Greenhouse climate 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 MCP3008, OutputDevice
from time import sleep

sensor = MCP3008(channel=0)                   # TMP36 on channel 0
fan = OutputDevice(17, active_high=False)     # relay boards switch on a LOW signal
heater = OutputDevice(27, active_high=False)

COLD, WARM = 20, 30                           # the comfortable range

while True:
    temp = (sensor.value * 3.3 - 0.5) * 100
    if temp < COLD:
        heater.on()
        fan.off()
        state = "COLD: heater ON"
    elif temp > WARM:
        fan.on()
        heater.off()
        state = "HOT: fan ON"
    else:
        fan.off()
        heater.off()
        state = "OK: both off"
    print("%.1f C   %s" % (temp, state))
    sleep(1)

How the code works

• The TMP36 gives 10 mV for each degree, plus 0.5 V. The formula turns the reading into degrees.
• Two relays switch two 230 V loads: a fan and a heater lamp.
• The heater and the fan never run at the same time: each branch switches the other one off.

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.