Light-curtain safety interlock

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, LED
from time import sleep

curtain = Button(17)                           # beam broken = pressed
estop = Button(5)                              # latched when pressed
start = Button(27)
motor = OutputDevice(22, active_high=False)
green = LED(23)
red = LED(24)

running = False
tripped = False

while True:
    safe = not curtain.is_pressed and not estop.is_pressed
    if not safe:
        if running or not tripped:
            print("SAFETY STOP: curtain broken or emergency stop pressed")
        running = False
        tripped = True
    elif start.is_pressed:
        if tripped:
            print("Safety circuit is clear again. Motor restarted by Start")
        tripped = False
        if not running:
            print("Motor RUNNING")
        running = True
    motor.value = running
    green.value = running
    red.value = tripped
    sleep(0.02)

How the code works

• safe is true only when the beam is clear and the e-stop is released.
• Any unsafe moment sets running to False and tripped to True. The motor stays off even if the beam clears again.
• Only a press on Start restarts the motor, and only while it is safe.

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.