If it enters the water drops on the humidity sensors E1 and E2 changes the resistance between the plates, which leads to the start of the generator, collected on the chip DD1. The signal from the oscillator is fed to transistor switch that controls the wiper motor.
The frequency of movement of the wipers depending on the moisture content of the windshield of the car, the more intense the rain, the less the resistance between the sensors and the greater the frequency of the pulses produced by the generator.
Humidity sensor are two aluminum plates of aluminum foil glued to the glass so that the wipers wiped water between them. Resistors R1 and R2, you can adjust the intensity of the wipers.
The size and the distance between the plates is chosen experimentally.
Author: A. Chmielewski; Publication: N. Bolshakov, rf.atnn.ru