Re-Triggering a 555IC without the output going low

[Otoole]

Hi All,

Wish I had a pen and paper, this one's easier drawn

I have a situation where I have a 'flickering' alarm closure which I need to effectively regulate into a solid closure.

I guess what I'm looking for is a monostable circuit with some way of re-triggering the time delay without sending the output LOW at any stage.

I can't let the 'flickering' alarm simply re-trigger the 555 on it's own unfortunately because that would set my auto-dialer (which sits on the end of this thing) would dial up my 5 pagers again :-(

Any suggestions or pointers would be MUCH appreciated.

Thank you all in advance.

Tim.

 

[derekmuller]

For monostable vibrators, a site I find useful with all types of nice circuits is Tony van Roon's site. Just search for it on a search engine. Hope this will help!

 

[melone]

First off, I would put your input signal through a schmidt-triggered device. Something like a 74LS244 should do the trick. Be careful, only the LS version of the 244 has schmidt-trigger inputs. If you are determined to use HC, HCT, etc., you will have to find another gate.

Second, why doesn't the LM555 timer work? Take a look at the application notes on the LM555 timer,

http://www.national.com,

and look at the monostable. I believe that the monostable will not retrigger on transitions during the "unstable" portion of the waveform. Otherwise, you will have to set-up either a latch circuit (R-S latch) or a simple D-Flip Flop using the output of the 555 as the clock signal.

Just a few thoughts. Keep us posted and good luck!!!

 

[electricuwe]

Just the last issue of EDN (Feb 21st 2002) contains an article on that topic. You can find it at:

http://www.e-insite.net/ednmag/index.asp?layout=article&articleId=CA196470

The task you decribe is usually called debouncing and you will find lots of other tips if you search for the term "debounce".

 

[TerryKing]

You may need a "retriggerable One-shot" such as a 74ls122 or 123, as I recall..
The TI app notes show this application...