Hello,

I have IC555 astable multivibrator. I configured it for 0.9 sec ON & 0.9 sec OFF. It is working fine also. There is one problem in it. When we start the IC 555 for first time the 1.2 sec high pulse will come and then circuit starts working fine for 0.9 sec ON & 0.9 sec OFF. I want to avoid this 1.2 sec High, it should be around 0.9 to 1.0 sec high pulse. Can any body help me in this?

Sagar

 

That is unavoidable. From the data sheet: "the (timing) capacitor charges and discharges between 1/3 Vcc and 2/3 Vcc." The initial period is long as the capacitor is charging from 0 volts.

 

You are fighting a classic 555 problem, charging the capacitor for the first time. Once the capacitor is charged the voltage bounces between 1/3 and 2/3 the power supply, but that first time the capacitor is 0 volts.

I helped someone not too long ago with something similar, but it took qty 2 555's. This is the thread...

http://forum.allaboutcircuits.com/showthread.php?t=34546

There are other ways to do this, but I don't have them handy. You could use a common collector to quick charge the cap, once the cap is over the emitter voltage it would not enter into the circuit again.

Have you read this?

Bill's Index

The 555 Projects

My Cookbook

LEDs, 555s, Flashers, and Light Chasers