tinyΩhms

Level 1 · Light · 25 min · ●●○○○

Tabletop Traffic Light

Three LEDs, three parallel lanes — build a traffic light and discover parallel circuits.

What you need

  • 1× 3×AA battery pack (4.5 V) L1
  • 1× Solderless breadboard L1
  • 6× Jumper wires L1
  • 1× Red LED (5 mm) L1
  • 1× Yellow LED (5 mm) L1
  • 1× Green LED (5 mm) L1
  • 3× 220 Ω resistor one per LED — every lane needs its own speed bump L1
  • 1× Slide switch L1

you'll learn: Circuits are loops · LED polarity · Series & parallel

4.5 V + switch 220 Ω ×3 each lane: own resistor, own current (~11 mA)
parallel lanes share the voltage, add up the current: 3 × 11 ≈ 33 mA total

🔧 Open in Workbench — test this exact circuit virtually, then rewire it however you like.

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. Along the way you'll meet 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 lane gets the full battery 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). Own lane, own speed bump — everyone glows properly.

Build it

  1. 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.
  2. Give each LED its own 220 Ω resistor connected to its long leg.
  3. Wire the free ends of all three resistors together into one breadboard row. That row is where the road splits — call it the junction.
  4. Connect the junction to one outer leg of the slide switch, and the switch's middle leg to the battery's red (+) wire.
  5. Wire all three LED short legs to another shared row, and connect that row to the battery's black (−) wire.
  6. 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.)
  • 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 pull three times the current, so a short circuit gets hot three times faster. Double-check the junction row before switching on.
  • If any LED stays dark, switch off before rewiring. Habit now, instinct later.

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