Build yours at home · Build 009
A switch with no moving parts
What we're doing
Build a switch with no moving parts. A tiny push of electricity from the button lets the transistor connect the battery to the light. No click.
About 30 minutes.
Goes with these lessons
No kit? Try this
Touch a touch lamp, or a phone screen. A tiny touch signal switches something bigger, with no click and nothing moving.
Get these parts
New to breadboards? See Getting set up- 4×AA battery holder with a switch (electronics shop or online)
- One NPN transistor (PN2222 or S8050), one push button, one LED, one 220 Ω resistor, one 1k resistor, the breadboard and jumper wires (Elegoo kit)
Fill each holder with the matching number of AA batteries (household).
Keep it safe
Battery power only, never household power. Use AA packs matched to the parts, not a 9V battery. A grown-up connects and disconnects the pack. Stop immediately if any part warms. Never join the pack's two metal ends: the wires can get hot. Coin cells stay with grown-ups, locked away. CPSC warns a swallowed button or coin cell can burn through a child's throat in about two hours; suspected swallowing needs emergency medical care immediately.
A grown-up checks the transistor's legs against the kit tutorial before switching on.
Steps
- 1
Find the transistor's legs. Hold it flat side toward you, legs down. On most kit transistors they are, left to right, E (out), B (signal) and C (in). Check the picture in the kit tutorial's transistor lesson.
- 2
With the switch off, push the battery's red wire into the + line and its black wire into the − line. Push the transistor's three legs into three rows next to each other.
- 3
Light path. Put the 220 Ω resistor from the + line into an empty row. Push the LED's long leg into that row and its short leg into the C row. Add a jumper from the E row to the − line.
- 4
Signal path. Put the button across the middle gap. Add a jumper from the + line to one button leg's row. Put the 1k resistor from the row of the button leg next to it, on the same side, into the B row.
- 5
Switch on and press the button. The light comes on, with no click. Let go: it goes off.
Not working? Light on without pressing? Turn the button a quarter turn. No light at all? Turn the LED round, then check the transistor's legs.
- 6
Hold the button down and pull the 1k resistor out. The battery is still connected, but the light goes off. The signal works the switch.
- 7
Switch off and take the parts out.
Talk while you build
- What happens when you press the signal button?
- Did anything click or move when the light switched on?
Spot it at home
- Phones, tablets and game controllers are full of transistors you can't see. A touch lamp switches on with a tiny touch and no click.
Why might a robot want a switch that never clicks or wears out?
- Streetlights and porch lights that come on by themselves at night: a light sensor sends the signal.
- Phones, tablets, TVs and game controllers: each is full of tiny transistors switching on and off very fast.
- A touch lamp that turns on with one light touch and nothing moving.
What could tell this switch to turn on, instead of a button?
Grown-up references
Electronics for Kids · Øyvind Nydal Dahl Projects that show how a transistor controls current.