Level 1 · Light · project 6 of 24 · 25 min · ●●○○○
Tabletop Traffic Light
Three LEDs on three parallel lanes, all lit by one switch — build a tabletop traffic light and discover parallel circuits.
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× Red LED (5 mm)
- 1× Yellow LED (5 mm)
- 1× Green LED (5 mm)
- 3× 220 Ω resistor one per LED — every lane needs its own speed bump
- 1× Slide switch
you'll learn: Circuits are loops · LED polarity · Series & parallel
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.
- One LED dark, others fine. Flip that LED — long leg toward +. Every lane also needs its own 220 Ω; three LEDs, three resistors, no shortcuts.
- All three LEDs dark. Battery pack switch first, then the slide switch's middle leg + one outer leg wiring.
- Two LEDs share one resistor and both look dim. Give each LED its own 220 Ω. Combining resistors starves the lanes — that's the whole point of parallel.
- Green looks dimmer than red. Normal at 220 Ω. Green needs ~2.1 V, red ~2.0 V, so green draws a hair less current. If you want them matched, drop green's resistor to a lower value later.
- Slide switch does nothing. Middle leg + one outer leg. The other outer leg is disconnected. Slide connects middle-to-outer.
What you're building
A traffic light: red on top, yellow in the middle, green at the bottom, all glowing at once — with one master switch. Yes, all at once: a light that changes by itself needs a chip that counts, and that's Level 6. Here the prize is the idea that makes big circuits possible: parallel paths.
The big idea: parallel lanes
In First Light your loop was one road. Here, the road splits into three lanes — one per LED — and joins back together before returning to the battery. Paths that split like this are called parallel. Each resistor-plus-LED lane gets the same full battery voltage across it (inside the lane, the resistor and the LED share that voltage), and each lane decides its own current.
That's why each LED gets its own 220 Ω resistor. One shared resistor would make the LEDs squabble over current (and because each color has a slightly different forward voltage, the reds usually win and hog it all). Each lane needs its own resistor, so each LED glows properly.
Build it
- Place the three LEDs in a vertical line on the breadboard: red, yellow, green — with a gap between each. Long legs all facing the same side.
- Give each LED its own 220 Ω resistor connected to its long leg.
- Wire the free ends of all three resistors together into one breadboard row. That row is where the road splits — call it the junction.
- Connect the junction to one outer leg of the slide switch, and the switch's middle leg to the battery's red (+) wire.
- Wire all three LED short legs to another shared row, and connect that row to the battery's black (−) wire.
- Flick the switch: all three lanes light at once.
Check it with numbers
Each lane behaves like its own First Light circuit: about 2.5 V ÷ 220 Ω ≈ 11 mA per lane. Three lanes side by side means the battery supplies roughly 3 × 11 = 33 mA in total — parallel lanes add up current. AA batteries handle that easily, but keep this rule in your pocket: it's the reason a real house doesn't dim when you plug in a second lamp, and the reason too many things on one plug is a problem.
🧠 Your challenge
No single right answer. That's the point.
- Real traffic lights never show all colors at once. Using only what's on the board, how can you make just one color light at a time? (There's more than one answer — moving a wire counts as a solution. So does thinking of a switch as a lane-picker.)
- Go further: put a switch in every lane (the push button counts as one) and become the traffic controller. Run the real sequence by hand — red, green, yellow, red — and time how long you'd give each. What would it take for the circuit to do that without you?
- What happens if two LEDs share one resistor lane? Try red and green together on one 220 Ω. Which one wins, and why might their different forward voltages explain it?
- Add the push button as a pedestrian crossing: green stays on, but pressing the button lights yellow. Sketch the loop before you build it.
For grown-ups: safety notes
- All battery-safe voltages — nothing here can shock you.
- Three lanes normally draw about three times one LED's current — around 33 mA. A short circuit is a different animal: it bypasses the resistors entirely and can pull far more than that, limited only by the battery and the wires, which get hot fast. Double-check the junction row before switching on.
- If any LED stays dark, switch off before rewiring. Habit now, instinct later.
Checked against
Keep building
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