Level 1 · Light · project 8 of 24 · 15 min · ●●○○○
Signal Pair
Two push buttons, two LEDs, one battery — no sharing. Each button lights its own lamp with its own current, independent of the other.
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
- 2× Push button (tactile) two independent "signal" keys — one for each lamp
- 2× 220 Ω resistor one per lane — a parallel load with no safety resistor is a parallel load with no safety resistor
- 1× Red LED (5 mm) the red "stop" lamp — long leg toward +
- 1× Yellow LED (5 mm) the yellow "caution" lamp — long leg toward +
you'll learn: Circuits are loops · Switches · Series & parallel · LED polarity · Ohm's law
Circuit map
The circuit you'll build, running on the Workbench right now — shown with the switches closed.
simulating…
If it doesn't work
The boring ways this circuit actually breaks — check these first before you unwire everything.
- Press the red button — nothing happens. Check the red LED direction first (long leg toward +). Then the red button: its two diagonal legs are the switch; between its two same-side legs is a permanent short. Yellow lane still working proves the battery is fine.
- Press the red button — BOTH lamps light. A stray wire or jumper connects (3,2) to (5,2) across the breadboard — the two LED anodes have become one node. Pull the extra wire; the two columns should be electrically separate above the ground rail.
- Both LEDs light together even with no button pressed. One of the buttons is wired between its two SAME-side legs (always closed instead of open-until-pressed). Rotate the button 90° on the breadboard and try again.
- One lamp is dimmer than the other. You have different resistors in the two lanes, or one LED is partially flipped. Both lanes should carry about 11 mA and look the same brightness.
- Lamp lights for a moment then goes out. Loose jumper somewhere in that lane — probably the ground return. Press firmly on every wire and try again.
- Both buttons warm up during a long press. You skipped the 220 Ωs. Without the resistors, an LED pulls close to a quarter-amp through the button contact. Put the resistors back before trying again.
What you're building
Two push buttons at the top of the breadboard. Red lamp in the middle, yellow lamp next to it. Press the red button: red lamp lights, yellow stays dark. Press the yellow one: yellow lamp lights, red stays dark. Press both: both light at full brightness, no change in either.
A railway-crossing signal, a hand-held "stop/caution" paddle, a two-channel beacon — any time you want two indicator lamps that run on the same battery but behave independently, this is the circuit.
The big idea: parallel lanes don't share
Signal Light mixed a slide and a button in parallel so either one could light the same LED. Signal Pair keeps the parallel idea but gives each switch its own LED and its own 220 Ω. That one change makes the two lamps independent: lane A can be on while lane B is off and vice versa.
Watch the battery, though. The lanes don't share wires, but they share the source. The battery sees whatever currents the lanes draw, added up:
- No buttons pressed: 0 mA
- Red pressed only: ~11 mA
- Yellow pressed only: ~11 mA
- Both pressed: ~22 mA
If you kept adding independent lanes — ten different-colour signals on ten different buttons — the battery would hand each one its own 11 mA, no questions asked, right up until the AA pack ran out of push. Battery push is shared; the lamps aren't.
Do the math
Both lanes are identical to First Light. The battery sits at 4.5 V. The LED eats about 2.0 V of that. The resistor burns what's left:
- Voltage left for the 220 Ω: 4.5 − 2.0 = 2.5 V
- Current through the lane: I = V ÷ R = 2.5 ÷ 220 ≈ 0.011 A = 11 mA
Press button A: this is exactly what flows through lane A. Press button B: exactly what flows through lane B. Press both: each lane still does its own 11 mA because the buttons, resistors and LEDs on this side have no idea what parts are doing on the other side. Only the battery notices — it has to supply the sum.
Why "independence" is more than a trick
Real electronics is full of lanes that look parallel but aren't independent. Hook two LEDs straight across one 220 Ω — the brighter one hogs the current and the dim one goes dark (that's why Do Not Disturb uses its own resistor for each lane too). Share one switch between two lamps and now the switch controls both — that's Traffic Light.
The move that keeps lanes independent is: give each lane its own switch AND its own current-limiting resistor. Then they're bolted to the same battery but their behaviour is their own.
