a free-running multivibrator(An astable multivibrator) based on 555 timer
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.