Posted on Dec 7, 2012

This astable will trigger itself and run free as a multivibrator. The external capacitor charges through RA and RB and discharges through R8 only. Thus, the duty cycle is set by the ratio of these two resistors, and the capacitor charges and discharges between 1/3 Vs and 2/3 Vs.

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The charge and discharge times, and therefore frequency, are independent of supply voltage. The free-running frequency versus RA, RB and C is shown in the graph.

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