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Digital Feedback Light Emitting Diode Control

IP.com Disclosure Number: IPCOM000080608D
Original Publication Date: 1974-Jan-01
Included in the Prior Art Database: 2005-Feb-27
Document File: 3 page(s) / 50K

Publishing Venue

IBM

Related People

Thomas, DC: AUTHOR [+2]

Abstract

Light-emitting diodes (LED's) lose their output efficiency gradually as they age and, in time, may lose up to 50%. It is well known that the higher the current carried in the LED, the faster the aging process occurs. Detector devices such as phototransistors and photodiodes also degrade with age and may lose as much as 25% of their sensitivity. The circuit described automatically compensates for the loss of efficiency in both the LED and in the photodetector, and for temperature variations as well, by adjusting the level of the LED bias current.

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Digital Feedback Light Emitting Diode Control

Light-emitting diodes (LED's) lose their output efficiency gradually as they age and, in time, may lose up to 50%. It is well known that the higher the current carried in the LED, the faster the aging process occurs. Detector devices such as phototransistors and photodiodes also degrade with age and may lose as much as 25% of their sensitivity. The circuit described automatically compensates for the loss of efficiency in both the LED and in the photodetector, and for temperature variations as well, by adjusting the level of the LED bias current.

In the circuit illustrated, a LED is used as a source of light which impinges through a grid onto a photodetector. Movement of the grid produces digital data output signals from the detector, in accordance with the varying patterns of light transmissive and opaque areas in the grid. In the home position, the grid is open and the LED bias current is lower than is necessary to activate the detector, when the small apertures which exist in the grid are interposed between the LED and the photodetector. This circuit gradually increases the LED bias current until the detector threshold is reached.

An output from the detector is then utilized to turn the current generator off and allow the bias current to gradually decay. When the bias current falls below the detector threshold, the detector output again changes state which causes the cycle to repeat itself. Using this system, the average LED current will be at the minimum threshold current required to make the LED and detector combination operate as a unit, with an open gap between them.

In the figures, the LED 1 and the photodetector 2 are optically coupled, although they are shown separated on the drawing to simplify the layout. The detector 2 contains a phototransistor, an amplifier and a Schmitt trigger, from which the detector output is taken. Assume that the detector 2 output is low, when the LED current flowing through Q3 is below the level necessary to create enough light from LED 1 to energize detector 2. The enable line 3 will be assumed to be normally in the high condition when the grid, not shown, is in the home position which leaves an open gap between LED 1 and detector 2. In this case, with the grid in the home position, transistor Q1 will be on and the output line 4 will be inhibited, because AND gate 5 requires an input both from the enable line 3, inverted in inverter 6, and an output from detector 2 before the output line 4 is enabled.

With Q1 on, the enable line 3 up and the detector 2 in a low condition, OR gate 7 will be activated and through it, Q2 will be in the on condition. The capacitor C1 will begin to charge as a result of current received when Q2 is on. Capacitor C1 charges through Q2 and R5. A portion of the voltage from capacitor C1, which is determined by the bridge R3 and R4, is applied to a voltage following operational amplifier 8. Operational amplifier 8 controls...