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Optical Sensor with Positional Hysteresis

IP.com Disclosure Number: IPCOM000082846D
Original Publication Date: 1975-Feb-01
Included in the Prior Art Database: 2005-Feb-28
Document File: 2 page(s) / 26K

Publishing Venue

IBM

Related People

Nasser, MH: AUTHOR [+2]

Abstract

In many digital position sensing applications, signal "bounce" caused by mechanical jitter or vibration can cause sensing errors (e.g., extra pulses in an emitter-wheel sensor). Positional hysteresis can often eliminate such errors in these applications.

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Optical Sensor with Positional Hysteresis

In many digital position sensing applications, signal "bounce" caused by mechanical jitter or vibration can cause sensing errors (e.g., extra pulses in an emitter-wheel sensor). Positional hysteresis can often eliminate such errors in these applications.

The figure illustrates a simple and inexpensive optical position sensor which incorporates positional hysteresis. The sensor consists of a light-emitting diode (LED) light source, a phototransistor T1, and an output buffer. A photodarlington or photodiode could also be used in lieu of the phototransistor.

When the optical path between the LED and the phototransistor T1 is blocked, the T1 current drops to a very low value, transistor T1 turns off and turns off T2. When the optical path is unblocked T1 turns on and turns on T2. Positive feedback through R(fb) provides positional hysteresis and speeds up the switching transitions. The positional hysteresis is accomplished by the changing of threshold levels which will activate the circuit, thereby preventing unintentional reverse crossing of the threshold.

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