Development of a Mid-Infrared Sensing system for Gas Detection

Agricultural products and process often release volatile compounds. Understanding the characteristics of the volatile can accurately define the quality of the products and the performance of the process. Near infrared (NIR) spectroscopy has been widely used for rapid determination and analysis of agricultural products. Recent research also showed that many agricultural related volatiles have stronger absorption in mid infrared (MIR) spectral range compared to that in NIR range. This paper introduces the design of a signal conditioning circuit for a gas detection system which comprised four mid-IR LEDs, one photo sensors, and a signal acquisition unit. The LED provided four different-band mid-IR light sources. A MIR Photoresistor with the sensitivity range of 1.1-4.5µm was used to collect the optical transmittance signal. The signal was amplified through a two stage instrumentation amplifier, and measured by the microcontroller’s ADC module. The sampling result can be sent out through RS232 port for data analysis to determine the composition and concentration of the sampled gas.