Evaluation circuit for optical touch sensor, performs stabilization of operating point of phototransistor and ambient light compensation, and attenuates signal behind high-pass filter capacitor based on low-frequency voltage at resistor

摘要

The evaluation circuit has a phototransistor (FT1) used as a receiving unit of reflective sensor, whose collector is connected to voltage terminal and emitter is connected to ground through resistor (R1). The voltage generated across the resistor is filtered through a high-pass filter capacitor (C1). The stabilization of operating point of phototransistor and compensation of ambient light are carried out indirectly by circuit unit (AS). The signal behind the high-pass filter capacitor is attenuated according to the low-frequency voltage at resistor based on the ambient light.