Design of CO2 gas detection circuit based on infrared sensor

With the progress of human society and the development of science and technology, people's living standards have been rapidly improved, and the scale of industrial production has also expanded rapidly, but at the same time, the emission of carbon dioxide has doubled, such as the greenhouse effect, accelerated land desertification, etc., which seriously affects and destroys the living environment of human beings. In addition, carbon dioxide is the main raw material for crop photosynthesis, and its content directly affects the growth of crops. In recent years, with the enhancement of people's environmental awareness and the progress of scientific and technological progress, how to quickly detect the content of carbon dioxide and reduce carbon dioxide emissions has become a problem of special concern to governments at all levels and people of insight. Therefore, it is of great significance to study and design carbon dioxide detection circuits.

At present, the main methods for detecting carbon dioxide include chemical method, electrochemical method, gas chromatography, volumetric titration method, etc. These methods generally have the problems of high price, poor universality, and low measurement accuracy. The sensor method has the advantages of safety, reliability, fast direct reading, and continuous monitoring. At present, various carbon dioxide sensors used for detection mainly include solid electrolyte type, barium titanate composite oxide capacitor type, conductivity change thick film type, etc. These sensors have poor selectivity for gas, easy false alarm, frequent calibration, and short service life. The infrared absorption type carbon dioxide sensor has the characteristics of wide measurement range, high sensitivity, fast response time, good selectivity, and strong anti-interference ability. For this reason, this design uses an infrared absorption type carbon dioxide sensor, and the entire circuit design strives to be simple and easy to use, fast direct reading, and low price.

1 Working principle of detection circuit

1.1 Working principle of infrared absorption type carbon dioxide gas sensor〔1〕

Infrared absorption type CO2 gas sensor is based on the principle that the absorption spectrum of gas varies with different substances. Different gas molecules have different chemical structures, and the degree of absorption of infrared radiation of different wavelengths is different. Therefore, when infrared radiation of different wavelengths is irradiated to the sample substance in turn, the radiation energy of certain wavelengths is selectively absorbed by the sample substance and weakened, resulting in an infrared absorption spectrum. Therefore, when the infrared absorption spectrum of a certain substance is known, the absorption peak of the substance in the infrared region can be obtained from it. When the same substance has different concentrations, there are different absorption intensities at the same absorption peak position, and the absorption intensity is proportional to the concentration. Therefore, by detecting the effect of gas on the wavelength and intensity of light, the concentration of the gas can be determined.

According to Beer-Lambert's law, the relationship between output light intensity, input light intensity and gas concentration is:

(1)

Where is the molar molecular absorption coefficient; C is the concentration of the gas to be measured; L is the action length of light and gas (sensing length). Transforming the above formula, we get:

(2)

The concentration of the gas can be known by detecting relevant data.

Figure 1 Carbon dioxide sensor probe structure

The structure of the infrared carbon dioxide sensor probe is shown in Figure 1. It is composed of an infrared light source, a measuring gas chamber, an adjustable interference filter, a light detector, a light modulation circuit, an amplification system, etc. The infrared light source uses nickel-chromium wire, which can emit 3-10μm infrared rays after being powered on and heated, including the strong absorption peak of CO2 gas at 4.26μm. In the gas chamber, carbon dioxide absorbs light of a specific wavelength emitted by the light source, and the detector can detect the absorption of infrared rays by carbon dioxide. The interference filter is adjustable, and adjusting it can change the wavelength band of the light wave it passes through, thereby changing the strength of the signal detected by the detector. The infrared detector is a thin film capacitor. After absorbing infrared energy, the gas temperature rises, causing the pressure in the room to increase, and the distance between the two poles of the capacitor will change, and the capacitance value will change accordingly. The greater the concentration of CO2 gas, the greater the change in capacitance value.

1.2 Design principle of detection circuit

Figure 2 Detection circuit principle block diagram

The principle block diagram of the detection circuit design is shown in Figure 2.

The detection circuit is composed of infrared carbon dioxide sensor, digital filter circuit, amplifier circuit, current stabilization circuit, single-chip microcomputer system, temperature compensation, etc. The basic principle of the design is that the infrared carbon dioxide sensor converts the detected carbon dioxide gas concentration into a corresponding electrical signal, and the output electrical signal is filtered and amplified, input to the single-chip microcomputer system, and after temperature and pressure compensation, the single-chip microcomputer system outputs it to the display device to display its measured value.

1.3 Design of detection circuit

Figure 3 Carbon dioxide detection circuit diagram

According to the above design principle, the designed carbon dioxide detection circuit is shown in Figure 3. The working principle is that the infrared sensor first detects the concentration of carbon dioxide gas and converts it into an electrical signal, the filter circuit extracts the electrical signal and outputs it to the amplifier circuit, and then it is processed by the single-chip microcomputer system and output, and then sent to the display circuit by 74AC138 to realize the detection of carbon dioxide gas concentration.

The circuit is composed of R1, R2, R3, R4, C1, C2 and an operational amplifier to form a filter circuit〔2〕, which introduces both negative feedback and positive feedback. When the signal frequency tends to zero, the reactance of C1 tends to infinity, so the positive feedback is very weak; when the signal frequency tends to infinity, the reactance of C2 tends to zero. This ensures that when the signal frequency is at any value between zero and infinity, the filter circuit can normally extract the corresponding electrical signal.

The amplifier circuit after the filter circuit is used to amplify the signal output by the filter circuit to a certain extent in order to drive the load. R6 and C4 are connected in series to form a correction network for phase compensation of the circuit.

The single-chip microcomputer system is mainly composed of MC14433 and 8031. MC14433 is a dual-integral A/D conversion chip, which is connected to the 8031 ​​single-chip microcomputer as shown in the figure. The conversion result QQ of MC14433 is connected to P1.0-P1.3 of 8031, and the strobe output pulse DS1-DS4 is connected to P1.4-P1.7 of 8031. The conversion result flag EOC is connected to the update conversion control signal input line DU on the one hand, and to the interrupt input line INT1 of 8031 ​​on the other hand, indicating that the single-chip microcomputer can use the interrupt method to read the A/D conversion result or the query method. The final result is sent to 74AC138 and drives the digital tube to display the specific value [3][4].

2 Detection Processing Procedure Flowchart

The flowchart of the detection processing program is shown in Figure 4. The MCS series assembly language〔5〕 is used for programming. Due to the guarantee of hardware design, the entire system can work in both loop query mode and interrupt management mode. function ImgZoom(Id)//Reset the image size to prevent the table from breaking { var w = $(Id).width; var m = 650; if(w

3 Conclusion

This design has been successfully used in the flower demonstration garden of Yan'an Agricultural Science Institute, and the operation effect is good. Practice has proved that this detection circuit is simple to operate, has numerical display, is small in size and easy to carry, is very intuitive, and can detect continuously and quickly. It can detect the content of carbon dioxide gas in various occasions indoors and outdoors at any time.

Figure 4 Detection processing flow chart

This circuit is simple in design, low in price, and highly universal, overcoming the shortcomings of frequent calibration, short service life, and high price.