part · Level 1 · Light · input
How a slide switch works
Slide switch — Opens or closes the loop. On means the loop is complete.
What it is
A switch is the simplest part in electronics: a piece of metal that either bridges a gap or does not. Closed, the loop is complete and current flows. Open, the loop is broken and everything stops — no matter how much voltage the battery has.
Your slide switch has three legs. The middle leg is the common: the slider always connects it to one of the two outer legs. Push the knob one way, middle joins the left leg; push it the other way, middle joins the right leg. Engineers call this SPDT — single pole, double throw — one input, two possible outputs.
For an on/off switch you use only two legs: the middle and either outer one. The third leg just sits there. For a "choose A or B" switch you use all three.
How it works
A switch does not use up any voltage or current — it is either a wire (a few milliohms, practically 0 Ω) or an air gap (billions of ohms, practically infinite). That makes the maths trivial and the position flexible: anywhere in the loop breaks the same loop.
Take First Light: battery + → switch → 220 Ω → red LED → battery −. Switch closed: the loop is 220 Ω plus the LED, so current is (4.5 − 2.0) ÷ 220 ≈ 11 mA. Switch open: the resistance is effectively infinite, so 0 mA, and the pack loses nothing while it waits.
The three-leg version becomes useful in the Traffic Light: the common leg on + and one LED on each outer leg means only one lane can be lit at a time — the slider picks. Either way the current through whichever lane is chosen is still 11 mA with its own 220 Ω.
Switches only pass a limited current before their contacts wear — small slide switches are typically rated around 0.3 A at low voltage — but your projects draw 30 mA at most, so that is never a problem here.
The numbers
| Legs | 3 — middle = common; outer = the two positions |
|---|---|
| Type | SPDT — single pole, double throw |
| Resistance closed | ≈ 0 Ω |
| Resistance open | ∞ — no current, no drain |
| Contact rating (typical) | ≈ 0.3 A — far above any project here |
See it working
The First Light circuit, simulated live. Open it on the Workbench to change anything.
simulating…
Mistakes everyone makes
- Using the two outer legs. They are never connected to each other, so the switch is always "off". Fix: middle leg plus one outer leg.
- Battery + on one outer leg and − on the other. Sliding to the middle position is fine, but the common now shorts the pack in one position if it is wired to a rail. Fix: put the common on the side that feeds the circuit, not directly across the battery.
- Legs too far apart for the breadboard. Some switches are 2 mm pitch. Fix: bend the legs gently or use short jumpers from each pin.
- "It only works in one position". That is correct — it is a switch. If you wanted both positions to do something, wire both outer legs.
Questions people ask
- Which pins of a slide switch do I use?
- The middle pin (common) and one of the outer pins. The slider connects the middle to whichever outer pin it is pushed toward.
- Does it matter where the switch goes in the circuit?
- No. A loop broken anywhere is broken everywhere, so the switch can sit before or after the LED with exactly the same result.
- What does SPDT mean?
- Single pole, double throw: one input that can be connected to either of two outputs. Using just one output makes it a plain on/off switch.
Projects that use this part
- 🔦 First Light — Build a real flashlight circuit and learn why every circuit is a loop. L1 · 15 min
- 🎈 Twin Reds — Two red LEDs on one loop — same current, half the leftover push, and a lesson in how voltages stack when parts sit end to end. L1 · 15 min
- 🚦 Tabletop Traffic Light — Three LEDs on three parallel lanes, all lit by one switch — build a tabletop traffic light and discover parallel circuits. L1 · 25 min
- 🚪 Do Not Disturb Sign — A green "chill" LED plus a switched red "leave me alone" LED — two lanes on one battery, and a switch that only touches one of them. L1 · 20 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. L1 · 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. L1 · 15 min
- 🚨 Signal Light — A slide switch and a push button in parallel — the slide is the steady beacon, the button is the extra flash. Either lights the lamp. L1 · 15 min
- 🛟 Escape Hatch — The Vault with a big red emergency button wired around both locks — turn BOTH keys, or hit the button, and the light comes on. L1 · 20 min
- 📻 Dial-a-Buzz — Dial-a-Brightness for sound — one knob rides the buzzer between full alarm and library silence, with no button in the way. L2 · 15 min
- 🚨 Arm & Ring — A doorbell that ignores you unless a hidden slide switch is armed first — series-AND with sound, plus an 'armed' LED that shows the state. L2 · 20 min
- 🔔 Twin Bells — One doorbell button rings two buzzers in parallel — one in your room, one in the kitchen — with a slide switch that mutes just the bedroom bell at night. L2 · 20 min
- 🔕 Silent Alarm — Three panic buttons wired in parallel, all inside an armed lane — any one of them fires the buzzer, and a small LED tells you the alarm is watching. L2 · 25 min
- 🪤 Trip Wire Alarm — A thin wire across a doorway holds the alarm silent. Break the wire and the buzzer screams — until you connect it back. L3 · 25 min
- 🏝️ Vacation Nightlight — The Automatic Night Light with a manual override — flip the slide switch off and the lamp stays dark even in a pitch-black room. L3 · 25 min
- 📚 Study Lamp — The Automatic Night Light with a force-on override — flip the slide switch and the lamp stays lit even in daylight, ignoring the sensor. L3 · 25 min
- 🌀 First Spin — Your first motor. A switch, a coil, and a diode that catches the voltage kick when the current stops. L4 · 15 min
- 🕸️ Trip Wire Fan — Trip Wire Alarm's brain wearing a motor. The wire is intact — the fan is silent. Snap it — the fan spins as the alarm. L4 · 25 min
verified against: SparkFun — Switch Basics · SparkFun — What is a Circuit?