Document sensing apparatus which detects voltage dropage and gen

Details Document sensing apparatus which detects voltage dropage and gen Document sensing apparatus which detects voltage dropage and gen

1990-09-17

1991-10-08

Nelms, David C.

Radiant energy

Photocells; circuits and apparatus

Optical or pre-photocell system

250214R, 226 45, G01N 904

Patent

active

050556703

ABSTRACT:
A document sensing apparatus includes a phototransistor (12) for generating a first or second voltage depending on whether or not, respectively, a document is present at a sensing location, and a capacitor (36) for providing a second voltage whose peak value is determined by said second value. A voltage comparator (30, 38, 40) compares said first and second voltages and provides an output signal indicative of the presence of a document at the sensing location if said first voltage is less than a predetermined fraction of said second voltage. A microprocessor (44) controls the operation of a circuit (71-1) which provides, as necessary, a charging current to said capacitor (36) so that, while a document is present at the sensing location, said second voltage is prevented from falling by a significant amount, thereby preventing problems occurring if a document remains at the sensing location for a prolonged period due to a feeding irregularity.

REFERENCES:
patent: 3488048 (1970-01-01), Dykaar et al.
patent: 3906232 (1975-09-01), Meihofer
patent: 4151410 (1979-04-01), McMillan et al.
patent: 4268746 (1981-05-01), Schroeder
patent: 4289406 (1981-09-01), Maddox
patent: 4607155 (1986-08-01), Nao et al.
patent: 4723072 (1988-02-01), Naruse
patent: 4892426 (1990-01-01), Steele
NCR Invention Disclosure Record No. 4472, pp. 2 & 6.

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