Patent ID: 7507982

Claim:
A signal detecting circuit which irradiates a pulse light from a light emitting element to a windshield of a vehicle, receives a reflected light by a light receiving element, processes the pulse signal from the light receiving element and inputs it to a processing unit in order to control a wiper of the vehicle, comprising: a current—voltage converter circuit for converting the pulse signal from said light emitting element to a voltage signal; an outside-light component reducing circuit provided in parallel with said current—voltage converter circuit for holding a constant outside light component included in an output signal of said current—voltage converter circuit and feeding it back to the input side of said current—voltage converter circuit; and a band-pass filter circuit/amplifier circuit for reducing a noise of the output signal of said current—voltage converter circuit and for amplifying the output signal; wherein said outside-light component reducing circuit includes a switch circuit connected to the output side of said current—voltage converter circuit, an outside-light component voltage holding circuit connected to said switch circuit for holding the voltage of said constant outside light component, and an outside-light component voltage-current converter circuit connected to said outside-light component holding circuit for converting the held voltage to an electric current and feeding it back to the input side of said current—voltage converter circuit.