Wall switch with a relay

Parts in this circuit

Wiring, pin by pin

Red = 5 V, orange = 3.3 V, black = GND, other colours = signals. “Pin” is the position on the 40-pin header; the GPIO number is what the code uses. Power the Pi from a 5.1 V supply and wire with the power off.

Python code (gpiozero)

from gpiozero import Button, OutputDevice
from signal import pause

# Most relay boards switch ON when the pin goes LOW.
# active_high=False hides this: relay.on() really switches the lamp on.
relay = OutputDevice(17, active_high=False)
button = Button(27)

def flip():
    relay.toggle()
    print("Lamp ON" if relay.value else "Lamp OFF")

button.when_pressed = flip
print("Press the wall button to switch the lamp")
pause()

How the code works

• A GPIO pin gives only a few milliamps. A relay board uses a small transistor and a coil, so a tiny signal can switch a big load.
• The lamp is wired through the relay contacts, not through the Pi. The Pi only sends the signal on the IN pin.
• Many cheap relay boards are active low: the relay clicks ON when the pin is LOW. active_high=False tells gpiozero about this, so your code can still say on() and off().
• when_pressed calls flip() each time the button goes down, and toggle() reverses the relay.

Parts to buy

The links open a search on each store, so you see what is in stock today. Check the model, voltage and price before you order.