An astable multivibrator using a 555 timer is a free-running oscillator that continuously switches between high and low states, producing a square-wave output. Unlike the monostable mode, it does not require a trigger input. Two resistors and a capacitor connected to the 555 determine the frequency and duty cycle of oscillation. The capacitor charges and discharges alternately through the resistors, causing the output to toggle between on and off states. The period of oscillation is given by T = 0.693 × (R1 + 2R2) × C. This circuit is widely used for clock pulses, flashing lights, tone generation, and waveform timing.
