Solar street light and mains complementary inverter photovoltaic based on single chip microcomputer

0 Introduction
The utilization of solar energy has become an urgent task in the current situation of increasingly tight energy supply, and how to achieve efficient utilization of solar energy under the premise of cost control is the purpose of many solar energy product designs. This project is aimed at realizing solar photovoltaic control of street lights commonly used outdoors and in parks. It uses a single-chip microcomputer control circuit to realize two ways of solar power supply and city power supply for street lights. It can not only solve the energy consumption problem of street lights, but also realize that ordinary 220V city power can ensure the normal power supply of street lights when there is insufficient light.

1 Structure of solar street light photovoltaic control system
Figure 1 is the structural block diagram of this project. The core is a single-chip microcomputer, and the main components are: solar cell control circuit, battery charging and discharging circuit, inverter circuit, and single-chip microcomputer control circuit.

In this control system, the output of the TL494 pulse width modulation (PWM) circuit is controlled by the single-chip microcomputer, and the PWM duty cycle is changed to adjust the output voltage of the inverter circuit. When the solar panel is working normally, the battery is in a charging state, and the voltage required for the load to work is obtained by the battery output after inversion. If the external light is insufficient and the battery charging voltage is lower than the set low value, the controller directly connects to 220V AC power for the load to ensure that the street lamp will not be off under any circumstances.

2 System hardware circuit design
In this controller, the hardware circuit design is the key, which mainly includes four parts: solar panel control circuit design; battery detection circuit design; inverter circuit design and single-chip microcomputer control circuit design.
2.1 Solar panel control circuit
The solar panel control circuit is the core of realizing solar power supply for street lamps. When there is light, the battery is charged through the solar panel. When the battery is working, the lighting of the street lamp is provided by the battery.

2.2 Design of battery detection circuit
When the solar panel is working, the battery is charged by the solar panel. When the street lamp needs to be lit, the DC power of the battery is inverted by the inverter to become the same amount of AC power required by the load for the load. The power supply voltage of each battery group is 12V. According to the power of the street lamp, there can be multiple groups of batteries, and the batteries are connected in series. The charging control circuit has a battery voltage detection function. When the battery voltage of the battery group is lower than the preset value, the solar panel starts to charge the battery. When the charge is saturated, constant voltage and constant current are achieved to maintain the dynamic stability of the battery. When the load needs power supply, the DC power generated by the battery is inverted by the inverter circuit and converted into AC power for the load.

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2.3 Inverter circuit design
The inverter circuit is used to convert DC power into AC power of various frequencies, which is highly efficient and clean. In this project, the inverter circuit mainly converts the DC power transmitted by the battery into the AC power required by the load. It adopts the half-bridge inverter method. The inverter circuit switch device used is the IRF3205 MOSFET field effect tube. The control method of the inverter circuit is PWM technology. The PWM modulation wave is generated by the TL494 dedicated sine wave pulse width modulation circuit.

2.4 Single-chip microcomputer control circuit design
The single-chip microcomputer is the core of the entire controller. It is mainly used to manage the solar panel control circuit, control the battery charging and discharging circuit, generate the PWM modulation wave required by the inverter device, and also undertake the battery status detection, as well as the overcurrent and overload protection in the circuit. In addition, it also has the sound and light alarm function. Figure 5 is a partial schematic diagram of the single-chip microcomputer control circuit. The single-chip microcomputer used in this controller is the S3C9454 series single-chip microcomputer produced by Samsung. This type of single-chip microcomputer has the characteristics of small size, low price and comprehensive functions, which is more suitable for large-scale product production.

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2.5 Protection circuit
The protection circuit is to limit the current of the circuit when the load is short-circuited or overcurrent occurs in the controller. In this controller, when the battery voltage is lower than 10.5V or the output current is greater than 10A, the circuit enters the self-protection state. Figure 6 is a partial schematic diagram of the protection circuit.

In the protection circuit, it is necessary to sample and compare the battery voltage and the output current. When the voltage is lower than the set requirement by setting the reference value, the load power supply mode is automatically switched from battery power supply to external AC power supply, thereby protecting the battery. In addition, once the load is short-circuited and overcurrent occurs, the circuit enters the protection mode and cuts off the external power supply to prevent overcurrent from damaging the circuit components.

3 System software design
The software of this system is designed in C language, which has the characteristics of wide application and strong data processing capability. The software control mainly realizes the following functions:
(1) Control of PWM output. The PWM modulation signal output by the TLA94 chip is controlled by the single-chip microcomputer. When the load changes or the protection state appears, the single-chip microcomputer sends a signal (220V OFF) to turn off the TL494 output, cut off the inverter circuit input, and ensure that the inverter output voltage is zero. Therefore, it is required in the software design to ensure that the single-chip microcomputer always samples the output signal state.
(2) Detection and control of the battery voltage. The normal working value of the battery voltage is 12±1V. When the battery voltage is lower than 11V, the circuit is required to immediately stop the battery from supplying power to the load output, otherwise it will affect the battery life. There is a special battery detection circuit in the system, and the single-chip microcomputer has been sampling the battery voltage detection value, that is, the Batter_Adin signal in the circuit. Once the voltage is found to be low, the single-chip microcomputer outputs a control signal (RY1) to switch relay 1 in time so that the load bulb power supply is transferred from the battery to the mains.
(3) Display and keyboard input function. In this system, the display function is divided into two parts: one is the digital tube display, which is mainly used to display the system working time; the other is the LED display. The state change of the LED represents the working state of the circuit, where the green light represents normal operation and the red light represents a fault. The display function of the system is mainly realized by the single-chip microcomputer software. In addition, considering the need to set the circuit mode in the circuit, a keyboard input circuit is added. There are three dip switches in this controller to realize the three-way remote control output of the street lamp. Figure 7 is the main program flow chart of this controller.

4 System Test
The system test mainly checks the battery voltage and circuit overcurrent protection status when the load is working continuously. After the controller is connected to the solar panel, the street light will be automatically switched on and off through the photoresistor. When the light intensity is lower than the set point, the street light will turn on automatically. If the light intensity is higher than the set point, the street light will also turn off automatically. Considering some emergencies, a manual operation mode is set in the system. If there is a short thunderstorm and the light intensity drops, manual operation is used to prevent the street light from turning on automatically.
4.1 Battery discharge protection test
When the battery supplies power to the load, its battery voltage will continue to decrease with the discharge time. When the battery voltage is 10.5V, the controller will cut off the battery's power supply to the load and switch to external 220V mains power supply. When there is light during the day, the battery is charged through the solar panel. When the voltage returns to 12.5V, the controller will automatically cut off the mains power supply and switch back to battery power supply mode.
4.2 Battery overcharge protection test
When the solar panel is charging the battery, if the battery voltage reaches 14.3V, the controller turns off the charging circuit, and when the voltage drops to 13.6V, the charging circuit is turned on again.
4.3 Load short circuit protection test
When the load power is greater than 50W, the controller turns off the load, and after a delay of 10s, it can automatically turn on the load again, and it can work normally for a 30W load (the load rated power designed by this controller is 35W).

5 Conclusion
The biggest advantage of this controller is that it realizes the function of dual-channel power supply, avoiding the situation that the street light cannot work normally when the battery voltage is insufficient. Through the optimization control of the single-chip microcomputer, the stable operation of the system is guaranteed.