My dad has asked me to build him an alarm that he can leave on his windowsill so that when it rains during the night, he can get up and close the windows. Moisture alarms are common, but they're designed to be submerged in water, and don't have sensors that would respond to a few raindrops sprinkling on them. This is a sensor that seems perfect for the application:
As I was thinking about it, I realized that, first of all, this hardly requires an Arduino. It can easily be built with a DC power supply, a potentiometer to control the sensitivity of the trigger, and a speaker module. I also figured that re-inventing the wheel is probably not the best way to go about this. I wondered if I could just take an existing moisture alarm and replace its sensor.
But all of the sensors I see online are 2-wire hookup, while the sensor I want to use is three-wire. That leads me to believe that the other sensors are basically being treated like digital inputs. The voltage is weakly pulled-up (or down) and as the sensor gets wet, resistance drops and voltage on the output pin drops (or rises) to a threshold that triggers the alarm. So now I am wondering what would be involved in converting the three-wire sensor to operate as a two-wire sensor.