555 multi-resonant swing circuit: The moment the circuit is connected to the power supply, because the capacitor C has no time to charge, Vc=0v, and the output Vo is high. At the same time, the collector output end (pin 7) is disconnected to the ground, the power supply Vcc charges the capacitor C, the circuit enters a transient state, and then the circuit repeatedly produces a periodic output pulse. The maintenance time of the two transient states of the multivibrator depends on the parameters of the RC charging and discharging circuits. The maintenance time of transient state Ⅰ, that is, the forward pulse width of output Vo T1≈0.7(R1 R2)C; The maintenance time of transient state II, that is, the negative pulse width of output Vo T2≈0.7R2C.
Therefore, the oscillation period T=T1 T2=0.7(R1 2R2)C and the oscillation frequency f=1\/T. The ratio of the forward pulse width T1 to the oscillation period T is called the duty cycle D of the rectangular wave, which can be obtained from the above conditions D= (R1 R2)\/(R1 2R2), if R2> > R1, then D≈1\/2, that is, the positive and negative pulse width of the output signal is equal to the rectangular wave (square wave).