Design of indoor air renewal system for sky garden based on STC12C65A60S2

    In order to solve the problem of air quality degradation in indoor air environment caused by long-term operation of central air conditioning, the air in the closed room cannot circulate. The concept proposed in this method is based on STC12C65A60S2, which uses sky gardens (i.e., building large green plant gardens on urban buildings) to purify and filter air, and further purify it with negative ion generators. When the air index reaches the preset index, it is then transported through pipelines to closed spaces such as offices, shopping malls, and home users for fresh air replenishment to achieve indoor air renewal. The system has the characteristics of simple circuit structure, low production cost, good economic benefits, and good air purification effect, meeting the basic requirements of the design.

    With the development of urban economy, central air conditioning has become an indispensable refrigeration appliance for various large commercial buildings such as office buildings, large shopping malls and hotels. However, one problem that all air conditioners will bring is that the air is not circulated and the indoor air quality is reduced due to the internal circulation of the indoor closed environment for a long time, which causes conditions such as "air conditioning disease". According to statistics from relevant experts, in an airtight room with air conditioning, after 5 to 6 hours, the indoor oxygen level dropped by 13.2%, the Escherichia coli increased by 1.2%, the red mold increased by 1.11%, the diphtheria increased by 0.5%, and other harmful respiratory bacteria increased to varying degrees. It is precisely those who work and live in such an environment for a long time that often get "air conditioning disease" unknowingly. It is based on the idea of ​​"air circulation". By allowing the outdoor air to pass through the purification and oxygen enrichment treatment of garden plants, the fresh air with suitable temperature and humidity is obtained, and the control circuit built is combined to provide air renewal services to users at the right time and under the right air renewal demand conditions. Improve indoor air quality. At the same time, centralized air renewal is more economical, and users only need to install air ducts and air valves.

1 System structure and hardware circuit design
    The system is based on STC's 51 enhanced single-chip microcomputer STC12C65A60S2 chip microcontroller as the core, considering that the performance of STC12C65A60S2 is sufficient to meet the circuit working requirements and the low cost characteristics. STC12C65A60S2 is selected as the best solution; the conceptual model is divided into: 1) Display part (12864 display screen), which is used to display the basic parameters of the garden and indoor greenhouse temperature, soil humidity inside the garden, and the user room number that is performing air update in real time; 2) Control and detection part (host computer written in VB language) is used to display various parameters of air freshness inside the air storage tank in real time: temperature, humidity, CO2 concentration, negative ion concentration, and the parameter value of the comprehensive air freshness obtained after the algorithm. We divide the value range into high, good, poor, and bad to divide the processed air level, and display the air quality level with different color indicators. There are control buttons on the host computer to control the mode and object of the system operation and set the working time. 3) Sensor circuit part, which is used to detect air quality parameters. The overall scheme of the system is shown in Figure 1.

    
1.1 DS1302 digital clock circuit The
    DS1302 clock circuit is used here to set the time point for changing the gas of the air renewal system and the duration of the ventilation work.
    DS1302 is a high-performance, low-power, real-time clock chip with RAM launched by DALLAS, USA. It can count the year, month, day, weekday, hour, minute, and second, and has a leap year compensation function. The working voltage is as wide as 2.5 to 5.5 V. The clock can work in 24-hour format or 12-hour (AM/PM) format. The interface between DS1302 and the microcontroller uses synchronous serial communication, and only three lines are used to connect it. The clock signal or RAM data can be transmitted one byte at a time or in burst mode to transmit multiple bytes at a time. There is a 31×8 RAM register inside the DS1302 for temporary storage of data. DS1302 is an upgraded product of DS1202 and is compatible with DS1202, but it adds dual power pins for main power supply /back power supply, and provides the ability to charge the back power supply with trickle current.

    The data is written into DS1302 at the rising edge of the next SCLK clock after the control instruction word is input, and the data input starts from the low bit, that is, bit 0. Similarly, the data of DS1302 is read out at the falling edge of the next SCLK pulse following the 8-bit control instruction word, and the data is read out from the low bit 0 to the high bit 7. The data read and write timing is shown in Figure 3.

1.2 DHT21 sensor circuit The
    DHT21 digital temperature and humidity sensor is a temperature and humidity composite sensor with calibrated digital signal output. It uses dedicated digital module acquisition technology and temperature and humidity sensing technology to ensure that the product has extremely high reliability and excellent long-term stability. The sensor includes a capacitive humidity sensing element and an NTC temperature measuring element, and is connected to a high-performance 8-bit microcontroller. Therefore, the product has the advantages of excellent quality, ultra-fast response, strong anti-interference ability, and extremely high cost performance. Each DHT21 sensor is calibrated in an extremely accurate humidity calibration room. The calibration coefficients are stored in the OTP memory in the form of a program, and these calibration coefficients are called in the sensor during the processing of the detection signal. The single-wire serial interface makes system integration simple and fast. The ultra-small size, extremely low power consumption, and signal transmission distance of more than 20 meters make it the best choice for various applications and even the most demanding applications. The product is a 4-pin single-row pin package. It is easy to connect and special packaging forms can be provided according to user needs. The circuit is shown in Figure 4.

