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Self Regulating Current Switch for Photodetectors

IP.com Disclosure Number: IPCOM000086122D
Original Publication Date: 1976-Jul-01
Included in the Prior Art Database: 2005-Mar-03
Document File: 2 page(s) / 37K

Publishing Venue

IBM

Related People

Brecke, RA: AUTHOR [+2]

Abstract

In a broken-beam optical object sensor, circuit 100 switches an output 101 reliably, despite variation in light-source intensity, photodetector sensitivity and ambient light conditions. Circuit 100 responds to a fixed fraction of the unblocked light, regardless of the absolute light intensity.

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Self Regulating Current Switch for Photodetectors

In a broken-beam optical object sensor, circuit 100 switches an output 101 reliably, despite variation in light-source intensity, photodetector sensitivity and ambient light conditions. Circuit 100 responds to a fixed fraction of the unblocked light, regardless of the absolute light intensity.

With a light beam falling on phototransistor T1, current I has a maximum value Im. The resulting voltage across diode-connected transistor T2 is coupled to the inverting input of operational amplifier A1. The output of A1 drives amplifier A2, causing transistor T3 to charge capacitor C1. The reference voltage across C1 is sensed by the noninverting input of amplifier A1, completing a feedback loop. Amplifier A1 also drives transistor T4, whose output 101 is fed back to A1. This positive feedback through resistor R9 provides a hysteresis to debounce output 101 when current is close to the switching threshold.

When phototransistor T1 output current I falls to half its quiescent value, the voltage on transistor T2 changes by 18mV, and the output of amplifier A1 becomes sufficiently positive to turn on transistor T4. This turnon threshold is established by resistors R1-R4 inclusive. (Because of the inherently logarithmic transfer function of T1, other fractional changes can be established by different absolute thresholds.)

The voltage on capacitor C1 will remain essentially constant for several seconds. But, if current 1 remains...