Discriminating paper sensor

摘要

A paper sensor uses two sample and hold circuits which are fired by the phase and antiphase of an oscillator that also drives an infrared light emitting diode (LED). These circuits sample the output from a phototransistor. If the outputs from the phototransistor are different when the LED is lit and unlit, then a comparator will give an output, thus synchronously filtering out unwanted infrared light that may cause a false paper reading.