GB1335444A - Photosensitive misfeed detector - Google Patents

Abstract

1335444 Photo-detector circuits XEROX CORP 19 April 1971 [28 Jan 1970] 20405/71 Heading G1A [Also in Division B8] A device for detecting the presence and correct positioning of a sheet being conveyed on a roller surface comprises a fixed plate 11, Fig.2, having angularly directed holes 17, 18 in which are mounted fibre e.g. plastics, optical light guide members 19, 20 through which light is directed towards the roller, and two photo-cells 23, 24 to receive light reflected from the roller. The receiving ends of the guide members project into a housing containing a lamp. A photo-transistor (35) Fig.4 (not shown) in the housing detects any failure of the lamp. The photo-cells 23, 24, Figs.2 and 6, form two arms of a bridge circuit connected to a D.C. power source and including a tap element and a resistor R1. The bridge circuit is connected to the base of a PNP transistor 60 and through a resistor R2 to the emitter of the PNP. A bias resistance R3 is also coupled to the base. The emitter is connected to the base of an identical transistor 61, and the collectors of the transistors are connected. A timing means comprises a capacitor C1 and resistors R4, RT which control the firing of a uni-junction transistor UJT suitably biased by resistances RT, R4, R5, R6, the resistor RT providing thermal compensation for ambient environmental variations. The output of the uni-junction is connected to the collector of a silicon controlled rectifier SCR including a relay 64 in series therewith. The logic control circuit of a printing machine, which is to be controlled according to the detection of the sheet feed, applies to and then removes power from the timing circuit during a specific period during which the condition of the sheet feed is to be detected. In normal operation, i.e. correct sheet feed, unequal reception of light by the photo-cells during the detection period causes the bridge to become unbalanced before the capacitor C1 has become charged sufficiently to trigger the transistor UJT, then both transistors 60, 61 conduct to discharge the capacitor, and a new timing cycle begins at the next detection period. In the case of incorrect feed, the two photo-cells both receive either light or no light and the bridge remains balanced, so that the capacitor C1 continues to be charged until it triggers the uni-junction UJT to fire the rectifier SCR and energize the relay 64, which produces a signal which stops the printing-machine. In the event of a failure of the lamp, the photo-transistor (35) in the housing, which is connected in a circuit block 63, produces a high resistance which prevents the transistors 60, 61 from discharging the capacitor.