The rain sensor cooperates with a microprocessor which activates the wipers. The sensor emits light which is reflected from the windscreen. A dry windscreen reflects nearly all of the light emitted by the sensor, whereas a wet or snow-covered windscreen absorbs some of the light, as a result of which not all of it makes it back to the sensor and, consequently, the wipers are activated. The sensor is damaged during windscreen replacement or it wears with time. In the event of its failure, the wipers work chaotically or cannot be activated automatically.