Research on GSM-based solar street light network monitoring system

　　0 Preface

　　In order to promote the use of renewable energy and accelerate the construction of a resource-saving and environmentally friendly society, the use of solar energy is gradually gaining people's attention. Solar street lights, as high-tech energy-saving products, are gradually replacing traditional street lights.

　　According to the characteristics of solar street lights, a networked monitoring system for solar street lights is introduced. That is, the slaves and the host are connected through the RS485 interface. The host monitors the working conditions and various operating parameters of the solar panels, batteries and LED lamp heads of each slave. Then, the host transmits the detection results to the monitoring center or related technical staff in the form of text messages or voice through the MC39i module, thus realizing the networked monitoring of solar street lights.

　　1 System Hardware Design

　　The current solar street light control systems are all independent photovoltaic control systems, which are mainly composed of six parts: solar cells , batteries, LED street lights, controllers, charging circuits, and discharge/load drive circuits [1]. The system structure diagram of the host is shown in Figure 1. The output of the solar panel is directly connected to the battery after being regulated by the CUK circuit. The system main control chip uses the DSPIC30F3011 microcontroller to realize the functions of solar panel voltage collection, battery voltage collection, CUK circuit control, LED lamp head control, 485 communication between the master and slave, and connection between the host and the monitoring center or staff.

Figure 1 Host system structure

　　1.1 Setting the maximum power point of photovoltaic power source

　　The voltage (current) of the photovoltaic power system varies greatly due to the influence of sunlight intensity and ambient temperature. In order to have greater flexibility and higher conversion efficiency when the load resistance changes greatly , the main circuit of the system uses the CUK circuit, which is based on the Boost-Buck circuit, and the first-level circuit realizes two-level voltage regulation.

　　The system adopts CCM working mode, the characteristics of which are very close to a DC-DC transformer with adjustable turns ratio. Energy storage and transmission are carried out during two switching actions and two loops at the same time, and the converter efficiency is very high. The change in the duty cycle of the switch tube in the CUK circuit is manifested as a change in the output impedance of the photovoltaic array, and the change in output impedance will affect the output characteristics of the photovoltaic array. Therefore, a certain output impedance corresponds to an output voltage value and an output current value. The MPPT technology is to change the output impedance of the photovoltaic array by adjusting the duty cycle of the CUK circuit, so as to seek the maximum value of the output power, which is the product of the output current and the output voltage.

　　1.2 Control circuit hardware design

　　The main control chip of the control circuit adopts DSPIC30F3011 single-chip microcomputer, and its main control functions include: solar panel voltage acquisition; CUK circuit gating control; battery voltage acquisition; unloading circuit control; LED lamp head control; RS485 communication; GSM module sending text message control; street light switch control; working mode control, etc. The main schematic diagram is shown in Figure 2, Figure 3 and Figure 4, where Figure 2 is the schematic diagram of the main control chip DSPIC30F3011. Figure 3 shows the voltage sampling circuit and CUK circuit. Since the solar panel voltage and battery voltage both vary from 0 to 35 V, and the A/D input voltage range of the single-chip microcomputer is 0 to 5 V, the sampled voltage is divided and transmitted to the A/D conversion channel of the single-chip microcomputer. The CUK circuit is used to adjust the maximum output power point of the solar panel, and its gating switch is controlled by the single-chip microcomputer PWM3 output.

Figure 2 DSPIC30F3011 principle

Figure 3 Voltage sampling circuit and CUK circuit

　　Figure 4 shows the LED lamp control circuit and unloading circuit. The microcontroller controls the LED lamp by monitoring the voltage of the solar panel and the battery, and adjusts the switch and brightness of the LED lamp through PWM0 and PWM1 respectively. When the battery voltage reaches 30 V, the microcontroller starts unloading by controlling the PWM2 pin to discharge the battery. [page]

Figure 4 LED lamp control circuit and load unloading circuit

　　2 Communication System Design

　　The overall communication connection diagram of the solar street light network monitoring system is shown in Figure 5. The DSPIC30F3011 microcontroller has dual serial ports. One serial port in the host communicates with the slave via RS-485, and the other serial port is used to control the GSM module, that is, to communicate with the MC39i module and control the MC39i to send text messages to the monitoring center.

Figure 5 Overall communication connection

　　2.1 Communication design between master and slave

　　Since the distance between solar street lights is tens of meters, the master and slave devices in this system are connected through RS-485 communication. The communication distance of RS-485 can reach hundreds of meters or even thousands of meters, with a maximum transmission rate of 10 Mb/s. In addition, multi-point communication can be realized, so that a local area network within a small range can be established [3]. Figure 6 is a hardware connection diagram of the connection between the DSPIC30F3011 microcontroller and the MAX485. The DSPIC30F3011 and the MAX485 are isolated by 6N136 to ensure the accuracy of data transmission. The master and slave devices both have serial ports connected to the MAX485, and the A, B and GND pins of each MAX485 chip are connected to each other. The master and slave devices continuously detect the voltage of the solar panel and the battery voltage. When low power occurs, the slave device will transmit information to the host device in a timely manner.

　　2.2 Communication design between host and monitoring center

　　The wireless measurement and control system based on GSM communication technology has the characteristics of good versatility, wide geographical coverage, no debugging and maintenance, low operating costs and flexible control methods. Therefore, GSM communication modules are used for information transmission between the host and the monitoring center.

