Switches & motion
Switch & push button
Two pieces of metal that touch or don't. Closed, a switch is a wire; open, it is a gap that stops current everywhere in the loop. A push button is only closed while you hold it.
Pins
- Either side
- : A simple on/off switch has no polarity.
- Tactile buttons
- : They have four legs joined in two pairs. Pressing connects one pair to the other; measure to be sure which legs are which.
Switches are named by poles and throws: SPST is one circuit on/off, SPDT picks between two. Small 6 mm tactile buttons are the usual input on circuit boards.
What to look for inside
- A switch in a loop with a lamp. It closes, stays closed, then opens again.
- Closed, metal touches metal and current flows all the way round. Open, the air gap stops it everywhere at once.
- Look at the moment it closes: the contacts hit, spring apart and hit again. That is contact bounce (slowed right down here; in reality it lasts a few milliseconds).
The numbers that matter
| Spec | Typical |
|---|---|
Current rating Above it the contacts arc and wear out. | 50 mA (tactile) to 15 A (mains) |
Voltage rating A switch rated for low voltage can arc across when switching mains. | 12 V to 250 V AC |
Bounce time How long the contacts chatter. Your debounce must wait longer than this. | 1 ms to 10 ms |
Where you’ll use it
- Power switches and mode selectors.
- Buttons read by a GPIO pin with a pull-up resistor.
- Limit switches that tell a machine it has reached the end of its travel.
Common mistakes
- Reading a button with no pull-up or pull-down. The input floats and reads random values while the button is not pressed.
- Forgetting to debounce. One press gets counted as three or four.
Lessons that use it
- Lesson 0.1: Your First Circuit: Voltage, Current & Resistance
- Lesson 4.1: Pull-Up Resistors & Button Debounce Explained
Go deeper
RC debounce: make the time constant longer than the bounce.