Path: utzoo!attcan!uunet!decwrl!wuarchive!zaphod.mps.ohio-state.edu!uwm.edu!linac!tellab5!laidbak!obdient!vpnet!cgordon
From: cgordon@vpnet.chi.il.us (Gordon Hlavenka)
Newsgroups: sci.electronics
Subject: DTMF Decoder IC
Message-ID: <27114b84-483.1sci.electronics-1@vpnet.chi.il.us>
Date: 9 Oct 90 05:55:03 GMT
References: <14793@mentor.cc.purdue.edu>
Lines: 26


After you get this thing replaced, do this:


input ----||---/\/\/\/---+----+------to decoder input
         1uF   1.2K     _|_   |
                        \ /  ---
                         V    A  (Silicon diodes, like 1N4148)
                        ---  /_\
                         |    |
                         +----+
                         |
                        GND

This should block the ring voltage for you...  It takes off any DC, does a
current limit (which shouldn't affect sensitivity since the decoder input is
Hi-Z) and then limits the voltages to 1 diode drop -- about .7V

Plenty of signal is left over for the decoder, but ring voltage is reduced
to non-injurious levels.

If you're looking for _any_ DTMF decoder, look for the SC11204 from Sierra.
(Sierra: 408-263-9300)

-----------------------------------------------------
Gordon S. Hlavenka            cgordon@vpnet.chi.il.us
Disclaimer:          Sierra who?  Never heard of 'em!