Hi Today Instructable is very simple and easy to build ASTABLE Multivibrator CircuitA Complete Step By Step Video Tutorial is also given in the video below.

Astable Multivibrator mode of 555 timer IC is also called Free running or self-triggering mode. Unlike Monostable Multivibrator mode it doesnt have any stable state, it has two quasi stable state (HIGH and LOW). No external triggering is required in Astable mode, it automatically interchange its two states on a particular interval, hence generates a rectangular waveform. This time duration of HIGH and LOW output has been determined by the external resistors (R1 and R2) and a capacitor(C1). Astable mode works as a oscillator circuit, in which output oscillate at a particular frequency and generate pulses in rectangular wave form.”