Temperature alarm with an analog sensor

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)      # TMP36 on channel 0
led = LED(17)
buzzer = Buzzer(18)
LIMIT = 30                       # alarm above 30 degrees C

while True:
    volts = sensor.value * 3.3
    temp_c = (volts - 0.5) * 100       # TMP36: 10 mV per degree, 0.5 V at 0 C
    print("Temperature:", round(temp_c, 1), "C")
    if temp_c > LIMIT:
        led.on()
        buzzer.on()
    else:
        led.off()
        buzzer.off()
    sleep(1)

How the code works

• The TMP36 gives 0.5 V at 0 degrees C plus 10 mV for every degree. So 25 degrees is 0.75 V.
• The MCP3008 reads that voltage and returns a number from 0 to 1 (0 to 3.3 V). It talks to the Pi over SPI.
• The formula (volts minus 0.5) times 100 turns the voltage into degrees C.

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.