CIRCUIT DIAGRAM OF 555TIMER IN ASTABLE MODE
An astable multivibrator, often called a free running multivibrator, is a rectangular wave generating circuit. The circuit doesnt need any triggering. The timing of the circuit is determined by the resistors and the capacitors used in it. The details of the astable multivibrator are:
Pin1 is grounded; Pins 4 and 8 are shorted, Pin 3 gives the timing output; Pins2 and 6 are short circuited and connected with a capacitor C1; at Pin 5 a bypass capacitor is used having value of 0.01uF; a supply of 3v is given to the circuit, the suppy can be more upto 9v. Pin 7 is connected through Pin 4 through a resistor R1and also with pin 6 through resistor R2.
The output pin 3 of the circuit is connected with a led having positive side in the pin 3 and negative side with the ground.
The overall oscillation period of the circuit an be determined by
T = 0.693.(R1 + R2).C1 + 0.693.R2.C1 = T,high + T,low
where T,high = 0.693.(R1 + R2).C1
T,low = 0.693. R2. C1
the frequency of oscillations= 1/T =1.44 / ( R1 + 2R2).C
PIN CONFIGURATION OF A 555 TIMER
Here is a video showing the working of a 555 timer in astable mode: