Hi all, i need to generate a pure square wave of 0V lo and 5V high of abt 2.5KHz...... i tried this circuit and it gave me a square wave of abt 0.5 or may be 1V lo..... i need a wave that has exactly 0V as lo..... 5V at hi is nt necessary it can be 3V or watever bt there has be be 0V as lo..... plz suggest a schematic

 

What do you want to drive and you need exactly 0V?

 

Just select a resistor for R1 that is on the small side compared to R2. The greater the ratio, the better the duty cycle.

For example, a 75KΩ and 1KΩ resistor set will produce a 50.33% duty cycle.

 

A BJT 555 will not get to 0v output.
A CMOS 555 might get near 0v output, but not quite there.

If you need precisely 0v output, you're going to have to tell us a lot more about your load requirement. Right now, we don't know anything about your load.