A signal processing circuit provided with an outside-light component
reducing circuit capable of reducing an outside light component in the
frequency separation method is provided. This is a signal detecting
circuit which irradiates a pulse light from an LED to a windshield of a
vehicle, receives a reflected light by a PD, processes the pulse signal
from the PD and inputs it to a processing unit in order to automatically
control a wiper for eliminating raindrops on the windshield of the
vehicle. This is provided with a current--voltage converter circuit for
converting the pulse signal from the LED to a voltage signal, an
outside-light component reducing circuit provided in parallel with the
current--voltage converter circuit for reducing the outside light
component included in the output signal of the current--voltage converter
circuit and a band-pass filter circuit/amplifier circuit for reducing the
noise in the output signal of the current--voltage converter circuit and
amplifying the output signal.