In Fig.1 shows a diagram of the humidity sensor for windshield wipers. When it enters the water drops on the humidity sensors E1 and E2 changes resistance between the plates, which leads to the start of the generator, assembled on a chip DDL.
The signal from the oscillator is fed to transistor switch that controls the wiper motor.
Fig. 1
The frequency of movement of the wipers depending on the moisture content of the windshield car, the more intense the rain, the less the resistance between the sensor and more the frequency of the pulses produced by the generator.
Humidity sensor are two aluminum plates of aluminum foil glued on the glass thus; that the wipers wiped water between them.
Resistors R1 and R2, you can adjust the intensity of work wipers. The size and the distance between the plates choose experimentally.