BUILT_IN_HYSTERESIS

Published:2009/7/6 7:54:00 Author:May | From:SeekIC

Will operate at all speeds in range from 20 kHz down to zero while still having suitable rise times for driving digital logic. When optical path is blocked, all three transistors are off and output is low. As light on Q1 increases, Q2 and Q3 begin turning on; rising collector of Q3 adds more current through R2 to LED, giving Q1 more light and driving Q1 into saturation. When light dims, Q2 begins to turn off and extra current is cut off, driving Q3 off. With this hysteresis action, there is no constant light level at which circuit will oscillate.-D. C. Hoffman, Optical Sensor Has Built-In Hysteresis, EDN Magazine, June 5, 1973, p 91.



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