Rain responsive windshield wiper control

A control circuit comprising in combination:

• (a) an optical sensor means for detecting the accumulation of moisture droplets on a transparent surface and producing a composite electrical signal having a component due to the presence of moisture and a component due to changes in ambient light reaching said sensor means, and producing an output signal having a component due to changes in ambient light;
• (b) means for sampling said composite electrical signal in a first interval when both components are present;
• (c) means for sampling the output signal from said optical sensor in a second interval when only said component due to changes in ambient light is present, said first and second intervals being taken close to one another in time;
• (d) differential amplifier means for forming a linear differential summation of said signals obtained during said first and second intervals;
• and
• (e) means for determining whether the output of said differential amplifier means exceeds a predetermined threshold.

In a motor vehicle of the type incorporating a windshield and a motor-driven windshield wiper, an improved windshield wiper motor controller for automatically activating the motor of said motor-driven windshield wiper upon detection of moisture droplets on the exterior surface of said windshield, comprising:

• (a) an opaque housing secured to the inside surface of said windshield, said housing having a first and a second plurality of bores whose longitudinal axes intersect at points at the exterior surface of said windshield and at an acute angle to the plane of the windshield, said first plurality of bores including individual radiant energy sources and said second plurality of bores including individual radiant energy sensors, individual ones of said sources being associated with individual ones of said sensors;
• (b) driver means for periodically energizing said radiant energy sources with pulses of a predetermined duty cycle, the interval when said radiant energy sources are "on" defining a rain-sense interval and the interval when said radiant energy sources are "off" defining an ambient light sense interval;
• (c) first means for sampling and holding the voltage output of said radiant energy sensors during said rain-sense interval;
• (d) second means for sampling and holding the voltage output of said radiant energy sensors during said light-sense interval;
• (e) differential summing means coupled to receive the outputs from said first and second means for sampling and holding, said differential summing means providing an output voltage, V.sub.diff, proportional to the difference between said voltage outputs sampled and held by said first and second means;
• (f) first window comparator means coupled to receive said voltage, V.sub.diff, for signaling when the amplitude of said V.sub.diff voltage falls outside of a predetermined range of amplitudes;
• and
• (g) rain-control oscillator means coupled to receive the signal output from said first window comparator means for driving said windshield wiper motor at a rate proportional to the frequency with which said V.sub.diff voltage falls outside of said range of amplitudes during successive rain-sense intervals.