Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!att!mtuxo!rolls!westmark!dave
From: dave@westmark.UUCP (Dave Levenson)
Newsgroups: comp.dcom.modems
Subject: Re: Device to indicate phone line in use?
Message-ID: <1394@westmark.UUCP>
Date: 24 May 89 00:35:26 GMT
References: <298@indri.primate.wisc.edu>
Organization: Westmark, Inc., Warren, NJ, USA
Lines: 27

Most modems are configureable for use on multi-line key telephone
systems.  When this option is selected, the modem closes a relay
contact accross the second pair in its phone cord (the black-yellow
pair if standard inside wire color-coding is followed on the RJ-11
jack).  You can build a busy-lamp using a low DC voltage and a
suitable indicator lamp switched by the black-yellow pair, and the
indicator will be lit whenever either modem is off-hook.  For
example:

            |---|    /--------------------------------> YELLOW
110 vac ====|   |==== 10vac                     ~
            |---|    \------>|-----/\/\/\/----->|-----> BLACK
              ^              ^   |    ^         ^   |
              |              |   |    |         |   |
           transformer     diode | resistor    led  |
                                 |  68k~            |
                                 |                  |
                                 |                  |
                                 V                  V
                                to other indicators like
                                the above: resistor + led


-- 
Dave Levenson
{uunet | rutgers | att}!westmark!dave	
...the man in the mooney