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PICList Thread
'[OT]Private telephone network'
1999\04\06@141449 by Barry King

Jean-Yves wrote:
> My wish is to be able to phone (and why not speak ?) between 2 or 3
> local telephones without going "out" to the ordinary and public
> telephone network. (In fact I wish my wife to call me for dinner
> when I am in the attic, making electronics or PIC assembler :-)
> I thought of a pic that could monitor ringing and make the adequate
> switches ... But perhaps, only some wiring could be enough ?
Uhh... If that is really ALL you want to do, you can connect two
standard telephone sets, in SERIES, with a 48 V DC power supply.
Then if you both pick up, you have an intercom.

If you want the phone to ring, you have to capacitor-couple a 60 V AC
(or so) signal onto the 48 V loop.  The easy way to do this is to use
an isolation transformer from the AC line.  But, I hesitate to
recommend experimenting with AC power-- you can get killed.  But
thats the minimal components, easy to do solution.

> The obvious answer is to buy and install a small PBX.
But that's overkill for an intercom...

> The question is: Can it be done with PICs?
> The answer is: Probably not.

I disagree.   It is if you're willing to stay in the analog domain
for the audio. How about RJ-11 jacks with digital control on the
outer pair, and standard loop-start style audio on the center two.

But that would require "star" (aka "home run") wiring.

Our commercial PBX uses a variant of this system, so the audio is NOT
digitized, just switched by a digital controller.  Don't forget,
plain old telephone service lines are analog (to the end of the local

Well within the capabilities of a PIC.  Yea!  We're back on topic!!

> For the PBX the task is much more difficult, like extension management,
> probably bus management, not to mention multi-line, music-on-hold, voice
> prompts, tapeless answering machine with multiple mailboxes, Caller-ID
> management, dialing memories, special commands (hold, transfer, conference
> call, forwarding, and many more), FAX and/or Modem tone recognition, and a
> bunch of other things that you expect from a modern digital PBX.

All worthy to add (once you can get the intercom working)!

If I wasn't so busy doing this current project...

Barry King
Engineering Manager
NRG Systems "Measuring the Wind's Energy"
Phone: 802-482-2255
FAX:   802-482-2272

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