Posted on Sep 24, 2012

The circuit outputs power to the drive motor when one or no sheets are being fed, but interrupts motor power when two or more superimposed sheets pass through the optodetector slot. The optodetector can be either an H2aB darlington interruptor module or an H23B matched emitter -detector pair. The output from the optodarlington is coupled to a Schmitt trigger, comprising transistors Ql and Q2 for noise immunity and minor paper opacity variation immunity. When the Schmitt is on, gate current is applied to the SC148D output device.

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The de power supply for the detector and Schmitt is a simple rc diode halfwave configuration chosen for its low cost (fewer diodes and no transformers) and minimum bulk. While such a supply is directly coupled to the power triac, this is precluded by current drain considerations (50 mA de for the gate drive alone). Note that direct coupling of the Schmitt to the output triacs is preferred, since RFI is virtually eliminated with the quasi-de gate drive.

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