Possibly use a constant current source and an optoisolator.

Using the constant current source makes sure that no matter how long the 
wire is the current remains constant at either end.

Dom
----- Original Message ----- 
From: "Ben Wheare" <piclist@bwgames.net>
To: <piclist@mit.edu>
Sent: Wednesday, October 24, 2007 5:55 PM
Subject: [EE] Detecting open/closed circuit over long distance


> Hi there,
>
> I'm trying to set up a circuit that'll detect power failure at a remote 
> site
> (approximately half a mile away by cable length).
> Currently, we have 8 twisted pairs between the main centre and the remote
> site.
>
> I'd like to use one of those to detect a mains power fail, and use a PIC.
>
> So far, my plan consists of a voltage-controlled relay (
> http://www.crouzet.com/c-lynx/html/en/c-lynx-voltage-control.htm - the
> MUS260ACDC) at the remote site, connected between mains and one of the
> pairs. If power fails, the relay is de-energized.
>
> However, I'm not sure how best to detect circuit open/closed state 
> remotely.
> My initial thought was to ground one end of the twisted pair, at the main
> centre end.
> Then I would have the other end (in the main centre) of the twisted paid
> connected to a PIC input pin, then I could read it (if PIN==0) then power
> ON, else power OFF.
>
> Would this work over such a long distance, or is there a better option 
> that
> I haven't thought of?
>
> I only really need it to drive a lamp, so I was thinking of eliminating 
> the
> PIC altogether, but it seems better to keep it in, then I can add more
> features (e.g. get it to page someone if necessary) - depends whether a
> PIC/relay solution would work over this distance.
>
> Suggestions?
>
> Thanks,
>
> Ben
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> 

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