Research on infrared signal processing circuit of large size infrared touch screen with interference rejection

摘要

Infrared touch screen realizes touch operation by locating touch position on display screen with occlusion of infrared light. Infrared transmission diodes and photodiodes are placed in the frame of the screen to form an equivalent infrared optical network on the surface of the screen. Large size infrared touch screen will increase the number of transmission diodes and photodiodes, as well as the scanning speed and transmission distance of infrared light, it causes signal interference increases and signal-to-noise ratio decreases. This paper presents an infrared signal processing circuit with interference rejection, which improve the signal-to-noise ratio while ensuring the signal processing speed. The infrared light is transmitted in switch mode at a fixed frequency. The infrared signal processing circuit uses resonant circuit to restore the transmitted infrared signal which has the same resonance frequency. The output of the infrared processing circuit intensity extracted by band-pass filter and switch lock-in amplifier. The experiment results show that proposed circuit has 150$mu$s fast response time, the output voltage varies with the ambient light illuminance by 1.5 percent per 10000 lux, and the circuit has well characteristic of frequency selection which limited noise bandwidth. The interference rejection is improved effectively.