Level 1 · Light · project 5 of 11 · 15 min · ●●○○○
The Vault
Two buttons, one loop — the LED only lights when BOTH are pressed at the same time.
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
- 4× Jumper wires
- 2× Push button (tactile) two "keys" that both have to be held down
- 1× 220 Ω resistor
- 1× Red LED (5 mm) the "unlocked" light
you'll learn: Circuits are loops · Switches · Series & parallel · 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.
- LED lights with only one button pressed. Your buttons ended up in parallel. Rewire strictly in series: + → button 1 → button 2 → 220 Ω → LED → −.
- Both pressed, LED still dark. Flip the LED (long leg toward +), then check the 220 Ω is present, then that both buttons bridge diagonally (not across a pre-connected pair).
- One button feels like it does nothing. Push buttons bridge DIAGONALLY. If both wires enter contacts on the same side, they were already joined.
- Neither button does anything. Battery pack switch, then both battery leads truly reaching the rails.
- LED glows faintly with only one button. A jumper is bridging the second button. Trace + to − with your finger — the ONLY complete path should be through both buttons.
What you're building
A two-person vault. There's a red "unlocked" LED, and two buttons — one for you and one for a friend. Press just one button: nothing happens. Press both at the same time: click, glow, unlocked.
Movies love this trick — two officers turning two keys at the same time to launch a rocket, two thumbprints for the villain's safe. You're about to build the real electrical version of it. It's also your first proper logic circuit — no chips yet, just wires.
The big idea: switches in series
In First Light your loop had one switch. Add a second switch on the same wire and the loop is only complete when both are on — because either one being open still leaves a gap somewhere in the circle. Engineers call this arrangement series: the current has to pass through each part in a single-file line.
That "both must be on" pattern has a name from the world of logic:
AND. Later at Level 6 you'll build the same idea from chips, and at
Level 7 you'll write it as code (if buttonA and buttonB:). For now
you're doing it with your fingers.
Build it
- Push button A in on the left of the breadboard so it straddles the center groove. Do the same with button B on the right.
- Battery red (+) → button A's top-left leg.
- Button A's diagonal leg (bottom-right) → button B's top-left leg. (Diagonal legs are always on opposite sides of the button's inside switch — same safe trick as in Morse Code Messenger.)
- Button B's diagonal leg → the 220 Ω resistor → LED long leg.
- LED short leg → battery black (−).
- Hold either one alone: dark. Hold both together: light.
Do the math
When both buttons are pressed the loop is exactly the same as First Light — one battery, one resistor, one LED, all in a line. So the current is the same too:
- Battery push: 4.5 V
- LED forward drop: 2.0 V
- Left for the resistor: 4.5 − 2.0 = 2.5 V
- Through 220 Ω: 2.5 ÷ 220 ≈ 0.011 A = 11 mA
Right in the LED's comfy zone. Adding switches to a loop never changes the current — as long as every switch is closed, the loop doesn't know they're there. Switches only ever break loops; they don't slow them down.
Where AND shows up in the real world
- A microwave oven won't heat unless the door is shut AND start is pressed. Both switches in series, exactly your circuit.
- A car horn works whether the key is in or not — that's OR, our next-door neighbour. (See the challenge below.)
- Every combination lock is a giant AND: all wheels must match.
Build the difference between "and" and "or" now, before you meet it in software or in a chip.
🧠 Your challenge
No single right answer. That's the point.
- What if you want the LED to light when either button is pressed (instead of both)? You'll need to rewire the two buttons so each one is a stand-alone gate into the loop. Draw the change before you build it. (This is called OR. Your traffic-light lanes were arranged the same way.)
- Add the slide switch as a "master arm" that has to be on as well as both buttons. That's three switches in series — AND of AND. What real-world thing needs three simultaneous confirmations to happen?
- The vault has one weakness: a determined person can just hold both buttons alone. How could you make it need two different people's fingers — one on each side of the room? (Hint: the doorbell project ran a button on the other side of a door with long wires.)
For grown-ups: safety notes
- Battery-safe voltages throughout — nothing on this circuit can shock you.
- Buttons only bridge the loop while pressed, so this circuit uses no current at all when nobody is playing with it. Safe to leave built.
- If the LED lights when only one button is pressed, one of the buttons is probably wired between two legs that are always connected inside. Rotate that button 90° on the breadboard and try again.
Checked against
Keep building
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