Home alarm system

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 MotionSensor, Button, LED, Buzzer
from time import sleep

pir = MotionSensor(4)
door = Button(22)          # pressed = door open (a reed switch on a real door)
arm_button = Button(27)
armed_led = LED(17)
siren = Buzzer(18)

armed = False
alarm = False

def toggle_arm():
    global armed, alarm
    armed = not armed
    alarm = False
    siren.off()
    armed_led.value = armed
    print("System ARMED" if armed else "System disarmed")

arm_button.when_pressed = toggle_arm

print("Press Arm / disarm. Then click the PIR or hold the door contact.")
while True:
    if armed and not alarm and (pir.motion_detected or door.is_pressed):
        alarm = True
        print("ALARM! Press Arm / disarm to stop")
        siren.beep(0.2, 0.2)
    sleep(0.1)

How the code works

• The program has two states: armed and alarm. The button flips armed. Only an armed system can raise the alarm.
• The "global" line lets toggle_arm change the variables of the main program.
• "not alarm" makes sure the siren is started once, not ten times a second.
• siren.beep(0.2, 0.2) makes the buzzer pulse in the background while the loop keeps checking the button.

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.