Detects the presence of water using exposed PCB traces. The sensor is made up of interlaced traces of Ground and Sensor signals. The sensor traces are weakly pulled up using 1 MΩ resistor. The resistor will pull the sensor trace value high until a drop of water shorts the sensor trace to the ground trace. This circuit will work with the digital I/O pins of an Arduino or you can use it with the analog pins to detect the amount of water induced contact between the grounded and sensor traces.
