R1, R2 and R3 are all oscillating resistors, but their functions are different. R2 is used to adjust the duty cycle of the square wave when the frequency is fixed, while R3 is used to adjust the frequency of the square wave, and C1 is the oscillating capacitor. Since the frequency of the square wave is actually determined by the charging and discharging cycle of capacitor C1, the frequency can be changed as long as the charging and discharging cycle of capacitor C1 is changed. It can be found from the circuit that when closing switches S1 and S2 and disconnecting S3, the circuit is a square wave with adjustable frequency by adjusting resistor R3. From the circuit, it is not difficult to find that the duty cycle of square wave is determined by the charging and discharging time of capacitor C1. Therefore, to change the duty cycle of output voltage, the forward and reverse charging time constants of capacitor C1 must be different, that is, the parameters of the two charging circuits are different. Unidirectional conductivity of diode can be used to guide the current flowing through different pathways, circuit, when closing switch S3, disconnect the S1 and S2, adjust the resistance R2, can change the duty cycle square wave, because the frequency of the circuit is determined by the total resistance R2, so change the resistor R2 does not change the frequency of square wave, so that you can get a duty cycle adjustable square wave signal.