If you are looking to detect rain drops or even water in general sensors are in place which are quite able of the job. However, as per Tindie seller cowtheory they are effective in detecting large droplets of water, and not enough to detect thinner layer of moisture droplets created by condensation. After a futile search for an instrument for measuring moisture on the Internet the company developed an idea for the Water Condensation Sensor board, an exclusive device designed specifically for this purpose.
It’s actually tiny circuit boards with extremely thin (.2mm) tracer that allows it to detect tiny condensation droplets. Utilizing it in conjunction with an embedded microcontroller is straightforward enough: Connect one of the pins to ground, and another to one of the analog or digital pins that is set as an input to pull up. When condensation touches the surface of the sensor panel, it joins the trace enough to draw the pin towards ground, which allows the microcontroller detect the state and act according to the need.
It is possible to see sensors used to measure the condensation that comes from showers or maybe to check the time when your mailbox will be dry to send out (Tindie) packages with no residual moisture. Of course, there is numerous other possibilities to do using a sensor. Maybe it will inspire an idea that isn’t there or revitalize an old idea!