router, your microwave, your TV in standby — all of them run a tiny LED off the main power rail so you can see at a glance that electricity is reaching the box. The alarm or display or motor only kicks in when something actually happens.
This project builds that pattern. A green pilot LED draws a steady 11 mA from the battery the whole time — no switch in its lane. Next to it, a red alarm LED and a buzzer sit silent until someone presses the push button; then both fire together.
The big idea: a lane with no switch is "always on"
So far every lane in every project has had at least one switch somewhere along it — a slide, a button, or something else. That's why things could be turned off. Remove the switch entirely and the lane is live for as long as the battery is connected. That's the definition of a pilot indicator.
Three independent parallel lanes hang off the + rail here. Each has its own math:
- Pilot lane: + → 220 Ω → green LED → −. No switch. Always on.
- Alarm LED lane: + → push button → 220 Ω → red LED → −. Button is the gate.
- Buzzer lane: + → push button → buzzer → −. Shares the same button as the alarm LED.
The push button sits before the alarm-LED-and-buzzer merge point, so pressing it fires both of those lanes at once while leaving the pilot untouched. Three lanes, two states (idle and alarm), one battery.
Do the math (idle — nobody pressing)
Only the pilot lane carries current.
- Pilot lane. Battery 4.5 V − green LED Vf 2.1 V = 2.4 V left across the 220 Ω. I_pilot = 2.4 ÷ 220 ≈ 11 mA. Standard green glow — identical to every other green-LED project you've built, because the arithmetic is identical.
- Alarm LED lane: button open, 0 mA.
- Buzzer lane: button open, 0 mA.
- Battery total: 11 mA idle. A set of fresh AAs (~2500 mAh) can sustain that for over 200 hours — more than a week of continuous "I'm plugged in" glow.
Do the math (alarm — button held)
Pilot stays unchanged (it doesn't know the button exists), and both alarm lanes come alive.
- Pilot lane: 11 mA, unchanged. Lane independence.
- Alarm LED lane: 4.5 V − 2.0 V red Vf = 2.5 V across 220 Ω → I_alarmLED = 2.5 ÷ 220 ≈ 11 mA. First-Light brightness.
- Buzzer lane: 4.5 V across 150 Ω → I_buzz = 30 mA.
- Battery total: 11 + 11 + 30 = 52 mA while the button is held. Still cheap on the batteries — a 15-second alarm costs about 0.2 mAh, nothing noticeable.
Notice what doesn't happen when the alarm fires: the pilot does not dim. Each lane sees the full 4.5 V between + and − all by itself, so firing one lane does not steal from another. The battery simply supplies the sum of the three. This is the "lane independence" lesson from Do Not Disturb and Doorbell, now with three lanes instead of two.
Where "pilot + alarm" shows up
- Smoke alarms have a tiny green blink every 30 seconds to say "I have power" — the loud alarm only fires on smoke.
- Routers and modems always-on green power LED, blinking status LEDs on top. The green is a pilot by exactly the same wiring pattern.
- Appliances in standby: TVs, microwaves, air conditioners — the little red or orange standby LED tells you the main switch is on, even when nothing's being asked for.
- Alarm panels in offices: a steady green "system armed" LED sits next to the much louder red strobe + siren that fire on trigger. Same two-tier approach to feedback.
Steady indicators are not decoration. They tell you the thing is ready, which is a different fact from doing something. Design a device without a pilot and the first user question is always "wait, is it on?" This project is the smallest-possible answer.
🧠 Your challenge
No single right answer. That's the point.
- Make the pilot red and the alarm green. Swap the two LEDs between lanes. Does anything about the circuit stop working? (Nothing changes electrically — currents are the same because both red and green Vf are near 2 V. But the perception changes: a steady red pilot is confusing because red is the colour we trained ourselves to read as "warning". Convention matters; physics doesn't. Design choices live on top of the math.)
- Add a slide switch in the pilot lane. Now you can turn off the pilot without unplugging the battery — useful for a bedside version where even 11 mA of green glow is too much at night. What does this do to the "always on" meaning of the pilot? (It becomes a "pilot I can disable". The circuit becomes less honest — the LED no longer reliably reports whether the battery is connected. Many appliances deliberately avoid this for exactly that reason: a disable-able pilot is a pilot you can't trust.)
- Pilot that dims instead of turning off. Replace the pilot's 220 Ω with a 10 kΩ. Compute the new current and predict what you'll see. (4.5 − 2.1 = 2.4 V across 10 kΩ → 0.24 mA. The LED is barely visible — a pinprick glow, like Night Pin at Level 1. A pilot you can leave on indefinitely in a dark room without annoying anyone. 50× less current than the standard version.)
- Alarm WITHOUT a buzzer. Pull the buzzer out of its lane; what did it change about the circuit? (Nothing — the alarm LED lane is still happy at 11 mA. The two alarm lanes are independent parallel paths. The buzzer's absence is simply "that lane has no current now".)
- Replace the push button with a slide switch. Now the alarm stays armed indefinitely instead of only while held. What's the practical downside? (You can forget the alarm is on, and the 52 mA draw will drain the AAs in roughly 48 hours instead of 10 days. Momentary buttons are the kinder battery choice for things that aren't supposed to stay on.)
- A second pilot. Add a second green pilot in parallel with the first one. What's the battery current now in idle? (2 × 11 = 22 mA — twice the standby draw. If this were a real appliance, you'd think about whether both pilots deliver different information or just repeat the same message. Redundant pilots are rare for a reason.)
For grown-ups: safety notes
- Battery-safe throughout. Worst case (button held) is 52 mA at 4.5 V — a 234 mW load, trivially inside what AAs deliver all day without warming up.
- The pilot is always on. This means the battery is draining whenever it's connected, even with the circuit "doing nothing". 11 mA for a week is about 1800 mAh — roughly 70 % of a fresh AA pack's capacity. Unplug the battery when you set the project aside for more than a day.
- The buzzer is loud at close range. First press, point it away from faces and warn anyone nearby. 30 mA at 3 cm is a startle-loud beep.
- Nothing in this circuit should get warm during normal use. 11 mA through 220 Ω is 29 mW — a tenth of its 250 mW rating. If either 220 Ω feels obviously warm, pull the battery immediately: you've probably wired a resistor across the battery rails directly (short-circuit path), and the resistor is dissipating 90+ mW trying to limit it.
- LED polarity is doubled up here — one per lane, both long-leg toward +. A reversed pilot stays dark the whole time (easy to spot). A reversed alarm LED stays dark on button press (easy to spot). Fix the one that's wrong without disturbing the one that's right.
- Buzzer polarity: + leg toward the merge point after the button. Reversed = silent even on press.
- Never wire this to mains. Battery-only, every project on this site, no exceptions.
Checked against
Keep building
You might also like
shares ideas with this one