    
    The power supply voltage of DHT21 is 5 V. After the sensor is powered on, it needs to wait for 1 s to pass the unstable state. No instructions need to be sent during this period. A 100 nF capacitor can be added between the power pins (VDD, GND) for decoupling and filtering. Single bus interface; DATA is used for communication and synchronization between the microprocessor and DHT21. The single bus data format is used, and the communication time is about 5 ms. The specific format is described below. The current data transmission is 40 bits, and the high bit is first out.
    Data format: 40 bit data = 16 bit humidity data + 16 bit temperature data + 8 bit checksum Example
    : Receive 40 bit data as follows:
    0000001010001100 0000000101011111 11101110
    Humidity data Temperature data Checksum
    Humidity high 8 bits + humidity low 8 bits + temperature high 8 bits + temperature low 8 bits = last 8 bits = checksum For
    example:
    00000010+10001 100+00000001+01011111=11101110
    Humidity = 65.2%RH Temperature = 35.1℃ When the temperature is below 0℃, the highest position of the temperature data is 1. For example: -10.1℃ is represented by 1000000001100101. After the user host (MCU) sends a start signal, DHT21 switches from low power mode to high speed mode. After waiting for the host start signal to end, DHT21 sends a response signal, sends out 40 bits of data, and triggers a signal acquisition. (Note: The temperature and humidity data read by the host from DHT21 is always the previous measurement value. If the interval between two measurements is very long, please read twice in succession to obtain the real-time temperature and humidity values)

2 Main software design
    software is mainly programmed in VB6.0. VB6.0 is a visual programming language developed by Microsoft with integrated visual database tools. Visual Bas IC 6.0 provides a complete set of tools and integrates databases in any application. It has powerful data access, internet programming, use of new controls, use of enhanced controls to improve performance and enhanced wizard functions. The upper computer software written in VB6.0 is mainly used to display the quality parameters of the garden and indoor air in real time, and as the control end of air renewal processing. The flow chart of the system software design is shown in Figure 6.

    

3 System Application
    The system is applied to indoor air renewal. The system selects the time point for air renewal and the duration of each air replacement by setting the clock (DS1302) or manually. At the same time, a photoresistor is used to qualitatively detect the light intensity in the current greenhouse. When the light intensity is weak, the respiration intensity of plants in the garden is stronger, so a common air inlet is used instead to ensure the oxygen content of the transported air. Temperature and humidity sensors (DHT21) are set at multiple points to compare the difference in air temperature between the garden and the indoor air. According to the collected data, the processor will select the appropriate air pretreatment method (such as humidification and dehumidification, heating and cooling). The air storage bin is equipped with a CO2 sensor to detect the CO2 concentration of the air sent from the garden to the air storage bin. If the CO2 concentration exceeds the preset value, the air will be sent back to the garden for a new oxygen enrichment process. The negative ion generator is used for the second air purification of the air in the air storage bin to increase the negative ion concentration of the air and further remove harmful particles in the air. Result: After the outdoor air is purified, the air that meets the purification index is transported to the user's room, and the air that does not meet the index conditions is sent back to the garden and air pretreatment is performed. Finally, the air flow in the closed indoor environment under the central air conditioner is updated regularly. In addition, the system is equipped with a soil moisture sensor, which is combined with a simple intelligent irrigation system composed of a solenoid valve. When the soil moisture is lower than the set value, the solenoid valve opens the pot head to water the garden plants, replenishing the water needed for growth and realizing the basic maintenance of the garden. Through actual application, it is found that the test results of the test system are accurate, stable and reliable. Part of the interface of the program panel is shown in Figure 7.

    

4 Conclusion
    The air renewal system of this system uses plants combined with electronic purification instruments to achieve indoor air renewal; the software design adopts modular design ideas to improve the reliability and maintainability of the system. While improving the air environment of large closed indoor environments under the background of central air conditioning, it embodies the harmony between man and nature. At the same time, centralized air purification is more energy-saving and environmentally friendly than individual purification by users. Practical applications show that the test system has the characteristics of accurate testing, stable and reliable, and friendly human-computer interface, which meets the design requirements.