Since I know there is at least one other person watching this forum that plays with Arduino, I figured I would post about my latest project. The two simple goals of the project was to: Auto adjust indoor humidity level based on current outdoor tempertures. Only activate the steam humidify if the Geo system was already running. The part list is pretty short: 1 - Arduino Mini-Pro (others types would work as well) 1 - MID400 (Detects when 24v fan line goes active) 1 - NTC temperture probe 1 - AM2301/DHT21 Temp/Humidity module (really just need the Humidity sensor) 2 - 10k resistor 1 - 4.7k resistor 1 - 5.6k resistor 1 - Relay (used to close the circuit to the humidifier) 1 - 5V DC power supply The programming/algorithm I choose to use was pretty simple: I hard coded the target indoor humidity levels based on the outdoor temperature. I got 'ideal' indoor humidity target levels from web sources. Only start the humidifier if the Geo fan is active. Once the humidifier is ON, keep it ON for at least 45 mins. Once the humidifier is OFF, keep if OFF for at least 45 mins. Activate humidifier at (target_humidity-1)... assuming min time OFF has been meet and the geo is ON. Deactivate humidifier at (target_humidity+1).... assuming min time ON has been meet. It seems to working out quite well. But sometime you get 'unlucky'... Here the geo unit shut down just a few mins after the humidifier started up but I decide it's best to keep the humidifier active.