Motor start/stop with a seal-in circuit

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 time import sleep

start = Button(27)                       # momentary, normally open
stop = Button(5)                         # real stop buttons are normally closed
overload = Button(6)                     # click it to latch the overload trip
motor = OutputDevice(17, active_high=False)   # contactor coil

# The ladder rung, written in Python:
#   run = (START or MOTOR) and not STOP and not OVERLOAD
# "or MOTOR" is the seal-in: once running, the motor holds itself on.
while True:
    run = (start.is_pressed or motor.value) and not stop.is_pressed and not overload.is_pressed
    if run != bool(motor.value):
        motor.value = run
        print("Motor RUNNING" if run else "Motor STOPPED")
    sleep(0.02)

How the code works

• Press and release Start. The motor keeps running because the MOTOR contact in parallel with START feeds the rung from the output itself. This is called a seal-in or holding contact.
• Press Stop: the NC contact opens, the rung loses power, and the motor drops out. Releasing Stop does not restart the motor. You must press Start again.
• The overload trip is also NC in series. Click it to latch it: the motor stops and cannot start until you click it again.
• The Python line "run = (start or motor) and not stop and not overload" is the same rung as the diagram.

Parts to buy

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