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Ambient Light-Operated Sensor Circuit

IP.com Disclosure Number: IPCOM000061664D
Original Publication Date: 1986-Sep-01
Included in the Prior Art Database: 2005-Mar-09
Document File: 2 page(s) / 35K

Publishing Venue

IBM

Related People

Cato, RT: AUTHOR

Abstract

This article describes an ambient light-operated sensor circuit which is insensitive to long term changes in ambient light levels. The circuit includes a phototransistor, the output of which is compared to a reference voltage to produce a digital (on/off) output signal. A feedback circuit adjusts the base current to the phototransistor as a function of long term ambient light levels to prevent switching other than in response to short term changes in the ambient light. The circuit includes a phototransistor 10 which is exposed to ambient light and provides an output signal (collector voltage) on its collector terminal 12 to one input of a comparator amplifier 14.

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Ambient Light-Operated Sensor Circuit

This article describes an ambient light-operated sensor circuit which is insensitive to long term changes in ambient light levels. The circuit includes a phototransistor, the output of which is compared to a reference voltage to produce a digital (on/off) output signal. A feedback circuit adjusts the base current to the phototransistor as a function of long term ambient light levels to prevent switching other than in response to short term changes in the ambient light. The circuit includes a phototransistor 10 which is exposed to ambient light and provides an output signal (collector voltage) on its collector terminal 12 to one input of a comparator amplifier 14. The amplifier 14 compares the collector voltage to a fixed reference voltage V1 and changes the state of an OUT signal when the collector voltage exceeds the reference voltage. In one application, the sensor circuit is used to initiate a system operation when an operator places a hand over the phototransistor 10 to at least partially block the ambient light. To render the circuit insensitive to long term changes in ambient light, a feedback circuit is provided. Amplifier 16 amplifies the difference between the collector voltage and a second reference voltage V2, which is slightly lower than V1. The output of amplifier 16 drives an RC time constant circuit consisting of resistor 18 and capacitor 20. The component values in the RC circuit determine how fast the feedbac...