Memorize that pair of rules. From Level 2's twin-buzzer alarms through Level 4's two-wheel robots, projects keep repeating it — one switch, one resistor, one output, one lane. Side by side as many times as you need.
Build it
- Power rails. Battery + to a top row, battery − to a bottom row.
- Lane A (red). From + rail: one leg of button A → diagonal leg of the button → the top leg of a 220 Ω → short leg of the resistor to row below → red LED long leg at that row, short leg one more row down → wire to − rail.
- Lane B (yellow). Repeat the same four parts in the next column over: button B → 220 Ω → yellow LED → − rail. Lane B's parts never share a hole with lane A's parts.
- Press red. Red lamp lights at full First-Light brightness; yellow stays dark.
- Press yellow. Yellow lamp lights; red stays dark.
- Press both at once. Both at full brightness. Nothing dims because nothing is shared.
Where you'll see this pattern
- Game-show buzzers: each team gets its own button and its own light. Quiz Buzzer (Level 2) adds a buzzer to each lane; this is its L1 ancestor.
- Dashboard warning lamps: each sensor on a car (fuel, temperature, door-ajar) gets its own switch and its own bulb — a parallel bank of independent lanes.
- Audio mixer channel indicators: every input gets its own level LED. Twelve channels is twelve copies of this circuit.
The topology scales. The physics doesn't change.
🧠 Your challenge
No single right answer. That's the point.
- Make a third lane. Add a slide switch, a 220 Ω, and a green LED alongside the red and yellow lanes — a Traffic Light-style trio where each colour has its own control. How does the battery total change when all three are on at once? (Answer: 3 × 11 ≈ 33 mA — exactly Traffic Light's current, since the circuits are the same once all switches are closed.)
- Add a shared slide "power" switch. Put a slide switch between the battery + and both lanes (one switch in front of the whole thing). Now the two push buttons only work when the slide is on — a safety master switch. Which real device uses this layout? (Camera flashes, synthesizer effects pedals, any "arm then fire" interface.)
- Series the two buttons instead of parallel. Instead of two independent lanes, chain both buttons in a row driving ONE LED. Now both buttons must be pressed for the LED to light — you've rebuilt The Vault. Compare the feeling: Signal Pair is "either", The Vault is "both".
- Try a 1 kΩ in lane A, keep 220 Ω in lane B. Predict first: red lamp is now dimmer (~2.5 mA) and yellow is still bright (~11 mA). Prove that pressing the yellow button has no effect on the red lamp. (Still independent — different brightnesses, no interference.)
- Shared resistor, two LEDs. Replace the two 220 Ωs with ONE 220 Ω that both lamps share — all three parts (button A to R220, button B to R220, LEDs to the same R output). Now press both buttons at once. What happens? (The LEDs end up in parallel across the resistor. Each one carries about 5 mA — half of First Light. Not the same as independent 11-mA lanes. This is why the "two LEDs on one resistor" shortcut is tempting and wrong.)
For grown-ups: safety notes
- Battery-safe. 4.5 V and up to 22 mA when both buttons are pressed won't shock you, and nothing warms noticeably.
- Two LEDs, two polarities, two chances to flip one. Long leg on BOTH LEDs points toward + (the top rail). Flat side points toward − (the bottom rail). If a lamp doesn't light when you press its button, the first thing to check is the LED direction. (A flipped LED breaks only its own lane here — the other lane keeps working, which is a nice debug clue.)
- Each lane needs its own 220 Ω. The temptation when you have two LEDs is "they're both small, one resistor is enough" — it is NOT. Two LEDs in parallel share whatever current the single resistor allows, and one almost always ends up hogging and burning out. The resistors are safety parts, one per lane.
- Push buttons are rated for about 50 mA at the voltages you're using here — a 11 mA signal lane is way under that, so they'll feel cool through long presses. If a button feels warm, something is wrong in that lane (short, flipped LED, missing resistor); release and check.
- Small parts stay away from pets and toddlers. Buttons, LEDs and resistor leads are all the same swallow-hazard shape.
- Never wire this to mains. The pack is the whole supply.
Checked against
Keep building
You might also like
shares ideas with this one