Design of laboratory security system based on GSM and LPC2138

Abstract: This paper introduces an intelligent automatic security system for university laboratories based on GSM and LPC2138. The system uses the high-performance ARM microprocessor LPC2138 as the main control chip. The system collects various environmental data of the laboratory through sensors, and sends information that endangers laboratory safety to the laboratory administrator through the GSM network, realizing intelligent monitoring of the laboratory.
Keywords: Global System for Mobile Communications; Laboratory; Security System

0 Introduction
The laboratory is an important part of the teaching process of colleges and universities, and is an important place for the cultivation of comprehensive application-oriented talents and for teachers to conduct scientific research. In recent years, with the expansion of the scale of colleges and universities, laboratories in various colleges and universities have also developed rapidly, and the proportion of laboratory equipment in fixed assets of colleges and universities has also increased. However, since many colleges and universities do not have experimental centers, laboratories mainly depend on colleges and departments and are relatively scattered. In this way, it is difficult to centrally realize the safety maintenance and management of laboratories.
Based on the above problems, this paper proposes the design of a laboratory security system based on GSM and LPC2138. The system uses the 32-bit ARM series single-chip microcomputer LPC2138 as the main control chip, and collects laboratory information through various sensors. If there are factors that endanger laboratory safety, a short message is sent to the monitoring center or laboratory administrator through the GSM module to maintain the laboratory in time, thereby improving the reliability of laboratory safety operation.

1 System overall structure
The overall structure block diagram of the system is shown in Figure 1. The system consists of three parts: control module, information acquisition module, and alarm module.

The control module is composed of a single-chip microcomputer chip, LCD display, key control and power supply circuit. The control module mainly controls and coordinates the normal operation of the system. The single-chip microcomputer chip processes and identifies the information sent by the sensor, and displays the key information on the LCD display. The user can view the current status of the laboratory through key control and LCD display, and can perform manual arming and disarming operations, which is convenient and flexible to use.
The information acquisition module is composed of various sensors and signal processing circuits. Its main function is to collect environmental information and safety information of the laboratory. The sensor signal is processed by the signal processing circuit and sent to the main control module for processing.
The alarm module is composed of sound and light alarm and GSM module. When receiving information that endangers the laboratory, it turns on the corresponding alarm mode according to the pre-set alarm processing mode.
2 System hardware circuit design
2.1 Main control module hardware circuit design
The main control module is mainly composed of a single-chip minimum system, key control circuit and LCD display circuit.
The single-chip microcomputer uses Philips' LPC2138, which is a microcontroller based on a 32/16-bit ARM7 TDMI-STM CPU that supports real-time simulation and embedded tracing, with 512kB embedded high-speed Flash memory, 128-bit wide memory interface and unique acceleration structure that enables 32-bit code to run at the maximum clock rate. It has strict control over the code size, so that it can use 16-bit Thumb mode to reduce the code size by more than 30%, while the performance loss is very small.
The key control and LCD display hardware circuit is shown in Figure 2. The key control circuit provides 4 independent keys, each of which has a composite function. Users can easily view the current status and historical alarm information of the security system through 4 keys and LCD display, can select the appropriate security mode, and can also realize the arming and disarming of the laboratory through keys.

2.2 Information acquisition module hardware circuit design
The hardware circuit of this module is mainly composed of various sensors and signal processing circuits, which can be roughly divided into two parts. One part collects laboratory environmental information, including temperature sensors and humidity sensors. The remaining sensors collect laboratory safety information, such as theft alarms and fire alarms.
The temperature and humidity are collected using integrated sensors, which do not require processing circuits and can directly read real-time temperature and humidity information.
The smoke sensor is used to detect smoke in indoor gas and is used for fire alarms in the system. The schematic diagram of the smoke sensor signal processing circuit is shown in Figure 3. The smoke sensor is generally installed on the ceiling of the room, and a scattered light smoke sensor CA3302B is used as the smoke sensor. Pins 1 and 4 of J1 are connected to the internal light source of the sensor, and pins 2 and 3 are connected to the photosensitive element of the sensor. In normal state, the 2nd pin of J1 is high level, that is, the 2nd pin of IC2A is high level. Since the 1st pin of IC1A is high level, it outputs low level, and then outputs high level in reverse through IC2B. When smoke enters the dark room of the sensor, the light scatters and shines on the photosensitive element, and its resistivity decreases. The 2nd pin of IC2A becomes low level, and it outputs high level, which is reversed through IC2B and finally outputs low level. The single-chip microcomputer can determine whether there is a smoke alarm or a fire alarm by detecting the output of IC2B.

