555 Timer - Astable mode

Astable operation is a free running oscillator without any stable state. The threshold terminal (pin 6) and the trigger terminal (pin 2) are connected together to form a self-trigger. This ensures that an external trigger is not required for its operation.

The pulsewidth is determined by two external resistors and one capacitors using the constant 0.693(R1 + 2R2)C1


The formular for calculating the on-time is 0.693(R1 + R2)C1
The formular for calculating the off-time is 0.693R2C1

 555 IC Pinout

PIN NO	DESCRIPTION
1	Ground
2	Trigger
3	output
4	Reset
5	Control voltage
6	Threshold
7	Discharge
8	Vcc

Below are pictures that shows the pulsewidth for different values of R1, R2 and C1