Build yours at home · Build 008
One loop switches another
What we're doing
Use one battery and a button to work a switch you can't touch. Press the button: a little magnet inside the relay clicks a switch shut, and a second battery lights the LED.
About 30 minutes.
Goes with these lessons
No kit? Try this
Listen for clicks. Put a parked car's hazard lights on and listen: in many cars that click is a relay (newer cars play the sound through a speaker). Listen for clicks when an old toaster, fridge or washing machine starts.
Get these parts
New to breadboards? See Getting set up- 4×AA battery holder with a switch, for the button loop (electronics shop or online)
- 2×AA battery holder with a switch, for the light loop (electronics shop or online)
- The kit's 5 V relay, one push button, one LED, one 100 Ω resistor, the breadboard, jumper wires and four female-to-male 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.
Two of the relay's pins (COM and NC) are joined inside. Never connect a battery straight across them.
Steps
- 1
Find the relay's pins. Its top has a small drawing: a coil, and a switch with three pins called COM, NO and NC. Match the drawing to the five pins underneath. The kit tutorial's relay lesson has a picture.
- 2
Push a female-to-male wire onto each coil pin, onto COM and onto NO. Leave NC empty. The relay doesn't fit the breadboard, so these wires reach it.
- 3
Button loop. With both holders off, push the 4×AA's red wire into the + line along the bottom edge and its black wire into the − line beside it. Put the button across the middle gap. Add a jumper from that + line to one button leg's row. Push one coil wire into the row of the button leg next to it, on the same side. Push the other coil wire into the bottom − line.
- 4
Switch on the 4×AA holder and press the button. Click! The coil became a magnet and pulled the switch over. Let go: it clicks back.
Not working? Clicks without pressing? Turn the button a quarter turn.
- 5
Light loop. Switch off. Push the 2×AA's red wire into the + line along the top edge and its black wire into the − line beside it. Put the 100 Ω resistor from the top + line into an empty row. Push the LED's long leg into that row and its short leg into another empty row. Push the COM wire into the LED's short-leg row. Push the NO wire into the top − line.
- 6
Switch both holders on. Press the button: click, and the light comes on. The button's battery never touches the light.
Not working? Light on before you press? That wire is on NC, not NO. Move it. No light at all? Turn the LED round.
- 7
Switch off the 2×AA holder and press again. It still clicks, but the light stays dark. The light needs its own battery.
- 8
Switch both holders off and take the parts out.
Talk while you build
- Which battery makes the relay click?
- Can it click when the light's battery is unplugged?
Spot it at home
- A car's hazard lights, the click when an old fridge or washing machine starts.
What clicks at home when something turns on?
- A car's start button: a small switch tells a relay to send big power to the starter motor. A lift button: a small press, a big motor.
- A washing machine or dishwasher: its small control board uses relays to switch the big pump and heater on and off.
- Smart plugs and timer switches: a tiny signal works a relay inside to turn a lamp on.
What small thing tells something big to turn on?
More for grown-ups · printed parts
Optional equipment
Prints that make the build easier. None are required. Every build works with tape and clips.
Instructables
The sounder is a relay we can watch working: a coil, a lever, a click.
Not printed yet
Printables
Two separate loops are easier to see when each part is its own brick.
Careful: Ball magnets are dangerous if swallowed. Grown-up assembly only.
Not printed yet
Grown-up references
Make: Electronics — Learning by Discovery · Charles Platt Moves from ordinary switches to a relay, where one circuit controls another.