Simple design of transmission type visibility measurement device

Visibility refers to the visible distance of the target object, which refers to the maximum distance at which the target object can be distinguished from the background when observing the target object. In recent years, due to various reasons, atmospheric visibility has become lower and lower around the world, the greenhouse effect has become increasingly obvious, haze, sandstorms, etc. have become hot topics, and the production of the national defense industry and the daily life of the people have been greatly affected. It is urgent to strengthen the measurement of CO2 and visibility in the air.

At present, the visibility instruments that have been developed internationally include two types: transmission type and scattering type, but the research on visibility measuring instruments in my country is not yet mature.

This paper proposes a simple design of a transmission type visibility measurement device, which is also equipped with a CO2 detection function. Using a single chip microcomputer control, the device integrates measurement and display. The instrument is simple and practical, and the measurement results are clear at a glance. It is small and easy to carry, with good real-time data and great flexibility. Experimental verification shows that this solution basically meets the needs of daily life.

1 Principle and block diagram of the measuring device

The device includes a visibility measurement system and a CO2 measurement system, as shown in Figure 1.

The attenuation of light is caused by the scattering and absorption of particles along the path of the light beam. In the visibility measurement system, the laser tube emits lasers, and the silicon photocell detects the light transmittance of the horizontal air column in the sampling space. The light signal is converted into a voltage signal. At this time, the change in voltage reflects the change in visibility. This device controls multiple LED lights through a single-chip microcomputer to directly reflect the visibility of the measured environment. In the CO2 measurement system, the CO2 sensor uses MG811, which uses the principle of solid electrolyte batteries. The potential difference (EMF) between the sensitive electrode and the reference electrode of the sampling sensor is input into the single-chip microcomputer after amplification and temperature compensation. The formula is corrected and converted into a CO2 concentration value and displayed on the LCD screen. A buzzer is used to alarm for high concentration CO2 conditions.

2 Hardware and Software Design

2.1 Hardware Design

The hardware circuit of the device can be divided into three parts: signal acquisition circuit, control circuit and display circuit. The main functional block diagram of the device is shown in Figure 2.

The signal acquisition circuit includes a laser transmitter, a photosensitive receiver and a CO2 sensor circuit, which are used to collect the signal to be tested. The control circuit is a signal amplification circuit and a data control processing unit, which are used to analyze and process the sampled data. The display circuit includes a visibility grading display circuit, a CO2 concentration display and an alarm circuit, which outputs the results in real time and directly reflects the visibility and CO2 concentration of the current environment. The main functional circuit diagram of the system is shown in Figure 3.

2.1.1 Signal acquisition circuit

The laser emitting end uses a laser emitting tube that emits red light with a peak wavelength of 650.0 nm. The fine-tuning instrument is used to align the receiving end and is located at one end of the sampling tube.

The receiving end uses a silicon photoelectric color-sensitive diode with an anti-reflection film with a peak wavelength of 650.0 nm to prevent stray light interference and is located at the other end of the sampling tube.

The laser emitted from the transmitter is received by the silicon photoelectric color-sensitive diode through the sampling space and converted into an electrical signal, which is amplified to the range of 0~3.5 V by the NE5532 adjustable amplifier circuit and sent to the microcontroller for processing. The adjustable amplifier circuit amplifies the signal voltage by about 200 times.

The CO2 sensor circuit and pin diagram are shown in Figure 4, including the MG811 CO2 gas sensor probe, LM393 voltage comparator, etc. The MG811 tests the CO2 concentration range from 0 to 10,000 ppm and can detect the CO2 concentration value in various environments.

2.1.2 Control Circuit

The device is controlled by the chip STC12C5A60S2 with built-in A/D conversion function, which is a low-power, high-performance CMOS 8-bit microcontroller that can provide sufficient IO ports for peripheral circuits. This design configures a 11.059 2 MHz crystal oscillator as the working clock of the main system to realize the collection and analysis of visibility and CO2 data and the control of the display part.

2.1.3 Display Circuit

The display circuit includes visibility display, CO2 concentration display and alarm circuit. The signal sampled by the visibility acquisition device is controlled by the control circuit to display the result on the LED light. In order to more intuitively display the visibility, the device sets the visibility from small to large to a 6-level display structure, with two red, yellow and green LED lights, respectively. The red light indicates very low visibility, the yellow light indicates low visibility, and the green light indicates good visibility.

The CO2 concentration display circuit is implemented with LCD1602 liquid crystal display. LCD1602 character liquid crystal display module is a dot matrix LCD specially used to display letters, numbers, symbols, etc. The single chip microcomputer controls the module to display CO2 concentration. When the visibility is lower than a preset value, the buzzer alarms.

2.2 Software Design

Figure 5 is a system flow chart. First, the LCD, A/D conversion and serial port are initialized, and then the visibility and CO2 concentration data collected by the sensor are converted into digital signals recognizable by the microcontroller through A/D conversion. The visibility is displayed through the LED, the CO2 concentration is displayed through the LCD, and the buzzer reflects whether the CO2 is higher than the preset value.

3 Experimental results and analysis

In order to more effectively test the measurement range and sensitivity of the device, dry ice was selected as an experimental auxiliary material. Dry ice sublimates in the air and produces a large amount of gaseous CO2 in a short period of time to simulate different CO2 concentrations in an unknown environment. At the same time, white smoke is produced to change visibility. The results obtained from the experiment are shown in Figures 6 and 7.

Comparing the changes of the sampling points in the two figures, it can be seen that the visibility dropped to the lowest during the period of high CO2 concentration, which is very consistent with the actual situation. At the same time, it can be seen that the visibility recognition of the device is sensitive, and the results also show that the device has good adaptability under high CO2 concentration conditions.

4 Conclusion

This paper proposes a design scheme for a transmission-type visibility measurement device that is easy to operate and has variable parameters, which realizes the graded detection of atmospheric visibility. When other external conditions remain unchanged, the lower the visibility, the smaller the induced DC voltage. At the same time, a CO2 detector is added to the device, which can detect the CO2 content in the environment while measuring the visibility in the atmosphere. The device has clear physical principles, simple structure, easy operation and adjustable parameters. It also has strong scalability. By adding other different harmful gas sensors, various harmful substances in the atmosphere can be detected in real time.