Level 1 · Light · project 7 of 11 · 15 min · ●●○○○
Handhold Chain
Three switches in a line — the LED only lights when all three are closed at once. AND, one step past The Vault.
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
- 5× Jumper wires
- 3× Slide switch three "keys" in a row — every one has to be on
- 1× 220 Ω resistor
- 1× Red LED (5 mm) the "everybody agreed" 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 never lights, even with all three slides on. A slide is between the wrong two legs. Middle + one outer leg on each. Then check the LED direction and the 220 Ω.
- LED lights with fewer than three slides closed. Two slides ended up in parallel somewhere. The whole point is series: + → slide 1 → slide 2 → slide 3 → 220 Ω → LED → −.
- LED very dim. Wrong resistor, or two stacked. Only ONE 220 Ω (red-red-brown) in the loop.
- Nothing at all. Battery pack switch first, then LED direction (long leg toward +).
- One slide flips the LED off, another on — random behaviour. The series chain has broken into a parallel mess. Rewire strictly in a straight line, one slide feeding the next.
What you're building
Three slide switches in a row. Behind each one, imagine a different person: one flips a switch on their side, the next flips theirs, the third flips theirs. Only when all three are on does the red LED light up. Any one of them flips off — even for a second — and the light dies instantly.
The Vault proved that two switches in series make an AND gate: the LED only glows when both are closed. This project makes the same point with three, and it's here to answer a question you may not have asked yet: is there a limit?
The big idea: series has no ceiling
Add a switch to your loop and the loop is only complete while that switch is closed. Add a second and both must be closed. Add a third and all three must be closed. There is no rule that stops you at three — you could chain ten, or thirty, or a hundred. As long as every single one is closed, the loop is complete and the LED shines.
That "every one, no exceptions" rule is the AND gate again, just
wider. Engineers write it as A AND B AND C — the LED only
lights when A, B, and C are all true. Later at Level 6 you'll
build the same idea from a chip; at Level 7 you'll write it as
if a and b and c:. The pattern is identical.
Do the math
When all three switches are closed the loop is exactly the same as First Light — one battery, one resistor, one LED, one path. Adding more closed switches doesn't slow the loop down; a closed slide switch is basically a bit of wire (a few milliohms of metal — practically nothing next to the 220 Ω resistor).
- Battery push: 4.5 V
- Red LED eats: 2.0 V
- Voltage left for the resistor: 4.5 − 2.0 = 2.5 V
- Through 220 Ω: I = 2.5 ÷ 220 ≈ 0.0114 A = 11 mA
Same 11 mA as First Light, The Vault, and every other one-loop-one-LED project. If you built a version with ten switches in series and closed every one, the LED would still shine at 11 mA. Series switches only ever break loops — they don't slow them down.
Now open any one switch and every part of the calculation goes straight to zero. There's no such thing as "half a loop": either the circle is complete or it isn't.
Build it
Line up the three slide switches next to each other on the breadboard. Each slide switch has three legs, but you only need two of them for on/off — the middle common leg and one outer leg (either one). Same trick as every switched project so far.
- Power rails: battery red (+) to a top row, battery black (−) to a bottom row.
- Enter switch 1: + rail → switch 1 common leg. Slide the switch so the slider is toward the outer leg you're using.
- Chain 1 → 2: switch 1 outer leg → switch 2 common leg with a short jumper.
- Chain 2 → 3: switch 2 outer leg → switch 3 common leg.
- Exit to the LED lane: switch 3 outer leg → 220 Ω → LED long leg. LED short leg → − rail.
- Flick every switch on: the LED lights. Flick any single one off: the LED dies. Watch how quickly the loop notices — this is not a slow "checking your work" pause; it's the speed of light-through-copper.
The "series has no upper limit" experiment
Once it works with three, add a push button in series somewhere in the chain — say, between switch 3 and the resistor. Now the LED needs four things at once: three slide switches on and somebody's finger on the button. Release the button and it dies; press the button while a slide switch is off and it stays dark. Every added part in series is another vote required. AND is unanimous.
Where wide AND shows up in the real world
- Missile silos in old movies: three officers each turning a key, all at once, before the launch button will work.
- Machine safety interlocks: a factory press won't come down until the door is shut and the emergency stop is unlatched and both operator buttons are held.
- Aircraft cockpits: dozens of "everything is ready" signals chained in series with the engine start.
Every one of them, at heart, is your Handhold Chain — a big series loop that only closes when everyone in the chain agrees.
🧠 Your challenge
No single right answer. That's the point.
- Predict, then measure. With all three switches closed, what's the current through the LED? Now guess the current when only one is closed and the other two are open. Now guess when two are closed and one is open. Write your three guesses down, then check each one on the breadboard. (Two of the three answers are the same — that's the whole point of AND.)
- Vote of the many. Chain a fourth slide switch — or the push button — into the loop. Does the LED get any dimmer with more switches in the chain? (It shouldn't, and the math above explains why. If it does, one of your switches has dirty contacts — try wiggling it and it'll click into place.)
- What OR looks like. Rewire the same three switches so any one of them alone completes the loop (side by side, not end to end). That's the third form of Panic Doorbell — three parallel switches. Which real-world situations want "any one of us can trigger it" versus "everyone has to agree"?
- Half-agreements. Can you rewire it so the LED lights when exactly two of the three switches are on — but not all three, and not one? (Hard. You'd need a switch that's off when pressed, called normally-closed. This is why chips exist.)
For grown-ups: safety notes
- Battery-safe throughout. At 11 mA the whole circuit sips practically nothing; fresh AAs run this for weeks of on-time.
- Slide switches don't get warm and don't wear out at these currents — small slide switches are rated around 0.3 A, and you're using 4% of that.
- If the LED lights when only two switches are on, one of your switches is wired between two legs that are always connected inside — rotate that switch 90° on the breadboard and try again. (Every switched project on this site hits this snag once, then never again.)
- If the LED never lights even with all three switches on, walk the loop with your finger from + rail through each switch, through the resistor, through the LED long leg, out the LED short leg, to the − rail. Whichever step your finger can't trace, that's the break.
- Don't chain switches across rooms with long wires and expect fast timing — long thin wires add real resistance, and thirty metres of doorbell wire in series will start to eat into your 2.5 V margin.
Checked against
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