As the number of cars continues to increase, the theft of cars has become a social issue that is generally concerned and urgently needed to be resolved. At present, various car anti-theft methods and products are emerging, but the problem of car anti-theft is still not completely solved. There are many problems with the traditional alarm system, such as short working distance, noise pollution caused by the alarm, and false alarms. The GSM car anti-theft device designed by this system is based on the GSM network with extremely high coverage and utilizes GSM wireless communication services and short message value-added services. It has the characteristics of simple implementation, low communication cost, high spectrum utilization rate and good confidentiality. Not only real-time, automatic, reliable and convenient to achieve communication alarm, but also breaking through the distance limit, has the characteristics of high technical content, intelligence, network.
System overall design The car alarm system is mainly composed of the detection module, the processor, the GSM communication module, and the owner's mobile phone. The overall block diagram of the system is shown in Figure 1. The detection module includes a pyroelectric infrared sensor and a vibration sensor. After the detection module detects the signal, the alarm short message is transmitted to the owner's mobile phone or a designated number mobile phone through the software processing system and the GSM network, thereby realizing the long-distance transmission of the alarm signal.
Figure 1 Block diagram of the overall structure of the system
System hardware design
1 GSM communication module
The GSM module TC35i is an industrial-grade GSM module that supports Chinese short messages. It works in GSM900 and GSM1800 dual-band and can transmit voice and data signals. It is connected to the SIM card reader and antenna through the interface connector and antenna connector. Its data interface (CMOS level) can transmit commands and data in both directions through AT commands, and it supports SMS in Text and PDU formats. Restart and fault recovery can be achieved through AT command or shutdown signal.
2 Pyroelectric infrared alarm circuit Pyroelectric infrared sensor is a new type of high-sensitivity detection element developed in the 1980s. It can detect the change of infrared energy radiated by the human body in a non-contact form and convert it into a voltage signal output. Amplifying this voltage signal can drive various control circuits, such as power switch control, anti-theft and fire alarm, and automatic detection. The pyroelectric infrared sensor is only sensitive to infrared radiation with a center wavelength of 9 to 10 μm, and can detect infrared information of human body radiation, and can be used as a monitoring device for human body intrusion into a car. Pyroelectric infrared sensors output electrical signals by detecting infrared rays emitted by humans or animals at an angle of 110 °. BISS001 type infrared finished components can be used, and the working distance is adjusted by the Fresnel lens. The application circuit is shown in Figure 2.
Figure 2 pyroelectric infrared alarm circuit
BISS0001 in Figure 2 is a sensor signal processing integrated circuit with higher performance. It is equipped with a pyroelectric infrared sensor and a small number of external components to form a passive pyroelectric infrared alarm. It is composed of an operational amplifier, a voltage comparator, a state controller, a delay time timer, and a blocking time timer. Application Specific Integrated Circuit.
3 Vibration sensor The vibration sensor can monitor the vibration of the car body in a special frequency band, and can generate an alarm when the car body is destroyed by external forces. Vibration sensor can use ND-1 type highly sensitive vibration displacement sensor. It is an all-round solid-state control device that combines vibration and displacement measurement. It miniaturizes the mechanical vibration sensor, carbonizes the vibrating body and seals it. It has high detection sensitivity to vibration and sound signals from the surrounding environment. Suppressed and has a strong anti-interference ability, its sensitivity can be adjusted by capacitance. Its working performance is more reliable. The output switch signal is directly connected to the TTL circuit or the input circuit of the single chip microcomputer, the circuit structure is simple, the output impedance is high, and the static working current is small.
Figure 3 Vibration alarm circuit diagram and its output waveform Figure 3 is a vibration detection circuit composed of MOST NOT gate circuit, and the output waveform is a vibration pulse signal with the same amplitude. This signal is sent to the single-chip microcomputer, and the single-chip microcomputer software detects the authenticity of the vibration signal, and judges the type of vibration by the pulse width and the number of pulses. If a single-chip microcomputer with a programmable port is used, when the port is programmed to the input state, the vibration signal can be directly detected, and the shaping amplifier circuit is omitted.
System software design
1 AT commands related to SMS
The command interface provided by the GSM module conforms to the GSM07.05 and GSM07.07 specifications. The AT Command interface defined in GSM07.07 provides a universal interface between mobile platforms and data terminal equipment. GSM07.05 specifies the short message in detail. When the short message module receives the short message from the network, it can send an instruction message through the serial port, and the data terminal device can send various commands to the GSM module.
Currently, Text and PDU (Protocol Data Unit) modes are commonly used for sending short messages. Using Text mode to send and receive SMS codes is simple and easy to implement, but the biggest disadvantage is that it cannot send and receive Chinese text messages. The PDU mode not only supports Chinese text messages, but also sends English text messages. There are three types of codes that can be used to send and receive text messages in PDU mode: 7-bit, 8-bit and UCS2 codes. 7-bit encoding is used to send ordinary ASCII characters, 8-bit encoding is usually used to send data messages, and UCS2 encoding is used to send Unicode characters. The encoding type is specified in the PDU data packet.
Figure 4 System flow chart
2 System flow The system flow chart is shown in Figure 4. The main program includes system initialization and loop detection. The delay subroutine is used to generate a certain delay, and then the alarm is started to prevent false alarms: the main program is used to control the alarm circuit and the short message sending circuit. When the MCU receives the interruption alarm signal, the short message sending circuit sends an alarm message to the owner in time. If everything is normal, the program is waiting in a loop.
Conclusion It has been verified that the use of a car anti-theft system based on GSM short message data transmission technology can not only improve the communication reliability of the alarm, but also the communication distance is basically not limited, so that users can monitor the car for a long distance. It is convenient for the car owner to take measures according to the situation and meet the car owner's anti-theft requirements. The system is simple to install, powerful, easy to apply, and easy to promote.
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