Two-hand press control

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, time

left = Button(5)
right = Button(6)
press = OutputDevice(17, active_high=False)     # clutch relay
ready = LED(27)
running = LED(22)

SYNC = 0.5                  # both buttons within 0.5 seconds
t_left = t_right = 0
armed = False

while True:
    now = time()
    if left.is_pressed and t_left == 0:
        t_left = now
    if right.is_pressed and t_right == 0:
        t_right = now
    if not left.is_pressed:
        t_left = 0
    if not right.is_pressed:
        t_right = 0

    both = left.is_pressed and right.is_pressed
    in_sync = both and abs(t_left - t_right) <= SYNC

    if in_sync and not press.value:
        press.on()
        print("Both hands in sync: PRESS RUNNING")
    if not both and press.value:
        press.off()
        print("A hand was released: PRESS STOPPED")
    if both and not in_sync and not press.value:
        print("Too slow: release both buttons and try again")
        sleep(0.5)
    ready.value = not press.value
    running.value = bool(press.value)
    sleep(0.02)

How the code works

• The program notes the time at which each button went down. A button that is up resets its time to 0.
• in_sync is true only when both are pressed and the two press times are less than 0.5 s apart.
• Releasing either button switches the press off at once.
• A hand held down for a long time and a late second hand never starts the press.

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.