Browse Prior Art Database

Automatic Light Intensity Control for Optical Sensing

IP.com Disclosure Number: IPCOM000077201D
Original Publication Date: 1972-Jun-01
Included in the Prior Art Database: 2005-Feb-25
Document File: 2 page(s) / 49K

Publishing Venue

IBM

Related People

Braun, RJ: AUTHOR [+2]

Abstract

This Automatic Light Intensity Control (ALIC) circuit operates in conjunction with an LED (Light-Emitting Diode) and a phototransistor, in a closed-loop feedback system. The LED current is automatically adjusted to provide the required light intensity for a phototransistor output (ALIC input) current for a white or black level state, respectively. The circuit can be used in a see-through configuration where a document interrupts the light beam, or in reflective optical sensing.

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Automatic Light Intensity Control for Optical Sensing

This Automatic Light Intensity Control (ALIC) circuit operates in conjunction with an LED (Light-Emitting Diode) and a phototransistor, in a closed-loop feedback system. The LED current is automatically adjusted to provide the required light intensity for a phototransistor output (ALIC input) current for a white or black level state, respectively. The circuit can be used in a see-through configuration where a document interrupts the light beam, or in reflective optical sensing.

Current from phototransistor PT is amplified and inverted in a Darlington stage including transistors T1, T2. R3 and C1 provide strong negative feedback, R1 and C1 filter out noise above a few KHz. T3, T4 and T5 produce a logic output signal with the switching threshold determined by the differential pair T6, T7. T8, T9 and T10 provide amplification of the LED current and together with D1 close the feedback loop.

The circuit has two stable states, the white level (WL) and black level (BL). In the WL, T1 and T2 are fully on, D1 forward biased. T11 is saturated and provides a current which drives T8, T7 and D1. T6 is off; T3, T4 and T5 are on, with the output level down (OV). A slow decrease in input current (dust, aging) causes VC and VB to rise and the LED current to increase, compensating the input current. If the input current decrease is large and sudden, e.g., changes from white to black background, VB cannot follow the change in VC...