Measuring unit for a rain sensor to detect the umiditiy on a surface, and a humidity sensor

The measuring device (1) for a moisture sensor for the detecting of moisture on a surface (4) has a radiation receiving unit (2) to receive radiation in dependence upon the quantity of moisture on the surface, and a measuring instrument connected to the radiation receiving unit and equipped with a condenser (5) which is periodically charged to and from a reference potential and transmits a periodic output signal, the period of which is determined by the charging and discharging and dependent upon the value of the received radiation. Independent claims are also included for the following: (A) a moisture sensor with the proposed measuring device; and (B) a method for measuring the quantity of moisture on a surface and for the generating of a periodic output signal depending upon the quantity of moisture.