With the continuous expansion of monitoring scope, traditional wired monitoring alarms can no longer meet the needs of users. However, with the development of the GSM network, the hardware implementation of the remote network monitoring and alarm system has become possible, and it is expected to replace the traditional monitoring and alarm system controlled by a single machine. This paper designs a remote wireless alarm monitoring system based on GSM network for the current needs. The basic idea is: Connect the MCU and SIM300 communication module through the serial port to realize the mutual communication between the MCU and SIM300. This article refers to the address: http:// 1.1 MCU control circuit 3 monitoring alarm process 3.2 Monitoring process 3.3 Control Process Through the debugging of software and hardware, the functions of the system can be well realized, which effectively ensures the safety of the car. The system combines the MCU with the SIM300 function module to make full use of the mature GSM mobile communication network, which overcomes many limitations in the general monitoring system. The idea can also be used in the fields of household appliances and large-scale agricultural production. The solution has many advantages such as saving wiring resources, wide control range, strong versatility and high cost performance. Christmas light
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The SIM300 module features a SIM card slot for communication with a variety of users. The system is connected with vibration, infrared, sound and other types of sensors. When the car is in an alarm situation, the sensor transmits the alarm signal to the single-chip microcomputer. After the MCU judges the processing, it sends an alarm message to the user, or directly dials the alarm phone. If the situation is urgent, the user can directly send the SMS password, and the MCU controls the relay to cut off the line or the oil circuit, so that the car stops running, thereby realizing remote control and effectively ensuring the safety of the car.
1 hardware circuit design
The hardware part of the design includes the MCU main control module, LCD display, button circuit, power supply circuit, signal acquisition circuit and control circuit. The system structure block diagram is shown in Figure 1. 
The system uses the STC89C52 microcontroller, a low-power, high-performance CMOS 8-bit microcontroller with 8 KB of system-programmable Flash memory. This type of single-chip microcomputer has a high cost performance and is widely used in various fields such as industrial control, instrumentation, communication equipment, and household appliances.
1.2 GSM module circuit
This system uses SIM300 module, in which SIM300's 16-pin is connected to the network indicator (LED), which is used to indicate the strength of SIM300 signal. The 17-pin is connected to P3.3 of the MCU. The signal makes the module in working state, 43 , 41 , 49 are serial communication interfaces, which are respectively connected with P3.0, P3.1 and P3.2 of the single chip microcomputer. Connect the 1, 5, 5, 7, and 9 pins to the 4.2 V power supply. The SIM300 and the MCU are connected through the serial port, and the MCU control system can send the AT command to the SIM300 module to control its working status.
1.3 keyboard, display and external storage circuit
The system uses a 4-row 3-column matrix keyboard, in which the row lines are connected to P1.0~P1.3, the column lines are connected to P1.4 ~ P1.6, and the column lines are connected to the +5 V power supply via a 10 kΩ resistor. A total of 12 buttons are set to 0~9, *, # and other button numbers.
The system adopts LCD12864 liquid crystal display module. The display module can display 4 lines of characters or characters per screen for displaying prompt information and mobile phone numbers.
This system requires the user's preset mobile phone number to be stored during use, so external memory must be available. Use AT24C02 when the storage information is not large.
1.4 Sensor and relay circuit
The system uses vibration sensor, infrared sensor and sound sensor to cooperate with each other, which effectively avoids false alarms and misstatement of the system.
The relay circuit is used to cut off the oil circuit or circuit and control the running state of the car. When the user sends a control command by the mobile phone, the output of a certain port of the single chip is a high level signal, and the relay can be controlled. When the alarm is released, simply send a release command to resume normal car work.
2 software design
The main program flow chart is shown in Figure 2. 
3.1 Setup Process
Set the mobile number and SMS center number when the user first uses it. Turn on the power, follow the prompts on the LCD, and use the keypad to preset the number. You need to set the local SMS center number and personal control number. The set number is stored in the external memory. After the preset program ends, the MCU sends an AT command to the SIM300 to control the sending of the verification message to the preset number. The setting is successful. If it is not used for the first time, if the number is not reset within 10 s after prompting the password, it will automatically Skip the setup number phase and start normal monitoring. The setup process is shown in Figure 3. 
In the working state of SIM300, if the car has abnormal conditions, such as the door is opened, the window is smashed, the strong vibration or sound generated, etc., only one of the sensors is triggered, so that the level of the single-chip pin connected to the sensor changes. (from low level to high level), this change is used as a signal, and it is confirmed by the MCU to be an alarm signal. The MCU will control SIM300 to send a text message to the preset personal monitoring number, for example: "Your car has an emergency situation. Please check in time. The monitoring process is shown in Figure 4. 
When the SIM300 is in working condition, if there is an emergency, such as a car being stolen, it is necessary to close the tank valve urgently to stop the car. The SMS message is "closed" and sent to the SIM card connected to the SIM300. After receiving the SMS message, the SIM300 stores the information in the SIM card. The MCU sends a command to read the information just received, including the signal code, the length of the message, and the content of the message. The MCU parses it into a PDU code and determines whether the signal code is a preset user control number. If not, the subsequent work is stopped, the command is considered invalid, and the MCU sends a command to the SIM300 to delete the short message; if it is a preset number, it checks whether the short message conforms to the preset command. If it does not match, stop the follow-up work and delete the short message; if it is, execute the corresponding command, that is, the level of the single-chip pin connected to the relay changes (from low level to high level), and delete the short message (make the SIM card Each time the received text message is stored in the first position, it is convenient to read it next time. The output signal is then amplified by the signal amplifier to reach the voltage that makes the relay work normally. The fuel supply switch of the fuel tank is cut off to stop the car. The control flow is shown in Figure 5. 