An astable multivibrator, often called a free-running multivibrator, is a rectangular-wave generating circuit. Unlike the monostable multivibrator, this circuit does not require any external trigger to change the state of the output, hence the name free-running. Before going to make the circuit, make sure your 555 IC is working. For that go through the article: How to test a 555 IC for working An astable multivibrator can be produced by adding resistors and a capacitor to the basic timer IC, as illustrated in figure. The timing during which the output is either high or low is determined by the externally connected two resistors and a capacitor. The details of the astable multivibrator circuit are given below.
Take a look @ 555 Ic Pin configuration and 555 block diagram before reading further.
Pin 1 is grounded; pins 4 and 8 are shorted and then tied to supply +Vcc, output (VOUT is taken form pin 3; pin 2 and 6 are shorted and the connected to ground through capacitor C, pin 7 is connected to supply + VCC through a resistor RA; and between pin 6 and 7 a resistor RB is connected. At pin 5 either a bypass capacitor of 0.01 F is connected or modulation input is applied.
Astable Multivibrator Operation:
For explaining the operation of the timer 555 as an astable multivibrator, necessary internal circuitry with external connections are shown in figure.