This circuit diagram shows how a 555 timer IC is configured to perform as an astable multivibrator. An astable multivibrator is a timing circuit whose 'low' and 'high' states are each unstable. As such, the output of an astable multivibrator toggles between 'low' and 'high' continuously, in result generating a train of pulses. This circuit is therefore conjointly known as a 'pulse generator' circuit.
In this circuit, capacitor C1 charges through R1 and R2, eventually building up enough voltage to trigger an interior comparator to toggle the output flip-flop. Once toggled, the flip-flop discharges C1 through R2 into pin 7, which is that the discharge pin. When C1's voltage becomes low enough, another internal comparator is triggered to toggle the output flip-flop. This once more|once more|another time|yet again|all over again} permits C1 to charge up through R1 and R2 and the cycle starts everywhere again.
C1's charge-up time t1 is given by: t1 = zero.693(R1+R2)C1. C1's discharge time t2 is given by: t2 = zero.693(R2)C1. Thus, the overall period of one cycle is t1+t2 = zero.693 C1(R1+2R2). The frequency f of the output wave is that the reciprocal of this period, and is therefore given by: f = 1.44/(C1(R1+2R2)), wherein f is in Hz if R1 and R2 are in megaohms and C1 is in microfarads.