Photosensitive document detector with automatic compensation

Claim 1

A compensating photosensitive document detector comprising: a photodetector; a light source for simultaneously illuminating a document path and said photodetector, said photodetector and light source being positioned with respect to said path such that documents traversing the path interrupt the illumination of the detector; first circuit means connected to said detector for converting the current output supplied by said detector to a corresponding voltage; second circuit means responsive to said first circuit means for continuously sampling the voltage signal supplied by said first circuit means and providing an output voltage corresponding to the peak voltage supplied by said first circuit means; and third circuit means responsive to said first and second circuit means for providing an output when the output voltages provided by the first and second circuit means bear a predetermined relationship to each other whereby the status of the document path may be determined, said third circuit means including; a voltage divider network connected to the output of said second second circuit means for providing a voltage which is a fixed percentage of the output from the said second circuit means and further providing an adjustment of the percentage, said voltage divider network comprising a potentiometer connected between the output of the said second circuit means and a reference potential, a voltage divider circuit connected betwEen said reference potential and the wiper of the said potentiometer whereby adjustment of the potentiometer wiper adjusts the voltage supplied across the voltage divider circuit and switching means for selectively connecting a first and a second point on the said voltage divider network to provide an output; a comparator having a first input connected to said switching means and a second input connect to the said first circuit means for providing an output whenever the voltage provided by the said first circuit means falls below the voltage provided by the said switching means, said switching means in its first position being used to calibrate the voltage divider network to secure an output from the comparator when a single document intervenes between the said light source and the detector and in its alternate position providing an output to secure an output from the comparator when two or more documents in overlapped relationship intervene between the light source and the photodetector.