Design of a temperature control system for automobile air conditioning

With the rapid development and maturity of wireless communication and information sensing technology, it has become possible to connect objects into the Internet of Things through information sensing equipment and networks to achieve services such as automatic identification, positioning, tracking, monitoring and management of objects. The application of Internet of Things technology in the national economy is becoming more and more extensive. In recent years, it has attracted widespread attention from academia and industry. It has become a global hot issue. Many countries have raised the development of the Internet of Things to a national strategic level. This article proposes an application design based on the GSM wireless network, which combines the user's mobile phone and car air conditioner into a small Internet of Things through a temperature sensor. The air conditioner in modern cars is generally controlled manually. The air conditioner can only be turned on or off when the driver enters the cab. This makes people feel abnormally hot or cold when they just enter the car in hot summer or cold winter because the air conditioner is not turned on. Therefore, it is on the agenda of people's daily life to design and manufacture equipment that can monitor the temperature in the car and remotely control the opening of the car air conditioning system in advance according to the monitored temperature. This design is proposed to meet this requirement.

1 System composition and working principle

The system consists of two parts. One part is the temperature acquisition and relay control part centered on the STC89C52RC microcontroller, and the other part is the remote data transmission part composed of the GSM mobile communication network, TC35I and the user's mobile phone. The system block diagram is shown in Figure 1.

Figure 1 System block diagram

The working principle of the system is: the user sends a temperature query command through the mobile phone, and the command is sent to the TC35I installed in the car in the form of a short message through the GSM short message service center. After receiving the command, the module transmits the command to the STC89C52RC microcontroller through the serial port. The microcontroller starts DS18B20 to collect the temperature information in the car, and then transmits the collected temperature information back to the user's mobile phone through the microcontroller and TC35I through the GSM short message service center. If the temperature in the car is too high or too low, the user can control the relay switch in the car through the command to turn on the air conditioner in the car to achieve the effect of cooling or heating.

2 System Hardware Design

The hardware design part of the system includes microcontroller module, DS18B20 temperature acquisition module, relay circuit switch control module, power supply module and TC35I module.

2.1 Microcontroller Module

The microcontroller module mainly completes temperature acquisition, relay switch control and serial communication with TC35I. Considering the driving ability, power consumption and cost performance of the module, the system uses the STC89C52RC chip of Hongjing Company, which has the advantages of low power consumption, strong control and anti-interference ability, and high cost performance. The microcontroller has 8 KB FlashROM memory, 512 B RAM and 2 KB EEPROM memory. It also integrates watchdog circuit and UART. It has the functions of in-system programming and in-application programming. It does not require a special emulator and programmer. Therefore, the selection of this controller can bring great convenience to the design of this system.

2.2 Temperature acquisition module

The temperature acquisition module in the system uses the high-precision and high-reliability DS18B20 temperature sensor produced by DALLAS. It has the characteristics of small size, low hardware cost, strong anti-interference ability and high precision. It uses single bus data communication, full digital temperature conversion and output, with a maximum resolution of 12 bits, an accuracy of ±0.5℃, and a detection temperature range of -55℃~+125℃, so it can meet the design requirements of this system. The connection circuit between DS18B20 and microcontroller is shown in Figure 2.

Figure 2 Connection circuit diagram of STC89C52RC microcontroller module and DS18B20

2.3 Relay switch control module

The relay switch module is composed of TLP521-4, ULN2803, SRD-12VDC and transistors. The signal output by the microcontroller is sent to the TLP521-4 optocoupler chip through the switch circuit composed of transistors, and then amplified by the ULN2803 Darlington tube to drive the SRD-12DC relay, thereby achieving the function of controlling various switches of the air conditioner. The circuit connection diagram of the relay switch control module and the microcontroller is shown in Figure 3.

Figure 3 Relay control module circuit diagram

2.4 TC35I Module

The TC35I module is a dual-band 900/1800 MHz highly integrated GSM module launched by Simens. It is compact in design and has very low power consumption, and can provide cost-effective solutions for many communication applications. It supports EGS900 and GSM1800 dual-band, and the data transmission content supports voice, data, short message and fax services. The communication interface uses RS232 (two-way transmission of instructions and data), and the power supply uses a single power supply voltage of 3.3 V~5.5 V. The scope of application includes: low-power communication equipment for portable computers, telemetry and remote sensing, remote information processing and communication and other industrial fields. The circuit connection diagram of TC35I and microcontroller in this system is shown in Figure 4.

Figure 4 TC35I module circuit connection diagram

2.5 Power Module

The system power module uses two chips, LM78L05 and LM2941S, to convert the external 12 V DC power supply voltage into the 5 V and 4.2 V voltages required by the system. The power connection circuit diagram is shown in Figure 5.

Figure 5 Power supply connection circuit diagram

3 System software design

The software module part of the system mainly includes the software design of GSM communication module, DS18B20 temperature acquisition module and relay switch control module. The system workflow is shown in Figure 6.

Figure 6 System workflow diagram

3.1 Programming of GSM Communication Module

The program of the GSM communication module is to send AT commands related to GSM short messages through the serial port of the microcontroller by TC35I to interact with the user's mobile phone. The specific workflow is shown in Figure 7. In addition, the AT commands [2, 5] executed in this system are shown in Table 1.

Table 1 AT commands executed in the system

3.2 Programming of DS18B20 Temperature Acquisition Module

The programming of the temperature acquisition module is also mainly completed by the microcontroller through a single bus to complete the initialization of DS18B20 and read the temperature information according to the instructions sent by the user's mobile phone and feed it back to the user's mobile phone. The work flow chart is shown in Figure 8.

The software design of the relay switch control part is mainly based on the switch instructions sent by the user from the mobile phone, which is achieved by the microcontroller by setting or resetting the relevant ports.

Figure 7 TC35I workflow diagram

Figure 8 DS18B20 workflow diagram

This paper proposes a design method for a remote automobile air-conditioning temperature control system based on the GSM short message center. After actual production and debugging, it is verified that the system can operate stably and reliably. The system is also easy to expand and has a long wireless transmission distance, and can be widely used in the field of remote control.