Using a DHT22 sensor, an Arduino, and a relay, turns a humidifier on and off based on current humidity. I programmed this for the Arduino Uno, though as far as I know there's no reason it couldn't ...

Solder a 1MΩ resistor between two pins on your board (in this sketch, pins D2 and D3 are used). Connect or solder your board as a direct replacement for the blue sensor module on the power supply PCB ...