Design of wireless display system based on GSM network

    In order to use the existing mobile communication network to control the remote electronic display screen, a solution for wireless control of the display screen based on the GSM network is proposed. The system consists of a single-chip microcomputer and a TC35 wireless receiver. The controller sends a short message through the mobile phone, which is transmitted to the receiving end via the GSM network and finally displayed on the display screen. The results show that the system has a simple structure and is easy to operate.

    LED display screens are widely used in work and life. LED display screens can be used as electronic bulletin boards to publish bus information, weather forecasts, various advertisements, instant news, traffic conditions, ticketing conditions, temporary notices, etc. It is an information dissemination window for merchants to guide customers, which helps to attract consumers and promote the timely dissemination of merchant information. The
    traditional LED display screen is connected to the PC through a serial port data cable, and then the display content of the display screen is controlled through the application software on the PC. Since a wired connection is used between the PC and the display screen, data transmission problems will be faced. The use of traditional wired data transmission methods requires the laying of dedicated communication lines, which wastes resources and has poor operability. When the display screen is far away from the PC control end, the environment does not allow the laying of cables, and data cannot even be transmitted at this time.
    This paper studies how to use wireless network to control the content to be displayed on the display screen, and proposes a solution based on mobile communication network. The system has a simple structure and is easy to operate and maintain.

1. System design task
    The whole system consists of two parts: sending and receiving. The working principle of the system is shown in Figure 1. The sending end is the mobile phone, which is the controller of the display screen. The mobile phone sends the content to be displayed on the display screen to TC35 through the GSM mobile communication network.

    The design of the receiving system uses 89C51 single-chip microcomputer and Siemens' GSM module TC35. After receiving the short message, the receiving end TC35 automatically executes the collection of short messages through the AT command burned on the single-chip microcomputer. After the single-chip microcomputer completes the SMS extraction, it sends the short message content to the LED display screen for display through the serial port data line.

   2. Design of the receiving end system hardware
    The receiving end system hardware includes power supply circuit, TC35 communication module circuit, SIM card circuit, and single-chip microcomputer system.

2.1 System power supply circuit
    The power supply circuit is mainly composed of two three-terminal voltage regulator modules, L7805 and LM317 . L7805 stabilizes the input DC voltage at 5 V to ensure the normal operation of the single-chip system. LM317 is an adjustable voltage regulator with an adjustable range of 1.2 to 37 V. Here, it is adjusted to an output voltage of +3.6 V through 1 kΩ and 2.2 kΩ resistors. The output is connected to the 1st to 5th pins of the ZIF (Zero Insertion Force) connector to provide a stable power supply for the TC35 module. The circuit is shown in Figure 3. [page]
2.2 TC35 module circuit The
    TC35 module is a high-performance communication module, which is mainly composed of 6 parts: GSM baseband processor, GSM radio module, power supply module (ASIC), flash memory, ZIF connector, and antenna interface.

    TC35 has 40 pins, which are brought out through a ZIF connector. These 40 pins can be divided into five categories, namely power, data input/output, SIM card, audio interface and control. The functions of each pin are shown in Table 1.

2.3 SIM card circuit
    The SIM card has 8 pins (as shown in Table 2), through which the information is transmitted between the mobile terminal interface and the power supply required for the SIM card to work is provided. There are 6 pins on the ZIF connector for the SIM card interface, as shown in Figure 5.

2.4 MCU communication circuit
    The MCU system includes the MCU and TC35 communication circuit and the MCU and display screen communication circuit.
    (1) MCU and TC35 communication circuit
    The data input/output interface of TC35 is a serial asynchronous transceiver that complies with the ITU-T RS 232 interface standard. It has fixed parameters: 8 data bits and 1 stop bit, no parity bit, and the baud rate is selectable between 300 and 115 Kb/s. The MCU can communicate directly with TC35 through the serial port, and the DTE (MCU) and DCE (TC35 communication module) can be directly connected to the RXD and TXD of the corresponding MCU. However, the operating voltage of TC35 is 3.6 V, and the MCU works at 5 V. In order to ensure the correct data transmission, a level conversion circuit must be added to the interface. The circuit is shown in Figure 6.

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    (2) Serial communication circuit between single-chip microcomputer and LED display screen
    The serial port of the display screen is RS 232 level, and its voltage value is -12～+12 V; the single-chip microcomputer is TTL level, and its voltage value is 0～5 V. When communicating, there must be a level conversion circuit between the two. Usually, a level conversion chip is used to convert the levels of the two. In this system, the MAX 232 conversion chip is used to realize the conversion between TTL level and RS 232 level.

3. The short message format at the sending end
   is edited by the mobile phone and transmitted wirelessly because the content displayed on the remote LED display screen loses the direct control of the traditional PC terminal software.

Therefore, the corresponding display effects, display modes and other control items must be agreed upon through specific symbols in the short message.
    The format of the short message edited by the mobile phone as the sender is as follows:

        Display mode: The mode in which the short message is displayed on the screen. The value range is 1 to 5. The specific representation is as follows:

    Movement speed: The movement speed of the text on the display screen, with values ​​ranging from 1 to 9. The larger the value, the faster the text moves.
    Dwell time: With values ​​ranging from 1 to 9. The larger the value, the longer the initial display time of the short message on the display screen.
    Connector: With values ​​ranging from 1 to 2. With 1, the short message is displayed without paragraphs. With 2, the short message is displayed in segments.

4 Conclusion
    This paper designs a mobile phone remote control LED display system based on the GSM mobile communication network. Through this system, the sender can roam in more than 100 countries around the world and control the LED display connected to TC35 through the GSM mobile communication network. The new system avoids the traditional LED display screen that can only update information through a computer online, which greatly facilitates the information publisher.