Thermostat fan with hysteresis

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, OutputDevice
from time import sleep

sensor = MCP3008(channel=0)               # TMP36 on channel 0
fan = OutputDevice(17, active_high=False)

ON_ABOVE = 28       # degrees C
OFF_BELOW = 26      # a gap (hysteresis) stops fast on-off switching

while True:
    temp_c = (sensor.value * 3.3 - 0.5) * 100
    print("Room:", round(temp_c, 1), "C   Fan:", "ON" if fan.value else "OFF")
    if temp_c > ON_ABOVE and not fan.value:
        fan.on()
        print("Too warm: fan ON")
    elif temp_c < OFF_BELOW and fan.value:
        fan.off()
        print("Cool enough: fan OFF")
    sleep(1)

How the code works

• The same TMP36 and MCP3008 as project 10 give the temperature in degrees C.
• If one limit were used, a reading that wobbles around 28 would switch the fan on and off many times. With two limits, the fan has to cool 2 degrees before it stops.
• The tests "and not fan.value" and "and fan.value" make sure on() and off() run only when the state changes.

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.