Abstract: An electronic control system for windshield wipers on motor vehicles for initiating wiping action in response to the occurrence and level of precipitation on the windshield. A sensor unit comprising a block containing an array of photo-sensors arranged in a bridge configuration and a corresponding array of light sources is mounted on the inside surface of the windshield at a location traversed by the path of the vehicle's windshield wiper blades. The photo-sensor elements are oriented to normally receive light reflected back from the glass/air interface at the outside windshield surface from the photo-emitters, but when rain is present on the windshield, the light rays are scattered to create an unbalanced condition. The unbalancing of the bridge creates a signal which is demodulated in a synchronous demodulator and then applied via an automatic gain control (AGC) circuit to a window comparator.