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The temperature alarm monitoring system is controlled by a single-chip microcomputer to control multiple temperature acquisition modules and collect data from multiple points. After corresponding processing, it is reported via SMS. In practical applications, there are many occasions where the ambient temperature needs to be monitored.
1 System structure
The mobile phone SMS temperature alarm system is mainly divided into four parts: power supply part, GSM module, temperature acquisition and single chip microcomputer. The system can monitor whether the ambient temperature is within the set range. If it exceeds the range, it can send an alarm message to the set mobile phone number via SMS. Its system structure is shown in Figure 1. 
This temperature acquisition part adopts AD590 to collect temperature, adopts precision resistance to perform I / V conversion and then amplifies, and adopts ADC0809 to perform analog-to-digital conversion. The power supply part converts the AC 220V to DC 24V and then to 5V through the transformer for power supply to other parts. The single chip microcomputer control adopts AT89C51ED2 single chip microcomputer to collect data and control the GSM module. The GSM module uses Siemens' TC35T integrated module.
2 Hardware system design
2.1 TC35T module
TC35 is a new generation of wireless communication GSM module launched by Siemens. It can quickly, safely and reliably implement data, voice transmission, short message service (Short Message Service) and fax in system solutions. The working voltage of this module is 3.3 ～ 6.5 V, and it can work in two frequency bands of 900 MHz and 1 800 MHz. The power consumption of the frequency band is 2W (900MHz) and 1 W (1800 MHz) respectively. The module has an AT command set interface, supports text and PDU mode short messages, the third group of Type 2 faxes, and 2.4 k, 4.8 k, and 9.6 k non-transparent modes. In addition, the module also has a phone book function, can talk to multiple parties, and has a roaming detection function, its common working modes are power saving mode and IDLE, TALK and other modes. The unique 40-pin ZIF connector enables power connection and bidirectional transmission of commands, data, voice signals, and control signals. Through the ZIF connector and the 50 Ω antenna connector, the SIM card holder and the antenna can be connected separately.
The TC35 module is mainly composed of six parts: GSM baseband processor, GSM radio frequency module, power supply module (ASIC), flash memory, ZIF connector, and antenna interface. As the core of TC35, the baseband processor mainly processes the voice and data signals in the GSM terminal, and covers all analog and digital functions in cellular radio frequency equipment. Without the need for additional hardware circuits, it can support FR, HR and EFR voice channel coding. The structure of TC35 is shown in Figure 2. 
TC35T is a GSM dedicated modem launched by SIEMENS. TC35T has implemented TC35 in industrial mobile phones, which can complete the transmission of voice, data, short message and fax in GSM network. TC35T has a standard industrial interface and a complete SIM card reader, so it is very simple to use. The main features of TC35T are as follows:
â—‡ Can work in GSM900 / GSM1800 dual frequency band;
â—‡ With voice, data, SMS fax transmission function;
â—‡ With analog voice interface;
â—‡ Can read and write SIM card;
â—‡ With standard RS232 interface;
◇ Working range voltage 8 ～ 30 V;
â—‡ Support GSM phase2 / 2 +;
â—‡ Support AT command.
2.2 AT89C51 microcontroller
AT89C51ED2 is a high-performance CMOS FLASH version of CMOS single-chip 8-bit microcontroller 80C51. AT89C51ED2 contains a 64-Kbyte FLASH storage area, which can be used to store code and data. The memory area can be programmed in parallel mode, or in serial mode through ISP or software. The programming voltage is generated internally through the standard Vcc pin. AT89C51ED2 retains all the functional characteristics of Atmel 80C52, including 256 bytes of internal RAM, a 9-source 4-level interrupt controller and 3 timers / counters. AT89C51ED2 provides 2048 bytes of EEPROM, which can be used for non-volatile storage of data. In addition, AT89C51ED2 also contains a programmable timer array and a 1792-byte XRAM, a hardware watchdog timer, SPI interface, keyboard interrupt, and a more general-purpose serial interface to facilitate multiprocessor communication (EUART) and acceleration mechanism (X2 Mode).
