Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!iuvax!watmath!egvideo!timk
From: timk@egvideo.UUCP (Tim Kuehn)
Newsgroups: sci.electronics
Subject: Re: How can I restrict outgoing phone calls? (2nd try)
Summary: it's not too hard
Message-ID: <2019@egvideo.UUCP>
Date: 22 May 89 04:00:00 GMT
Reply-To: timk@egvideo.UUCP (Tim Kuehn)
Organization: A Box in the Basement, Kitchener, ON
Lines: 45

In article <5170029@hplsla.HP.COM> tomb@hplsla.HP.COM (Tom Bruhns) writes:
>
>Well, this is something I would like to do also (as part of a more
>grandiose phone plan).  So far my thoughts lead to a project that is
>probably bigger than $370, unless it could be done in volume.  If I
>do it, it will be a labor of love.  But your posting got me thinking
>about a simpler solution that may work for you.  Here it is:
>
>Put a box in series with the phone line.  

..technical details deleted....

>
>Tom Bruhns
>tomb%hplsla@hplabs.hp.com


Actually - a design for something like this is pretty simple. I'm putting
something together that does this same kind of function (plus a good number
of other bells and whistles) and can suggest the following (for any 
hobbyist/HW guy out there).

Quite simply, what you want is a pulse/code decoder, a DTMF decoder, a
on/off hook relay (for the pulse/code decoder and to detect off/on hook)
a cutoff relay, and a uC with enough EPROM for the SW to decode what's going 
on the line to hang to phone up if more than seven digits are dialed (or 
a "0" by itself, or other 'illegal' type phone call). 

Basically you take interface the DTMF decoder and the pulse count/decode
lines to the telco, program the uC to only allow seven digit codes to go
through and watch for an intial "0" immediately after on-hook is detected, 
and that's it!

Including a PS, chips, and other miscellany, it should run between $75 - $100 
US to build one from scratch. The hardest part would be programming the 
uC. (I'm using an 68HC05C8 for my project). 

Hope this gives some idea(s) for actual construction of such a device.

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