Level 1 · Light · project 6 of 11 · 20 min · ●●○○○
Do Not Disturb Sign
A green "chill" LED plus a switched red "leave me alone" LED — two lanes on one battery, and a switch that only touches one of them.
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)
- 1× Solderless breadboard
- 6× Jumper wires
- 1× Green LED (5 mm) the always-on "chill" lane — long leg toward plus
- 1× Red LED (5 mm) the switched "do not disturb" lane — long leg toward plus
- 2× 220 Ω resistor one per lane — every LED needs its own speed bump
- 1× Slide switch lives in the RED lane only — the green lane never sees it
you'll learn: Circuits are loops · Series & parallel · Switches · 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.
- Green LED off with red on, or vice versa. Flip whichever is dark. Each LED's long leg goes to + of its own lane.
- Slide switch turns off BOTH LEDs. The switch lives ONLY in the red lane. If it's in the shared + rail, both lanes run through it — move it back into just the red lane.
- Green LED never comes on. Green LED wired backward, or its 220 Ω is missing. Every LED lane needs its own speed bump.
- Both LEDs dark. Battery pack switch off, or the shared + rail isn't reaching either 220 Ω. Trace from the battery + wire all the way to both LEDs.
- Red LED lights even with the slide off. A jumper is bypassing the slide. Only the slide should feed the red lane's 220 Ω.
What you're building
Tape this to your bedroom door. The green LED is always on whenever the battery is plugged in — a friendly "yes, I'm home, all good." Flip the slide switch and the red LED lights up alongside it: do not disturb. Flip it back and the red goes off — but the green never flinches, because the switch doesn't live in its lane.
That "switch in one lane only" idea is what makes this project worth building. In First Light the switch controlled the whole circuit. In Tabletop Traffic Light one switch controlled all three lanes at once. Here, one switch controls exactly one lane and leaves the other alone.
The big idea: lane independence
When two lanes are in parallel, they share the same + rail and the same − rail — but nothing else. So whatever happens in one lane (an open switch, a popped LED, a resistor swap) doesn't reach the other lane. Each lane obeys Ohm's law on its own.
Look at the loop for each lane:
- Green lane:
+ rail → 220 Ω → green LED → − rail. A clean loop with no switch in it. As long as the battery is plugged in and the loop is unbroken, current flows. - Red lane:
+ rail → SWITCH → 220 Ω → red LED → − rail. Same loop, plus a gap you control. Slide switch on → gap closed → red glows. Slide switch off → gap open → red dark.
The green lane doesn't know the switch exists. It's not in the same loop.
That's exactly how the front of a laptop works: the little green "power" LED lights up the moment the battery has charge, and the white "Caps Lock" LED only lights when you press the key. Two LEDs, same battery, different switches (or in Caps Lock's case, a signal from the keyboard).
Do the math
Each lane is a mini First Light. Same battery push, same rules.
- Green lane current: the green LED "eats" about 2.1 V, so the resistor drops 4.5 − 2.1 = 2.4 V. Ohm's law: I_green = 2.4 ÷ 220 ≈ 11 mA. Right in the LED's happy zone.
- Red lane current (when the switch is closed): red LED eats 2.0 V, resistor drops 4.5 − 2.0 = 2.5 V. I_red = 2.5 ÷ 220 ≈ 11 mA. Same story, tiny difference from the green because the LED colors have slightly different forward voltages.
- Battery current — switch open: only the green lane is closed, so the battery supplies ≈ 11 mA. Fresh AAs run this for many weeks.
- Battery current — switch closed: both lanes are closed, so the battery supplies 11 + 11 ≈ 22 mA. Parallel lanes add up their currents — the same rule as Tabletop Traffic Light, just with two lanes instead of three.
Build it
- Power rails: battery red (+) to a top row on the breadboard, battery black (−) to a bottom row.
- The green (chill) lane — no switch, always on: + rail → 220 Ω → green LED long leg. Green LED short leg → − rail. Plug in the battery: green LED lights immediately. There's no switch to wait for.
- The red (do-not-disturb) lane — has the switch: + rail → slide switch → 220 Ω → red LED long leg. Red LED short leg → − rail. Slide the switch on: red joins the green.
- Test the independence: slide the switch off. The green LED should still be lit. If it isn't, the switch is somewhere in the loop it shouldn't be — check that the + rail feeds both lanes and the switch only sits in the red lane.
The order of parts within a lane doesn't matter electrically — the resistor could sit after the LED instead of before, and it would still limit current the same way. But putting the resistor before the LED is the habit engineers form because it makes the diagram easy to read left-to-right: push, speed-bump, load.
The bug that isn't a bug
Notice that when the switch is open, the red LED goes dark but the green LED doesn't get any brighter. A beginner sometimes expects "less stuff to power → more current for the rest." That would be true if the two lanes were in series (single-file). In parallel, each lane pulls its own current based on its own resistor, and one lane going dark just means the battery supplies less total current — it doesn't hand any extra to the other lane.
Save that intuition. It's the same reason plugging one lamp out of a wall outlet doesn't make the other lamps brighter. All household loads are in parallel, and each one draws what it needs based on its own resistance.
Where independent lanes lead
Almost every real gadget has this pattern. Your phone charger has a tiny orange LED that lights the moment it's plugged in — always on, no switch. The main phone circuit is a separate lane that only runs when the phone is connected. Same idea, larger scale.
At Level 2 you'll swap the red LED for a buzzer and the slide switch for a push-button, and you'll have built a doorbell with the same circuit shape.
🧠 Your challenge
No single right answer. That's the point.
- Which lane wins the swap? Move the slide switch out of the red lane and into the green lane instead. Now which LED is "always on" and which one you control? (This is the whole project running backwards. It also proves that lane independence works both ways.)
- Swap and predict. Change the green lane's 220 Ω resistor for the 1 kΩ one. Predict before you build: does the red LED change at all? Does the green LED get dimmer, brighter, or stay the same? Now measure the two lane currents in your head using Ohm's law. (The red lane shouldn't budge — this is proof of lane independence.)
- Add a "quiet time" third lane. Add a yellow LED with its own 220 Ω and its own push-button in parallel with the other two — a "come in gently" indicator. Now you have three independent lanes with three different switch structures: always-on, slide-on, press-and-hold. Which lane costs the most battery current when everything's active? (Add the three lane currents.)
- Break independence on purpose. Rewire it so the slide switch now sits before the split into the two lanes — up on the shared + rail, like the switch in Tabletop Traffic Light. Now what does it control? (A single "master arm" switch that cuts both LEDs at once. When would that be useful? When would the independent-lanes version be better?)
For grown-ups: safety notes
- Battery-safe throughout — even both lanes on together (~22 mA) is easy for fresh AAs, and 4.5 V won't shock you.
- If a lane refuses to light: switch off first, then check the LED polarity (long leg toward +). It's the single most common build mistake and it's harmless to flip — the LED just doesn't glow when it's in backwards.
- Never wire an LED straight to the battery without its 220 Ω. A bare LED across 4.5 V pulls hundreds of milliamps for a heartbeat and pops. Not dangerous to you, but wasteful.
- If both LEDs stay dark whenever the battery is plugged in, check that the + and − rails actually reach both lanes. A missing jumper on one rail is the second most common build mistake.
- You can leave the always-on green LED lit for hours safely — none of the parts warm up at 11 mA. But there's no reason to leave it running when you're not using the sign; the batteries are yours, spend them thoughtfully.
Checked against
Keep building
You might also like
shares ideas with this one