Gauge to estimate the fall in a certain period of time the rain on the windshield glass, the amount of automatic wiper system, rain sensor signal is determined based on need wiper interval. The device consists of a transmitter and a receiver diode component. When the weather is dry, there is a beam of light is always from one sensor to another sensor (the spread of the beam was one hundred percent.) If the glass wet (beam propagation rate of less than one hundred percent), the direction of the beam shift occurs, is proportional to the degree of deviation and rain. Its working principle is to reach the receiver, the less light, wiper faster. It should be noted is that when you remove the key from the ignition after, to system recovery. When the windshield wiper when the duration (more than seven round-trip), the rain detector will automatically light up headlights (based on models may be)