　　The DSPIC30F3011 microcontroller samples and compares the solar panel voltage and battery voltage. When the sampled value is lower than the set value, it sends a text message "solar panel voltage is insufficient" or "battery voltage is insufficient" to the monitoring center. The microcontroller can also monitor the working status of the street lamp. When an abnormality occurs, it will be sent to the monitoring center in the form of a text message.

Figure 6 DSPIC30F3011 and MAX485 wiring

　　The GSM module uses MC39i, which is an industrial-grade GSM module that supports Chinese short messages and can transmit voice and data signals. It is connected to the SIM card reader and antenna through the interface connector and antenna connector respectively. The data interface of MC39i can transmit commands and data bidirectionally through AT commands. The optional baud rate range is 300 b/s to 115 kb/s. It supports SMS (Short Message Service) in Text and PDU formats, and can achieve restart and fault recovery through AT commands or shutdown signals [4].

　　The MC39i module has 40 pins, which are connected through a ZIF (Zero Insertion Force) connector. These 40 pins can be divided into five categories, namely power supply, data input/output, SIM card, audio interface and control. Pins 1 to 5 of the MC39i are positive power input pins, pins 6 to 10 are power ground, and pin 15 is the start pin IGT. After the system is powered on, in order to make the MC39i enter the working state, a low pulse greater than 100 ms must be added to IGT, and the level drop duration cannot exceed 1 ms. Pin 18 RXD and pin 19 TXD are TTL serial communication pins, which need to communicate with the microcontroller or PC. MC39i uses an external SIM card. Pins 24 to 29 are SIM card pins. The SYNC pin, pin 32 of MC39i, is a control pin with two working modes. One is to indicate the power growth during transmission, and the other is to indicate the working status of MC39i. The AT command AT+SYNC can be used to switch. Pins 35 to 38 are voice interfaces [5].

　　The power input of MC39i adopts the switch-type adjustable high-performance microwave circuit dedicated voltage regulator chip LM2941S. The start pin IGT can be controlled by the microcontroller software, or its potential change can be controlled by a button. The 18-pin RXD and 19-pin TXD are directly connected to the asynchronous serial ports RXD2 and TXD2 of the DSPIC30F3011 microcontroller to realize the microcontroller's control of MC39i sending and receiving instructions. The 24-29 pins are directly connected to the corresponding pins of the SIM card to facilitate the detection of whether the SIM card is properly inserted and to complete the function of sending text messages. The SYNC pin can be connected to an external light-emitting diode to detect whether the module is in working condition. [page]

　　3 Software Design

　　3.1 System Software Design

　　The system uses DSPIC30F3011 microcontroller for monitoring and processing. The microcontroller monitors the voltage of the solar panel and the battery in real time. If the voltage of the solar panel is greater than the set value, it means that the light intensity is large enough. The microcontroller cuts off the power supply of the LED lamp head and the solar panel charges the battery. If the voltage of the solar panel is less than the set value, the battery will power the LED lamp head. First, the battery voltage is detected. If it is large enough, the battery will power the LED lamp head. If it is less than the lower limit, the microcontroller controls the MC39i module to send a text message "Battery low power". If the battery voltage is higher than the upper limit, the unloading circuit will be started to prevent the battery from overcharging. The system flow chart is shown in Figure 7.

Figure 7 System flow

　　3.2 GSM Software Design

　　AT commands can be used to control the MC39i module. The microcontroller uses AT commands to initialize the MC39i module and send and receive short messages. There are two modes for controlling short messages: PDU mode and Text mode, but the Text mode does not support text, so the design uses the PDU mode. Send the AT command "AT CRLF" to the MC39i module (CR stands for carriage return and LF stands for line feed) through the microcontroller asynchronous serial port. If the MC39i module sends "CRLFOKCRLF" to the microcontroller, it indicates that the MC39i module is connected normally. Then the microcontroller sends "AT+CMGF=0CRLF" to the MC39i module to set the SMS mode to PDU format. If the MC39i module replies "CRLFOKCRLF", it indicates that the setting is successful. Then the microcontroller sends "AT+CMGS=26 CRLF" to the MC39i module to set the total byte length of the SMS to 26. If "CRLF>26" is received, it indicates that the setting is successful. Finally, the microcontroller sends specific SMS information to the MC39i module, such as sending the SMS "Solar panel low power" to the monitoring center. The corresponding number of the SIM card of the monitoring center is 1364217302X, and its corresponding PDU The data is: 0891683108200205F011000B813146123720 FX0008A712592A963380FD677F4F4E75351A. Among them: 08: SMS center address length; 91: SMS center number type; 68: China code (after swapping); 3108200205F0: Tianjin SMS center number (after filling in F at the end, swap every two digits, the actual number is "13800220500"); 11: file header byte, default is 11; 00: information type, default is 00; 0B: called number length; 81: called number type; 3146123720FX: called number (after swapping, the actual number is 1364217302X); 0008: 00 Flag protocol 08 means using Unicode encoding; A7: Valid days = A7-A6; 12: SMS content length; 592A Tai; 9633 Yang; 80FD Energy; 677F Board; 4F4E Low; 7535 Electricity; Finally, the SMS content ends with 1A, 1A is the sending end mark.

　　4 Conclusion

　　Here, the system transforms the existing solar street light controller, integrates the maximum power point setting of the photovoltaic power source into the solar controller, realizes the communication connection between the master and slave machines with the help of serial communication technology, and realizes the communication connection between the host and the monitoring center with the help of GSM technology, and finally realizes the network monitoring of the solar street light control system. Therefore, the system not only improves the utilization efficiency of solar energy, but also realizes wireless data transmission between solar controllers, and improves the use value of the existing solar street light controller.