Machine stack light with start, stop and e-stop

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, LED, Buzzer
from time import sleep

start = Button(27)
stop = Button(5)
estop = Button(6)            # latched in = emergency (click it to latch or release)
red = LED(22)
yellow = LED(23)
green = LED(24)
horn = Buzzer(18)

state = None

def set_state(new):
    global state
    if new == state:
        return
    state = new
    print("State:", state)
    red.off()
    yellow.off()
    green.off()
    horn.off()
    if state == "READY":
        yellow.on()
    elif state == "RUNNING":
        green.on()
    elif state == "EMERGENCY":
        red.blink(0.4, 0.4)
        horn.beep(0.4, 0.4)

set_state("READY")
while True:
    if estop.is_pressed:
        set_state("EMERGENCY")
    elif state == "EMERGENCY":
        set_state("READY")            # e-stop released: a new Start is needed
    elif state == "READY" and start.is_pressed:
        set_state("RUNNING")
    elif state == "RUNNING" and stop.is_pressed:
        set_state("READY")
    sleep(0.05)

How the code works

• The program always has exactly one state. set_state() changes it, switches all lights off, then turns on only what that state needs.
• The e-stop is checked first in every loop, so it beats Start and Stop at any time.
• After the e-stop is released the machine goes to READY, not RUNNING. The operator must press Start again. This is a safety rule on real machines.
• The "if new == state: return" line stops the lights from being reset on every loop.

Parts to buy

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