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Double Threshold Mark Detection Circuit

IP.com Disclosure Number: IPCOM000096609D
Original Publication Date: 1963-Aug-01
Included in the Prior Art Database: 2005-Mar-07
Document File: 2 page(s) / 47K

Publishing Venue

IBM

Related People

Bene, JF: AUTHOR

Abstract

This circuit compares the signal output of a photoelectric transducer, sensing a mark position. It determines whether the signal indicates that mark or no mark exists, or that decision cannot be made as to the presence of a mark because of a poor erasure, light mark or smudge.

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Double Threshold Mark Detection Circuit

This circuit compares the signal output of a photoelectric transducer, sensing a mark position. It determines whether the signal indicates that mark or no mark exists, or that decision cannot be made as to the presence of a mark because of a poor erasure, light mark or smudge.

Photoelectric transducer 1, sensing a mark position on a document such as a student's answer sheet, generates a current. This is proportional to the light reflected from the document and modulates the current flowing out of the base of T1 to produce a signal. The signal is amplified by transistors T1 and T2 serving as Class A amplifiers.

The output from T2 is a varying negative voltage. This is applied as an input on lines 3 and 4 across capacitors to a lower threshold clamp and upper threshold clamp, respectively. The lower threshold clamp includes resistor 5, diode 6 and potentiometer 7. The upper threshold clamp includes resistor 8, diode 9 and potentiometer 10. These are adjusted to provide two different reference potentials against which the input signal is compared. The lower and upper threshold clamps are each connected to the base of their respective transistors T11 and T12 through normally reversely biased isolation diodes 13 and 14. T11 and T12 are held in a normally conducting state by voltage divider network 15 or 16.

When the input signal on line 3 becomes more positive than the lower threshold voltage, diode 13 is forwardly biased. Such causes T11 to turn off thus providing a negative input to inverting And 17. Similarly, if the same input...