Hi Rob,
Have a look at the CMOS 4538 we use it every where to detect loss of
signal very straightforward to use you set C and R to give you a pulse
width slightly longer than the ones you are detecting so they will keep
on retriggering the monostable so the output will be up while they are
there.
To deal with the negative pulses I would use a transistor (PNP probably)
or for less hassle an inverting op-amp with gain set at 1 or you can
even decide to condition the pulses with it.
This is the link for the datasheet:
http://www1.jaycar.com.au/images_uploaded/CD4538BC.PDF

But like some people said use a micro like that you can play with the
pulse width that you are detecting and the output of the circuit as much
as you can. I had a project where I needed a 250uS every 250mS, I first
used a 556 to generate the times and it was an absolute pain to get the
times right and then when I decide to change them even worse. So I
decided to use a micro to do it ( 68hc08) and I just built the circuit
and it took half hour to write the code, changing timing now is just
easy and I do not need to get the soldering iron out. 
Hope it helps.
Best regards
		Luis  



-----Original Message-----
From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf
Of Rob Robson
Sent: 30 March 2006 18:29
To: pic microcontroller discussion list
Subject: [EE]: looking for retriggerable 555 monostable circuit

I'm looking for a 555-based monostable (one-shot) circuit whose output
will 
STAY high as long as short negative-going pulses are present at the
input. 
The textbook 555 monostable circuit doesn't do this.  I have a spare NPN
on 
the board to utilize if necessary.  Has anyone come across a 555 circuit

that will do this?

Many thanks,
RR 


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