Scheme design of CO monitoring and alarming instrument based on single-chip microcomputer and CO gas sensor

introduction

Carbon monoxide (CO) is a colorless and odorless gas. When CO enters the human blood circulatory system, it will quickly combine with hemoglobin to form carboxyhemoglobin, which occupies the position of binding oxygen, so that hemoglobin loses the function of transporting oxygen, leading to C0 poisoning due to insufficient oxygen supply. The important hazard of C0 is air pollution and human health. People can have headaches, tinnitus, fatigue and other symptoms even in a low-concentration CO environment. If CO poisoning is severe, it can impair thinking and feeling in mild cases, and weaken physical activity, while in severe cases it can cause brain damage and even death. Therefore, designing a CO monitoring alarm with high sensitivity, reliable performance, and simple operation, timely, accurate and effective detection of CO in living conditions and industrial production processes, is an important issue related to human life safety.

1 System design plan

1.1 System composition

C0 gas monitoring alarm is composed of CO gas sensor, conditioning circuit, single chip microcomputer, display circuit, buttons, signal output circuit and application system software. The block diagram of CO gas monitoring and alarm system design is shown in Figure 1.

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1.2 Selection of MCU

ADuC816 single-chip microcomputer is a new type of single-chip microcomputer introduced by ADI in the United States. It is a high-performance micro-converter that combines ADI’s years of experience in producing A/D and D/A converters with mature 8051 single-chip technology. It has very powerful functions. ADuC816 integrates 2-channel 16-bit ADC, 12-bit DAC, 3 timer/counters, 10 interrupt sources/2 priority interrupt levels, dual sensor excitation current source, 8 KB Flash EEPROM program memory, 256 bytes on-chip RAM and 640 Byte data Flash EEPROM. The external data memory is addressed in groups, and the address space is up to 16 MB. Therefore, the selection of ADuC816 microcontroller can not only meet the high requirements of CO gas monitoring and alarm instruments, but also does not require external A/D, D/A converters and external memory. This is of great significance for simplifying the peripheral circuit design of the entire system and improving the anti-interference ability of the system.

1.3 Selection of CO gas sensor

There are many types of CO gas sensors. The semiconductor CO gas sensor mainly uses oxide semiconductor as the basic material to adsorb gas on the surface of the semiconductor, and uses the resulting electrical conductivity change to determine the CO gas concentration. Compared with other gas sensors, the semiconductor CO gas sensor has the advantages of fast, simple, and sensitive, but its gas selectivity is poor, and it is susceptible to interference from other gases during operation. The electrochemical potential CO gas sensor can convert the gas to be measured into an electrical signal through an electrochemical reaction for direct detection. Among them, in order to improve the measurement and eliminate the influence of some uncertain factors in the measurement environment, a third electrode, a reference electrode and an external constant potential working circuit can be used. This type of sensor uses a dilute sulfuric acid solution as a liquid electrolyte, which can not only work with other measurement and control equipment at room temperature, but also has a large output signal amplitude, high sensitivity, convenient use, and low price. At the same time, it can avoid direct contact between CO and oxygen. And there is a danger of explosion. The schematic diagram of the structure is shown in Figure 2.

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7E/F type CO gas sensor is electrochemical potential type. It detects the electrode potential related to the volume fraction or partial pressure of the gas to be measured, and its influence mechanism is based on the electrochemical equilibrium established by the electrochemical oxidation-reduction reaction that occurs on the electrode. The half-cell chemical reaction equation is expressed as follows:

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"The current generated by the above reaction is proportional to the concentration of CO gas. The measured current is converted into a voltage signal with a standard resistance, and after amplification and A/D conversion, it is directly displayed as the volume fraction of CO on the digital tube.

In addition, the measuring range of the 7E/F CO sensor can reach 0～2 000×10-6, and the gas selectivity is very good. When the mixed gas containing CO and H2 is passed into the sensor, the sensitive electrode will generate a signal proportional to the concentration of the two gases, while the signal generated by the auxiliary electrode is mainly due to the presence of hydrogen ions. In addition, the built-in filter can remove the interference of N0, NO2, H2S and SO2. Even if there are one or two interfering gases, very accurate measurement results can be obtained. The measurement error, response time, repeatability, etc. all meet the CO monitoring alarm in this article The requirements of instrument design.

