Research On Signal Readout Circuit Of NDIR CO₂ Sensor

With the sustained global spread of the novel coronavirus,there is an increasing demand for detecting gas concentrations and medical monitoring of CO₂ levels in human exhalation in enclosed environments.In order to meet the needs of high-precision gas concentration detection,this article designs a gas sensor dedicated circuit based on the principle of Non Dispersive Infrared（NDIR）,and designs a cylindrical gas chamber structure based on MEMS light sources and detectors.Completed the design of the CO₂ sensor and achieved high-precision CO₂ detection of 1.6ppm.This article focuses on the NDIR detection technology of CO₂ and carries out the following work:（1）A circuit system architecture of the NDIR sensor is proposed,which is divided into four parts:infrared light source driving circuit,signal amplification circuit,power supply circuit,and signal acquisition circuit.The working principle and design ideas of each part are elaborated,and the design and simulation of the infrared light source driving circuit and the signal filtering and amplification circuit,and the design of the signal acquisition circuit system are completed to verify the feasibility of each module.The key points and ideas of the PCB layout and wiring design are also introduced,and the overall circuit design of the NDIR sensor is completed.（2）A firmware program for NDIR sensors based on ESP32 is proposed,which implements digital to analog conversion and I²C driver programs for analog-to-digital conversion chips,designed a driver program for infrared modulation light sources,and designed a monitoring software upper computer program based on Matlab GUI.Based on the UART communication protocol,a program was designed for packaging data collected by NDIR sensors and analyzing Matlab serial port data.The program achieved data communication between ESP32 and Matlab,as well as real-time curve drawing and processing of voltage information,temperature and humidity information,and CO2concentration information.（3）A testing device for testing NDIR gas sensors was built based on an MFC gas flow meter and a constant temperature and humidity chamber.Control temperature and humidity variables,and conduct precision,accuracy,and stability tests on the designed CO₂ gas sensor.Compare it with commercial CO₂ sensors,and confirm that the NDIR CO₂ gas sensor designed in this paper has the advantages of high resolution,high accuracy,and high stability.It can be used for detecting low concentration carbon dioxide gas.