555 Astable

Circuit taken from the Texas Instrument LM555 data sheet. This circuit will trigger itself and free run as multivibrator. The external capacitor charges through RA + RB and discharges through RB. Thus the duty cycle may be precisely set by the ratio of these two resistors. The capacitor charges and discharges between 1/3 VCC and 2/3 VCC. The charge and discharge times, and therefore the frequency are independent of the supply voltage.