High speed paper tape reader

Posted on Jul 26, 2012

When computer peripheral equipment is interfaced, it is convenient to work with logic signal levels. With a nominal 4 V at the output dropping to - 0.6 V on illumination, this circuit reflects the requirements of a high-speed, paper tape optical reader system. The circuit operates at rates of up to 1000 bits per second. It will also operate at tape translucency such that 50% of the incident light is transmitted to the sensor, and provide a fixed threshold signal to the logic circuit, all at low cost. Several circuit tricks are required. Photodarlington speed is enhanced by cascode constant voltage biasing.

High speed paper tape reader
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The output threshold and tape translucency requirements are provided for by sensing the output voltage and operating to 2000 bits per second at ambient light levels equal to signal levels.

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