The pyroelectric infrared sensor is used to detect whether there are human activities in the laboratory, that is, it belongs to the anti-theft alarm. The product P228 of Hamamatsu Corp. is used in this article. The schematic diagram of the signal processing circuit is shown in Figure 4. Pin 1 of P2288 is the power pin connected to the +5V power supply, pin 2 is the output pin, and pin 3 is grounded. The effective detection range is about 7m. Under normal conditions, if there is no human activity, pin 2 of P2288 has no output, and the output port of the processing circuit remains at a low level; if there is human activity within its detection range, the output pin of P2288 outputs a weak current, which is converted into a voltage value by R1 and added to the input end of a single-tube common-emitter amplifier circuit. After amplification, it is sent to the comparator composed of LM324 to generate a high-level output.
When someone breaks the glass, the glass break sensor is subjected to mechanical vibration and will give a signal. This signal can be used for alarm after amplification and processing. This sensor can be used as a reference alarm signal for theft alarm together with door magnets and window magnets.
2.3 Alarm module hardware circuit design
The sound and light alarm uses an alarm with a warning light, and the relay controlled by the single-chip microcomputer controls the working power of the alarm.
The GSM communication module uses TC35. TC35 is a new generation of wireless communication GSM module launched by Siemens, which can quickly, safely and reliably realize data, voice transmission, short message service and fax in the system solution. The working voltage of the module is 3.3~5.5V, and it can work in the two frequency bands of 900MHz and 1800MHz. The power consumption of the frequency bands is 2W (900M) and 1W (1800M) respectively. The module has an AT command set interface and supports short messages in PDU mode. TC35 and LPC2138 are connected through UART.

3 System software design
When designing the system software, modular design is adopted, and the program flow chart is shown in Figure 5. According to the different functional modules of the system, corresponding subroutines are designed. Specifically, it can be divided into three subroutines: the main program and GS-MS communication, key control, and LCD display.

The main program includes equipment initialization, data collection, information processing and related subroutine calls. During data collection, the signals of infrared and smoke sensors are set as external interrupts, and the fire alarm interrupt has a higher priority than the burglar alarm interrupt. The signals of glass break sensors, door and window magnets, temperature and humidity sensors are polled and detected and collected. Information processing processes the collected information, identifies the alarm information, selects the appropriate alarm mode according to the security status, and sends the current laboratory safety status to the LCD display.
In addition to starting the sound and light alarm, the processing of fire alarms and burglar alarms also needs to send short messages through the GSM communication module to notify laboratory management and maintenance personnel. The TC35 communication module used in the system supports PDU message mode. According to the communication protocol of the system, the main way of sending information is to send data frames. A complete data frame includes four parts: start flag unit, command unit, CRC check unit, and end flag unit. When using PDU mode, a data frame can contain 140 bytes (70 Chinese characters) of data, and Chinese characters are encoded according to UNICODE. When sending data, the format is: service center address / PDU type / receiving main address / protocol identification code / data decoding scheme / valid period / user data length / user data; when receiving data, the format is: service center address / PDU type sender address / protocol identification code / data decoding scheme / time when the service center receives the message / user data length / user data. When designing the software, the corresponding AT command can be written according to the PDU data format to realize the sending and receiving of short messages.
The key control subroutine is set as an interrupt service subroutine, and the priority of the key interrupt is lower than that of the fire alarm and theft alarm. It can realize two major functions: query and setting. The query mainly queries the current information and historical alarm information of the system: the setting function can set different defense states in addition to the deployment and disarming of the laboratory. According to the characteristics of school work, the security mode can be set to semester mode and holiday mode. In these two modes, the security level is the same, and the difference is the alarm method. In the semester mode, the alarm information is mainly handled by the laboratory administrator; during the winter and summer vacations of the school, the holiday mode is adopted, and the alarm information is handled by the on-duty personnel of the school security department.

4 Conclusion
This system uses the GSM wireless network to realize centralized monitoring and management of decentralized laboratories. It can monitor the operating status of the laboratory in real time, detect alarms in time for quick elimination, and maintain the laboratory. It realizes the integrated, intelligent, and networked monitoring of university laboratories, saves human resources, improves the reliability of safe operation of university laboratories, and enables the laboratory to perform its functions smoothly.