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Detection Circuit

IP.com Disclosure Number: IPCOM000094423D
Original Publication Date: 1965-Jan-01
Included in the Prior Art Database: 2005-Mar-06
Document File: 2 page(s) / 28K

Publishing Venue

IBM

Related People

Cook, KA: AUTHOR [+2]

Abstract

Holes or spots carried by document 1 are detected by sensing light from lamp 2 reflected off the surface of 1. When no hole or spot lies in the light path, reflected light impinges upon the control electrode of light-actuated, silicon-controlled rectifier 3. This places 3 in a state of conduction and drops the voltage at gate 5 of field effect transistor 4, connected as a source follower, to a negative potential. This decreases the flow of current through R2 and drops the voltage at source 6, which, due to the voltage division of R2-R3, reverse biases diode 7. This places 8 in saturation and maintains output terminal 10 at ground potential.

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Detection Circuit

Holes or spots carried by document 1 are detected by sensing light from lamp 2 reflected off the surface of 1. When no hole or spot lies in the light path, reflected light impinges upon the control electrode of light-actuated, silicon- controlled rectifier 3. This places 3 in a state of conduction and drops the voltage at gate 5 of field effect transistor 4, connected as a source follower, to a negative potential. This decreases the flow of current through R2 and drops the voltage at source 6, which, due to the voltage division of R2-R3, reverse biases diode 7. This places 8 in saturation and maintains output terminal 10 at ground potential.

Current flow through 3 is kept below the holding value by use of high impedance R1. Thus, when a hole or a spot cuts off the reflected light, 3 is immediately switched to its high impedance state. This places a positive potential determined by voltage divider R1-R4 on gate 5, increasing current flow through R2. This raises the potential at 6 and forward biases 7, causing 8 to be cut off, generating a negative going output pulse at 10.

The use of field effect transistor 4 is instrumental for the optimum operation of the circuit. One reason is that its control input is voltage sensitive rather than current sensitive. This permits the use of the high impedance at R1 necessary for the proper low current operation of 3. Additionally, the low impedance at the output of 4 permits generation of the current levels ne...