The fully static design of AT89C51ED2 can reduce system power consumption. It allows the clock frequency to be reduced to any value (including DC) without causing data loss. AT89C51ED2 has two software-selectable operating modes, so it can reduce the active state of the 8-bit clock divider and further reduce power consumption. In idle mode, the CPU is frozen, and peripherals and interrupt systems continue to work. In power-down mode, the RAM contents are saved, and all other functions are invalid.
As can be seen from the above description, AT89C51ED2 can meet the requirements of this system. Other features of AT89C51ED2 are as follows:
â—‡ Compatible with 80C52;
â—‡ Compatible with 8051 instruction system;
â—‡ There are 6 8-bit I / O ports (64-pin or 68-pin version);
â—‡ There are 4 8-bit I / O ports (44-pin version);
â—‡ There are three 16-bit timers / counters;
â—‡ With 256 Bytes Shaanxi cache RAM;
â—‡ It has 9 interrupt sources and 4-level interrupt priority;
2.3 AD590 temperature sensor
The AD590 is a 2-port integrated temperature sensor that can produce an output current proportional to absolute temperature. Within the power supply voltage range of 4 to 30 V, the AD590 can be used as a high-impedance constant current regulator with a ratio of 1 μA / K. The AD590 can be used in temperature sensing applications below 150 ° C, which is currently the working range of conventional electronic temperature sensors. The inherent low cost of monolithic integrated circuits, coupled with the absence of peripheral support circuits, make the AD590 the most attractive option for many temperature measurement occasions. In addition, linear circuits, precision voltage amplifiers, thermal resistance measurement circuits, and cold junction compensation circuits are no longer required in the AD590 application. The characteristics of AD590 are as follows:
â—‡ Linear current output: 1μA ï¼ K;
◇ Wide temperature range: -55 ℃ ～ + 150 ％;
â—‡ The power supply voltage range is 4 ~ 30V, can withstand 44V forward voltage and 20V reverse voltage;
â—‡ With high output impedance, can withstand power supply drift and ripple;
â—‡ With multiple precisions to choose from.
3 Software design
3.1 Overall process
After powering on, the main program first initializes the microcontroller and GSM module. The initialization of the single chip microcomputer includes setting the serial port working mode, baud rate, and initializing variable parameters and flag bits. GSM module initialization includes restarting, closing the display, setting to not display detailed header information in the return value in TEXT mode, selecting the text message format as TEXT mode, opening the serial port to prepare to receive data, and then starting ADC to collect temperature data, and then proceed Data processing, and finally judge whether to alarm. The overall software flow of its system is shown in Figure 3. 
3.2 TC35T control commands
The TC35T modules of SIEMENS all support the AT command set specified in GSM07.05. Using its serial interface, the single-chip microcomputer can send a series of AT commands to the module to achieve the purpose of controlling the module to send SMS. When using a single-chip microcomputer, programming must pay attention that the commands it sends and the responses it receives are ASCII codes of characters. The AT commands involved in sending and receiving short messages using the single-chip microcomputer to control the GSM module are listed in Table 1. 
The GSM short message service SMS mainly uses signaling channels for transmission, which is unique to the GSM communication network. It does not need to dial to establish a connection, but only sends the information to be sent plus the destination address to the short message service center, and then sends it to the final destination after the SMS service center has completed the storage. Therefore, when the destination GSM terminal is not turned on, general information will not be lost. The amount of information per SMS is limited to 160 bytes. The subroutine flow chart of sending SMS by controlling TC35T through single chip is shown in Figure 4. 
4 Conclusion
This article focuses on the use of SMS in the GSM network to give a temperature alarm monitoring system, and gives specific hardware and software implementation methods. The system can not only realize the traditional temperature alarm, but also use the GSM network to realize remote alarm and multi-point temperature monitoring. In the actual production and life, many occasions need to monitor the temperature, so the system has a good application prospect.
GSM (GlobaI System for Mobile communicaTIon) system is a relatively mature and perfect, and the most widely used mobile communication system based on time division multiple access technology. The GSM-based SMS Short Message Service is a wireless application that transmits short messages on the mobile network, and is a process of storing and forwarding information on the mobile network. Since the public GSM network has achieved networking and roaming worldwide, the establishment of the above system does not require the establishment of a dedicated communication network, so the SMS application with real-time data transmission has been rapidly popularized.