Introduction With continuous economic development, people's living standards have significantly improved; especially with the improvement of material living standards, security issues have increasingly attracted people's attention. To protect their property, many apartment residents have installed security doors and bars on their basements and the doors and windows of their homes. However, while these devices are effective in preventing theft, they also bring many drawbacks: for example, losing the key to the security door can cause significant trouble for the homeowner; in case of an emergency, these security devices can also hinder escape and fire rescue. With the continuous advancement of science and technology and the widespread application of cutting-edge technologies, many intelligent alarm systems have emerged, providing more convenient and reliable protection for residents' property. In this paper, the author designs a GSM-based intelligent residential alarm system that meets the needs of modern residential security. II. System Working Principle The system utilizes pyroelectric infrared sensors to monitor the protected area in real time. The microcontroller is the core control component of the entire system. The alarm signal triggered by the sensor is input to the microcontroller after passing through the signal processing circuit. The microcontroller judges the signal and then issues commands to make the system work in a certain sequence through the actuator. This includes: when the intelligent detector detects an anomaly, on the one hand, it uses on-site sound, light, and electric alarms to deter criminals; on the other hand, it introduces advanced GSM communication technology and uses the GSM wireless communication network to transmit the situation at home to the homeowner. After receiving the information, the homeowner can immediately take effective emergency measures, such as calling the local property management or 110 emergency center to handle the incident. In addition, this system can switch between armed and disarmed states through encryption and decryption. The system structure is shown in Figure 1. [align=center] Figure 1 System Structure Diagram[/align] III. Main Components of the System 3.1 Pyroelectric Infrared Sensor In the field of electronic anti-theft, pyroelectric infrared sensors are welcomed by users due to their good concealment, low power consumption, low price, and stable technical performance. Its working principle: The human body temperature is generally 37 degrees, which will emit infrared rays with a wavelength of about 10μM. The infrared probe of the sensor works by detecting the infrared rays emitted by the human body. Its sensing source usually adopts pyroelectric elements. When the temperature changes after receiving infrared radiation from the human body, it will lose its charge balance and release charges outward. After a series of signal processing circuits, it can generate an alarm signal [1]. In order to be sensitive only to infrared radiation from the human body, the radiation surface of the sensor is usually covered with a special Fresnel filter to enhance the concentration of infrared rays and effectively limit environmental interference. 3.2 GSM Module This design uses the new generation wireless communication GSM module TC35 launched by Siemens. The working voltage of this module is 3.3～5.5V. It can work in two frequency bands of 900MHZ/1800MHZ. It has an RS232 communication interface, which can be conveniently connected to PC and microcontroller for communication, and safely and reliably realize the data, voice transmission, short message service and fax in the system solution. Among them, SMS is the most widely used communication method at present because it is simple to implement, has low communication cost, good confidentiality, is directly aimed at users' mobile phones and is not restricted by geographical location[2]. In this alarm system, the user's mobile phone is used as the receiving terminal. When an alarm occurs at home, the mobile phone will automatically receive an alarm SMS from the intelligent alarm system, so that remote alarm can be realized in real time, reliably and quickly. 4 Design of Intelligent Alarm System 4.1 Hardware Circuit Design 4.1.1 Monitoring Circuit Design This part is the core of the entire system. When the sensor detects the infrared radiation emitted by the human body, it outputs a thermoelectric signal. Since the signal is very weak and mixed with interference signals, a special signal processing circuit (using an operational amplifier and amplifying circuit with bandpass filter function) is designed to separate the thermoelectric signal and amplify it thousands of times. Through the optimization design of parameters, the interference caused by the flowing hot air is further eliminated by the comparator circuit. Then the thermoelectric signal separated after two-stage amplification is analyzed and judged. In order to prevent false operation, the signal will be compared with the threshold level to generate a high-level signal output. Finally, the