Both the AC and DC OpenSprinklers internally run on 3.3V DC, it’s just that one is powered by 24V AC, the other is powered by 9V DC. It’s pretty easy to modify the code to process serial communication. You may also consider I2C. But before that, what kind of sensor do you have? OpenSprinkler has several spare digital and analog pins, you can likely hook up your sensor directly to one of the spare pins.