555 timer

in a 555 timer why do we conect the freq, comp capacitor only at pin no 5 can't noise affect
at other terminals?

Posted by Anonymous (farhani22@indiatimes.com) on 15/07/2003, at 17:08 GMT

Stopping interference

Yes, interference can affect the other terminals, but in most 555 circuits these are connected to other components.

Normally, if an input pin of an IC like the 555 is not going to be used, we 'tie' it to one of the supply rails (Vs or 0V) with a piece of wire so that it can not be affected by interference.

For example, if you didn't need to use the Reset input (pin 4) in a circuit, you would connect it straight to Vs, since it won't then reset the 555.

The control input (pin 5) is a bit different though, because connecting it to one of the supply rails would change the timing characteristcs of the 555. That's why it is connected to 0V via a capacitor.

Posted by Tim Surtell on 15/07/2003, at 19:59 GMT

Interference

I am using a 555 on a project and I am not using the Treshold and Discharge pin. So what you mean is that I need to conect this pins to ground?

I took the diagram from the internet and does pins was not conected...........

THX,

Posted by Anonymous on 19/08/2003, at 01:04 GMT

Not those pins though!

No, you don't connect these pins to ground if not in use, they are exceptions as they are not like normal digital inputs - they are normally used for the timing components in 555 circuits.

Posted by Tim Surtell on 19/08/2003, at 22:36 GMT