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A universal active filter design method is proposed for the shortcomings of multi-components, bulk mass, complex debugging, cumbersome design and bad filtering characteristics of the conventional active filter. The system which mainly contains MAX275, Surplus MCU SPCE061A and digital potentiometer can implement 2nd-, 4th-, 6th- and 8th-order band pass or low pass filter response, such as Butterworth, Bessel and Chebyshev. The parameters are setted by the keyboard and displayed by the LCD.

The output power of solar cells varies with variable temperature, light intensity and loads. In order to solve the shortcomings such as long time and high cost of using solar cells to experiment, this paper successfully uses a DC power supply and a variable resistor to simulate the solar cells output characteristic curve, its feasibility is analyzed theoretically. The basic principle of the maximum power point tracking (MPPT) is described in detail. An improved disturbance method with variable voltage step is introduced, it inherits the advantages of traditional disturbance voltage method, overcomes the shortcoming of slow response of traditional method, reduces the power loss caused by maximum power point due to continuous disturbance, and realizes the maximum power point tracking for solar cells. The proposed method improves the system's speed and efficiency. The experimental results show that the system can track the maximum power point accurately and quickly.