sir,
I want to generate a square wave using 555(in astable or monostable mode) in such a way that when an input(high/low) occurs it should generate sqare wave continuosly and when the input disappears it should stop generating.please suggest any idea.

 

Use the 555 in astable mode. Use the reset pin to control whether it outputs a square wave or not. When the reset pin is high it will work however when it is low it will stop.

 

Pin 4 is a enable/disable for the timer, so it is the digital input you are looking for.

Have you seen these?

The 555 Projects

 

thank u i will try and see its working or not