1.4 Selection of signal amplifying circuit scheme

Because the selected gas sensor senses the change of CO gas concentration, it converts the concentration information into a current signal. Therefore, a dedicated signal amplifying circuit must be used, and the requirements for the circuit are very strict. Based on this requirement, the three electrodes used are: working electrode (S), counter electrode (C), and reference electrode (R). CO sensor signal amplifying circuit is shown as in Fig. 3.

2 Hardware circuit design

Hardware circuit design includes: power supply circuit design, program circuit design, display and button circuit design, alarm circuit design and other circuit (reset circuit, clock circuit) design.

2.1 Power supply circuit design

The main task of the power supply is to first convert the 220 V AC power to DC +9 V through measures such as rectification and filtering, and then stabilize the +9 V voltage to +5 V through the three-terminal voltage stabilizer 78M05 and supply it to the ADuC816 microcontroller and its peripherals. Each chip is used, and the circuit is shown in Figure 4.

2.2 Program Circuit design

ADuC816 single-chip microcomputer contains a full-duplex serial interface, and the RS-232C standard is used in serial communication. During communication, MAX232 chip must be used for level conversion. The circuit is shown in Figure 5. This circuit cooperates with the J4 terminal of the 8051 read program memory dedicated control line PSEN to complete the program from the computer to the 8 KB Flash EEPROM program memory in the ADuC816 chip. When programming, it is necessary to turn off the power of the microcontroller system, plug in the short-circuit block of J4, and then power on the microcontroller system, and the ADuC816 enters the program state. Run the computer software to transfer the HEX file RXD to the ADuC816 TXD. This is the convenience of this measurement and control system. When modifying the program, neither an expensive programmer nor the opening of the casing to plug in and out the chip is required.

2.3 Display and button circuit design

The CO gas monitoring alarm must have a circuit that displays the CO concentration in the gas to be measured. The display circuit adopts the serial-parallel conversion chip 74LSl64 and the digital tube LED. The button circuit is composed of buttons and pull-up resistors. The main function is to set the alarm point value. The circuit is shown in Figure 6.

2.4 Circuit design of alarm function

When the CO gas concentration reaches the alarm point, an alarm is given through two red LED light-emitting diodes and a buzzer. Two of the red LEDs represent alarm and secondary alarm respectively. The output alarm circuit controls the buzzer to alarm by controlling the relay. The control voltage of the relay is required to be 5 V, and the controlled voltage is 220 V.

3 Software design

Software design includes A/D conversion, display, button setting, etc. The main flow is shown in Figure 7. There are three keys in total, namely the setting key, the plus key and the minus key. Use different keys to control the instrument to set different functions. The process is shown in Figure 8.

4 Calibration

4.1 Homing

When the CO gas monitoring alarm does not start to work or is placed in the air without CO, the concentration value displayed after the A/D conversion is not necessarily zero. At this time, no matter what the value is, it is regarded as zero, and this value is regarded as the "zero point" of the instrument. The actual measured value is obtained through [(current A/D value-zero value)/coefficient] through the button, display, and calibration module flowchart in Figure 8.

4.2 Calibration

The calibration work line can be expressed by the linear equation y=Kx+b. Due to the influence of the gas sensor 7E/F and the amplifier circuit used in the design, the 2.5 V voltage is used as a reference for measuring the CO gas concentration, and the corresponding calibration fits The straight line is:

Y=K(2.5-x)+b

Among them, the value range of x is 0～2.5 V, and the value range of y is 0～2 000×10-6. After several sets of data are measured after calibration with standard CO gas, the values of K and 6 can be obtained by the method of undetermined coefficients.

Conclusion

CO, as a highly toxic gas, pollutes the atmosphere and affects human health. Through the debugging and use of the alarm instrument designed in this article, the entire instrument runs stably, and the required functions have been basically realized. When the instrument is running, test with a mixed gas containing CO, it can display the concentration of CO in the gas, and perform a secondary alarm.