Cold-room temperature alarm

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 MCP3008, LED, Buzzer
from time import sleep

sensor = MCP3008(channel=0)
ok = LED(27)
alarm = LED(17)
buzzer = Buzzer(18)

LIMIT = 8                  # degrees C
HOT_READINGS = 5           # how many readings in a row must be hot
hot = 0

while True:
    temp = (sensor.value * 3.3 - 0.5) * 100
    if temp > LIMIT:
        hot += 1
    else:
        hot = 0
    if hot >= HOT_READINGS:
        ok.off()
        alarm.on()
        buzzer.on()
        print("ALARM! %.1f C for %d readings" % (temp, hot))
    else:
        ok.on()
        alarm.off()
        buzzer.off()
        print("%.1f C  (hot readings: %d)" % (temp, hot))
    sleep(1)

How the code works

• hot counts the readings in a row above the limit. Any normal reading sets it back to zero.
• A short door opening makes one hot reading only, so it does not trigger the alarm.
• Only after 5 hot readings do the lamp and the buzzer start.

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.