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Hopper-Full Sensor Circuit for Printer

IP.com Disclosure Number: IPCOM000102596D
Original Publication Date: 1990-Dec-01
Included in the Prior Art Database: 2005-Mar-17
Document File: 2 page(s) / 60K

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Related People

Presley, DR: AUTHOR


Disclosed is a Light-Emitting Diode (LED)/photodetector circuit for indicating when a printer paper hopper is full.

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Hopper-Full Sensor Circuit for Printer

       Disclosed is a Light-Emitting Diode (LED)/photodetector
circuit for indicating when a printer paper hopper is full.

      It was desired to detect hopper-full by shining the light beam
across the width of the paper as shown in Fig. 1. It is necessary to
prevent the circuit from falsely indicating hopper-full when a sheet
of paper falls through the light beam.

      Using inexpensive LEDs and photodetectors, the LED requires a
drive current of 0.5 A to generate the light intensity required to
guarantee it would be detected at the large distance (10 inches).
However, if the LED is driven constantly at that current, it gets too
hot and its life is drastically reduced.

      The circuit shown in Fig. 2 pulses the LED at a current of 0.5
A.  The LED is pulsed on every 0.1 seconds and remains on for 200
microseconds.  This timing keeps the duty cycle and on-time of the
LED low enough to prevent excessive heating but the on-time and light
intensity is sufficient for the photodetector to respond to it.

      The 8-bit counter shown in Fig. 2 advances each time the LED is
pulsed, but it is reset to zero each time the light beam is detected.
The beam must be blocked for 128 LED pulses (or 12.8 seconds) before
it indicates hopper-full and latches in this condition.  This
prevents it from responding to a sheet of paper falling slowly
through the light beam.

      Energy is stored in a capacitor (C2) to pulse...