t=1.1RC - gives wrong delay

I set my monostable 555 timer delay time with t= 1.1*R*C (t= 1,100,000 ohms * .0001 F * 1.1) it should have given me 121 seconds. What I am geting is closer to 165 seconds. What am I doing wrong?

Thanks,

Posted by hvanflee on 28/07/2003, at 23:55 GMT

Component Tolerance or Capacitor Leakage

This could just be due to the values of the components not being exactly what they should be, but is more likely to be due to capacitor leakage since you are using a 100uF electrolytic capacitor and a large value resistance.

The 555 datasheet actually says not to use electrolytic capacitors, but they will normally work ok.

Try John Hewes's suggestions on solving a similar problem at http://www.eleinmec.com/board_message_view.asp?messageID=56

Posted by Tim Surtell on 31/07/2003, at 19:55 GMT