Level 1 · Light · project 8 of 11 · 15 min · ●●○○○
Two-Door Signal
Two slide switches in parallel, one shared LED — either switch alone lights the lamp. The OR gate, at Level 1.
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 two long ones let one switch live in a different doorway
- 2× Slide switch the front-door switch and the back-door switch — either one alone lights the lamp
- 1× 220 Ω resistor
- 1× Red LED (5 mm) the shared "someone left a door open" lamp
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 only when both slides are on. The slides are in series. Rewire in parallel: each slide's output should join the SAME rail feeding the 220 Ω and LED.
- One slide works, the other doesn't. Middle leg + one outer leg on each slide. The other outer leg is disconnected.
- LED never lights. Flip the LED (long leg toward +), then confirm the 220 Ω is in the lane, then the battery pack switch.
- LED lights even with both slides off. A jumper is bypassing the switches. Both slides should be the ONLY paths from + to the 220 Ω.
- LED dim. Wrong resistor. 220 Ω = red-red-brown.
What you're building
Two slide switches, one at each door, sharing one red lamp on the kitchen wall. Flip either switch on — the lamp lights. Flip the other one on as well — the lamp stays lit, at the exact same brightness. Flip both off — the lamp goes dark. Only when no switch is closed anywhere does the loop finally break.
The Vault proved that two switches in series make an AND gate: both switches have to agree. This project builds the mirror image — two switches in parallel make an OR gate: either one is enough.
Fridges do this. Cars do this too: any door → dome light on. Every "any one of these switches can trigger it" system in the world is this pattern.
The big idea: parallel switches = OR
In First Light there was one switch. To break the loop you opened that one switch — no choice about which one.
Put two switches in parallel and you get a second path. The loop now has two routes from + to the resistor: through switch A, or through switch B. Electricity is lazy — it flows through any available path. So the loop is complete as long as at least one switch is closed.
- Only switch A closed → path exists (through A) → LED on.
- Only switch B closed → path exists (through B) → LED on.
- Both closed → two paths exist → LED on.
- Both open → no path anywhere → LED off.
Engineers write that as A OR B — the LED lights when A, B, or
both are true. Level 6 will build the same idea from a chip; Level 7
writes it as if a or b:. Same pattern in every layer.
Do the math
When at least one switch is closed, the loop is exactly First Light — one battery, one resistor, one LED, one open path. Extra closed switches in parallel don't hurt: they just make an even lower-resistance route to the same place.
- 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 whether one switch or both are closed. A closed slide switch is basically a bit of metal — a few milliohms. Whether you add one such wire in parallel or two, the resistor and the LED still see the same 4.5 V push. Two paths carry roughly half the current each, but the total delivered to the LED is the same 11 mA.
That's the "fixed load" rule: a lamp designed to sip 11 mA sips 11 mA regardless of how many redundant paths deliver it.
Build it
Line up the two slide switches side by side on the breadboard. Each slide switch has three legs; use the middle common leg and one outer leg — same as every switched project so far.
- Power rails: battery red (+) to a top row, battery black (−) to a bottom row.
- Fork the plus rail into two branches: run a short jumper from the + rail to switch A's common leg, and another jumper from the + rail to switch B's common leg. Both switches now see + on their entry side.
- Merge the switch outputs onto one row: switch A's outer leg and switch B's outer leg both land on the same row on the breadboard — that shared row is the "someone closed a switch" node.
- The LED lane: from that merge row → 220 Ω → LED long leg → LED short leg → − rail.
- Slide one switch on: the LED lights. Slide the other on too: nothing changes — same brightness. Slide both off: dark.
If you have two long jumpers, run one switch across your desk to a different corner of the room and pretend it's a second door. Flip that far switch — the lamp lights across the room.
Wide OR: no upper limit
Add a third slide switch in parallel with the other two. Same pattern — a jumper from + to switch C's common leg, and switch C's outer leg to the merge row. Now any of the three lights the lamp. Three doors, one shared lamp.
The rule holds: adding a fourth, or a fifth, or a fiftieth doesn't change the LED brightness. Every added parallel path is another "any of us can do it" vote — still 11 mA when at least one closes.
OR vs AND — same parts, different wiring
The three-parts side-by-side view worth staring at:
| Circuit | Wiring | LED lights when… |
|---|---|---|
| The Vault (AND) | switches in series | both switches are closed |
| Two-Door Signal (OR) | switches in parallel | at least one switch is closed |
Same two parts, same LED, same resistor. The layout picks the logic. Once you can look at any switched circuit and instantly tell "series = AND" from "parallel = OR", you can read most doorbell, alarm, and safety-interlock diagrams on Earth.
🧠 Your challenge
No single right answer. That's the point.
- Predict, then measure. With switch A closed only, what's the LED current? With both closed? With both open? Write down all three guesses, then check them on the breadboard. (Two of the three answers are the same — that's the whole point of OR.)
- Three-input OR. Add the push button as a third parallel path (short jumper from + to one button leg, other leg to the merge row). Now three inputs, still one lamp. Any of the three triggers it. Is the LED any brighter than with just one? (No. The load decides current, not the number of paths.)
- What the truth table says. Fill in the OR truth table (A, B, LED on?) — all four rows. Then fill in the AND truth table for The Vault next to it. Only one row differs between OR and AND. Which one, and why?
- Two lamps, one switch each — the opposite plan. Rewire the same parts so each switch controls its own LED (two LEDs, two resistors, two switches, one battery). That's Do Not Disturb Sign again. Compare: two lamps + two switches (independent) vs one lamp + two switches (OR). Which one would a car dome light use? Which one would a "front-door + back-door + hallway all light the same porch bulb" system use?
- Bigger LED trick. Swap the red LED for a green one. Does anything about the OR behavior change? (No. The colors matter for the numbers — green eats 2.1 V instead of 2.0 V, so it draws 10.9 mA instead of 11.4 mA — but the OR pattern is the same. Logic is independent of the load.)
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, whether one switch is closed or both.
- Slide switches don't heat up at these currents — small slide switches are rated around 0.3 A, and you're using under 4% of that.
- If the LED lights when no switch is closed, one of the switches is wired between two legs that are always connected inside — rotate the offending switch 90° on the breadboard and try again. Common-plus-outer is the right pattern; outer-plus-outer is the trap. Every switched project on this site hits this snag once, then never again.
- If the LED never lights even with both switches on, walk the loop with your finger from + rail through one switch, across the merge row, through the resistor, through the LED long leg, out the short leg, back to − rail. Whichever step your finger can't trace, that's the break.
- Long parallel wires across a room add a little real resistance, but well under 1 Ω even at ten metres — nothing near the 220 Ω in the lane. So the far-door version behaves the same as the side-by-side version.
Checked against
Keep building
You might also like
shares ideas with this one