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Operability Checking Circuit

IP.com Disclosure Number: IPCOM000085769D
Original Publication Date: 1976-Jun-01
Included in the Prior Art Database: 2005-Mar-02
Document File: 2 page(s) / 34K

Publishing Venue

IBM

Related People

Ernst, LM: AUTHOR [+2]

Abstract

Many optical detectors depend upon the absence of reflected light to indicate a safe condition. Should the light source burn out, it is impossible to detect a subsequent fault. This circuit indicates a failure if the light source fails.

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Operability Checking Circuit

Many optical detectors depend upon the absence of reflected light to indicate a safe condition. Should the light source burn out, it is impossible to detect a subsequent fault. This circuit indicates a failure if the light source fails.

Fig. 1 shows one use of such an optical detector. Xerographic drum 10 carries a toned image to the transfer station, whereat this image is transferred to a sheet of paper. If the sheet fails to detach from the drum, light is reflected by a light-emitting diode (LED) to a photocell.

A number of similar LED/photocell detectors may be included in the xerographic machine, with the LED's connected in series as shown in Fig. 2.

The string of LED's are energized from a pulsating voltage source such that emitted light energy 11 pulsates. The energy from one of the LED's is optically coupled to a photocell in the form of light-sensitive transistor 12. So long as the LED string has continuity, and the pulsating voltage source remains operative, a pulsating signal is supplied to operational amplifier 13.

The resulting amplified pulsating signal is converted to a DC signal by diode 14 and capacitor 15. As long as the pulsating signal persists, capacitor 15 remains charged, and transistors 16 and 17 are conductive and nonconductive, respectively.

Should an LED or the voltage source fail, capacitor 15 discharges through its parallel resistors, and transistor 17 becomes conductive. Conduction of this transistor provi...