signals generated by different probes are introduced to the INT1 port of the microcontroller through an NOR gate, waiting for an interrupt to execute the corresponding action; when there is no interrupt, the system is in standby mode, which can reduce system power consumption. The circuit structure is shown in Figure 2. [align=center] Figure 2 Monitoring signal circuit structure diagram[/align] 4.1.2 Sound and light alarm system design When the microcontroller detects that someone has entered the protected area, the intelligent alarm system will sound an alarm through a buzzer and flash a red LED at the same time to deter criminals. A high voltage generator is also introduced in this design. When the normally open contact of the connected relay is closed, the voltage can be boosted to make the secondary output high voltage, which is led to the alarm device housing. If the perpetrator intends to damage the alarm device, when he touches the housing, even if he is wearing gloves, the high voltage can penetrate the gloves and discharge through the human body. This will effectively prevent the criminal from committing further crimes, but will not cause permanent damage. 4.1.3 GSM communication system design The communication between the microcontroller and TC35 generally adopts serial working mode 1, and the data format is 10 bits. Among them, there is 1 start bit (0), 8 data bits (least significant bit first), and 1 stop bit (1) [3]. The baud rate value depends on the overflow rate of timer T1 and the value of the SMOD bit in the special function register PCON. Since the TXD and RXD levels of the microcontroller are TTL levels, and the TC35 communicates through the RS-232 interface, the TTL level and the RS-232 level are incompatible with each other. Therefore, level conversion must be performed when the two are interfaced. The MAX232 chip is used here. [align=center] Figure 3 Level conversion circuit diagram[/align] 4.1.4 Arming and disarming state switching is performed by encrypting and decrypting the system to switch the arming and disarming state. During the system initialization process, a specific password is set in the microcontroller. When a family member returns home, they can disarm the system by correctly entering the password. At this time, no alarm will be generated when moving freely indoors. When going out, the keyboard can be operated to put the system back into the arming state. It should be noted that: when the system is armed, after the alarm signal is detected, the system needs to wait for a period of time before checking the existence of the signal again. If the signal exists and the system is not disarmed, the corresponding alarm will be generated (sound, light and electricity effects and SMS notification to the homeowner). This delay is very necessary because it takes a period of time for the homeowner to leave the self-set zone. If there is no delay, the homeowner will also generate a false alarm when passing through the zone. 4.2 Software Design The microcontroller mainly realizes the functions of monitoring infrared rays emitted by the human body, executing corresponding sound, light and electricity alarms, performing remote communication, and switching between armed and disarmed states. In software programming, modules are divided according to function and the assembly language of the 51 series microcontroller is used to implement them. For the GSM communication part, the microcontroller mainly sends standard AT commands to the TC35 module through the serial port and sends and receives SMS messages through specific commands. The specific sending and receiving process of SMS messages is automatically implemented by the TC35 module and is not controlled by the microcontroller [4]. The relevant flowcharts 4 and 5 are listed below. [align=center]Figure 4 Main Program Flowchart[/align] [align=center]Figure 5 Interrupt Program Flowchart[/align] V. Conclusion The innovative features of this system are: when an alarm occurs in a residential security zone, the system can automatically trigger an audible and visual alarm to provide a stronger deterrent to intruders; it also remotely notifies homeowners of the alarm via a GSM module, enabling them to take immediate and effective emergency measures, thus improving the anti-theft effect; in addition, encryption and decryption measures are added to switch between the system's arming and disarming states, truly realizing the function of providing services to homeowners on demand. This system can also be expanded to the entire building, or even the entire residential community. Therefore, it has broad application and development potential. References [1] Yuan Youxin, Huang Xiaoyan, et al. Design of car anti-theft alarm system based on GSM [J]. China Water Transport. 2007.3,5(3):142-143. [2] He Qiao. Principles and applications of single-chip microcomputer [M]. Beijing: China Railway Publishing House, 2004.3 [3] Yuan Tianyou, Xie Yue. Design and implementation of remote residential intelligent monitoring system based on GSM [J]. Microcomputer Information. 2006,5-1:95-96,53