For the 555 square wave oscillator circuit to obtain a square wave output, the circuit of Figure 9 can be used. It consists of a diode VD connected in parallel with RB on the basis of the circuit of Figure 8. When RA = RB, the charge and discharge time constants of C are equal, and the output obtains a square wave. The frequency of the square wave is f=0.722 / RAC ( RA = RB )
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The diode method can obtain a pulse oscillation circuit with an adjustable duty cycle. 
On the basis of this circuit, the addition of potentiometers in the RA and RB loops and the use of series or parallel 555-pulse oscillating circuits are often used as AC sources, with oscillation frequencies ranging from a few Hz to a few megahertz. Because the circuit is simple and reliable, it is extremely widely used.
