Design of Gas Alarm Based on Single Chip Microcomputer

Introduction

At present, with the continuous development of mining technology, the safety of underground operations is increasingly guaranteed, but there are still many mining enterprises with low mechanization levels, which pose a potential threat to the life safety of on-site mining workers. In particular, there are still hidden dangers in the detection and alarm of gas. There are still many major accidents caused by gas leakage every year. Gas is a gas formed in the coal-forming process and stored in large quantities in the coal seam. It is the most harmful gas in coal mines. Gas is a colorless and odorless gas, the main component of which is methane (CH4), with a density of 0.716kg/m3. The harm to the human body is that it can cause suffocation and death if it exceeds the time limit. During underground mining, a certain amount of CH4, CO and SO2 gases often leak in the well. The latter has a small content and is easily soluble in water. After being treated by water spraying during coal mining, it becomes acid. However, the first two gases have a large content and are almost insoluble in water, and are flammable and explosive gases.

Due to the danger of gas itself and the huge harm it causes to people's production and life, the detection and alarm of gas is a necessary task. Gas alarm refers to the use of gas sensor technology to compare the detected gas concentration with the standard value. When it is higher than a certain concentration value, a corresponding sound and light alarm is issued to remind the personnel who are working to take corresponding measures, organize personnel to evacuate or ventilate the mine to avoid unsafe accidents, which plays a very important role in the safety of the current mining industry. The author has designed a low-cost combustible gas alarm design that can monitor the concentration of gas in the mine, display the measurement results, make judgments on the current environmental status, and issue alarm information.

System Function Design
System Design

The gas alarm designed in this design consists of six parts: sensor, LCD display, sound and light alarm, control circuit, A/D conversion and power module. The gas sensor used in the sensor part can sense a certain gas in the environment and convert information related to the type and concentration of the gas into an electrical signal. This electrical signal is a continuously changing analog signal that needs to be converted into a discrete digital signal through A/D conversion. The control circuit is based on a single-chip microcomputer, which can process and judge the collected digital signals, use a certain algorithm to calculate the composition and concentration of the gas to be detected, and send it to the LCD display for display. When the concentration of the detected gas exceeds the set alarm threshold, an audible and visual alarm is given. This system can store the detected data and the set threshold parameters and has its own power supply. The system block diagram is shown in Figure 1.

Figure 1 Overall functional block diagram

According to the portable requirements of the use environment for product design, the core component of the control circuit uses the AT89LV51 low-voltage microcontroller, and the sensor uses the MQ5 gas sensor. In order to reduce the occupation of the microcontroller port and further expand other functions, the A/D conversion part uses TI's TLC1543, and the LCD display uses dm12232f. The A/D conversion and LCD are connected to the microcontroller in serial mode. The sound and light alarm circuit uses a buzzer and a light-emitting diode for alarm. This design can meet the application of different occasions. The test results are stable and reliable. The 10-bit TLC1543 A/D conversion chip can meet the accuracy requirements of the system measurement. The system circuit is shown in Figure 2.

Figure 2 System circuit diagram

The system uses AT89LV51 microcontroller as the core of the control part. AT89LV51 is a low-voltage, high-performance CMOS 8-bit microcontroller produced by ATMEL. It contains 4K bytes of repeatedly erasable read-only program memory (PEROM) and 128 bytes of random access memory (RAM), which is compatible with the standard MCS-51 instruction system. It has a built-in general 8-bit central processing unit (CPU) and Flash storage unit. AT89LV51 can work under a power supply voltage of 2.7~6.0V. The core device TLC1543 of the A/D conversion part has three control input terminals CS, I/O CLOCK, ADDRESS and a data output terminal DATA OUT that follows the serial peripheral interface SPI protocol. The 51 series microcontroller does not have a built-in SPI interface, but the SPI protocol can be simulated by software. In terms of hardware, the five ports of TLC1543, CS, I/O CLOCK, ADDRESS, DATA OUT, and EOC, are connected to the five I/O ports of AT89LV51. The alarm module uses a buzzer as a sound alarm device to remind users that the current gas concentration has exceeded the warning line and they should stop working immediately and take appropriate measures to avoid danger. It can effectively remind the staff of the working environment around them and help them improve their safety awareness in the alarm module circuit. When the level of the P0.7 port is low, the transistor is cut off; when the level of the P0.7 port is high, the transistor is turned on and the buzzer generates an alarm sound.

In this design, since the main component of gas is methane, the gas sensor used is MQ-5, which is suitable for monitoring devices for liquefied gas, methane (natural gas), and coal gas in households or industries. It has excellent resistance to ethanol and smoke interference, good sensitivity to liquefied gas, natural gas, and city gas; almost no response to ethanol and smoke; fast response recovery characteristics; long service life and reliable stability; simple test circuit, etc. Its structure and appearance are shown in Figure 3.

Figure 3 Structure and appearance of MQ-5

The structure and appearance of MQ-5 gas sensor are shown in Figure 3 (structure A or B). The sensitive element composed of micro AL2O3 ceramic tube, SnO2 sensitive layer, measuring electrode and heater is fixed in a cavity made of plastic or stainless steel. The heater provides the necessary working conditions for the gas sensor. The packaged gas sensor has 6 pins, 4 of which are used for signal extraction and 2 for providing heating current.

The wiring of MQ-5 in the design is shown in Figure 4. In actual measurement, the corresponding correction value can be calculated according to its equivalent circuit, where Ro represents the equivalent resistance of the measured gas in the cavity, and RL is the external load resistance, which is used to adjust the output analog voltage range. The specific value should be determined according to the input range of the A/D converter. The input range of TLC1543 is 0~5V, so RL can be adjusted to this range to ensure that the measurement range is sufficient.

Figure 4 Equivalent circuit of MQ-5

Gas test result calculation

For the calculation of the sensor adjustment parameters, you can follow the steps in Table 1 and make adjustments in combination with the sensor's characteristic table.

Table 1 Methane test calculation

Among them: RL = 20KΩ; Ro = 14.43KΩ; C = 86; RL - load resistance; Ro - sensitive body resistance; C - constant, adjust the display range.

In the process of sensitivity adjustment, the MQ-5 gas sensor has different resistance values ​​for different types and concentrations of gas. Therefore, when using this type of gas sensor, the adjustment of sensitivity is very important. It is recommended to calibrate the sensor with 1000ppm isobutane or hydrogen.

Conclusion

This design considers the convenience of carrying and practical alarm. The finished product can be easily embedded in the mine lamp, and the design cost is low. The design of the intelligent gas alarm for the mine lamp based on the single-chip microcomputer leaves room for the selection of hardware. While adding related hardware, the software part only needs to change a few parts to realize other functions, making the system function more perfect. The A/D converter is 11 channels, which can expand other sensors, such as sensors for monitoring temperature and humidity in the mine. And the system can expand the wireless transmission module to send the detected information to the safety control center in time, and cooperate with other safety monitoring equipment in the mine to better ensure the safety of underground operations.