Browse Prior Art Database

Self Stabilizing Photodetection Circuit

IP.com Disclosure Number: IPCOM000077223D
Original Publication Date: 1972-Jun-01
Included in the Prior Art Database: 2005-Feb-25
Document File: 2 page(s) / 29K

Publishing Venue

IBM

Related People

Walewski, LG: AUTHOR

Abstract

Large phototransistor PTX collector current variations caused by lamp voltage supply or phototransistor temperature variations are greatly reduced, so that PTX switching occurs only for lamp to PTX light-beam interruptions.

This text was extracted from a PDF file.
At least one non-text object (such as an image or picture) has been suppressed.
This is the abbreviated version, containing approximately 68% of the total text.

Page 1 of 2

Self Stabilizing Photodetection Circuit

Large phototransistor PTX collector current variations caused by lamp voltage supply or phototransistor temperature variations are greatly reduced, so that PTX switching occurs only for lamp to PTX light-beam interruptions.

The circuit provides a switched output in response to an intervening object in the light-beam path, as shown as an arrow from the lamp to the base of PTX. PTX collector current Ic versus PTX base-emitter voltage Vbe or lamp voltage VL is exponentially related.

In this circuit, a decrease in VL and lamp intensity is offset by an increase in Vbe and vice versa, thus holding Ic steady for supply. voltage Vs variations. Putting a diode or thermistor in series with R2 will compensate for base-emitter voltage temperature variations. VL and lamp intensity stabilize the PTX collector current, thus avoiding the need for a balanced differential pair of phototransistors or regulation of lamp voltage to discriminate between small lamp intensity variations and power supply or temperature variations. The current through R5 is much greater than the PTX emitter current. As the supply voltage decreases the voltage at the PTX emitter decreases, as well as the lamp intensity. As the lamp intensity decreases, base drive to the PTX decreases and, therefore, its collector current decreases. The decrease in the pTX collector current reduces the IR drop across its load resistor R4.

The reduced IR drop across R4 tends to raise t...