Electronic document detector circuitry with feedback control

Claim 1

An electronic document detector comprising: a light source; a current source including a light sensitive element spaced from said light source and arranged for receiving at least a portion of the light output of said light source for providing a current having a magnitude directly relative to the amount of light received by said element, the space between said light source and said element defining a path in which documents to be detected traverse, whereby as each document traverses said path said element is shielded from said light source; a current sink coupled to said current source having an input and an output for providing at said output an intermediate control voltage having a magnitude related to the magnitude of the current provided by said current source and within a first range below a given level in the presence of a document within said path and within a second range above said given level in the absence of a document from said path; a voltage detector coupled to said output of said current sink for providing a document indicative signal when said intermediate control voltage falls below said given level into said first range; and intermediate control voltage restoring means coupled between said current sink output and said current sink input for restoring said intermediate control voltage to a predetermined level within said second range after each document traverses said path; whereby, with said intermediate control voltage restoring means restoring said intermediate control voltage to said predetermined level within said second range after each document traverses said path, said current sink is reset for operation between documents to a level which assures proper intermediate control voltage transitions between said second range and said first range not withstanding variations in light output levels of said light source due to aging of said light source or the collection of dirt on said light source.