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Re-Triggering a 555IC without the output going low

Re-Triggering a 555IC without the output going low

Re-Triggering a 555IC without the output going low

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.


RE: Re-Triggering a 555IC without the output going low

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!

RE: Re-Triggering a 555IC without the output going low

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, 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!!!

RE: Re-Triggering a 555IC without the output going low

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

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