OLE@SRI-NIC.ARPA (Ole Jorgen Jacobsen) (10/30/85)
SRI International: (415) 859-4536 Home: (415) 325-9427 Message-ID: <12155147608.12.OLE@SRI-NIC.ARPA> ReSent-Date: Wed 30 Oct 85 20:04:26-EST ReSent-From: The Moderator <TELECOM-REQUEST@MIT-XX.ARPA> ReSent-Sender: JSOL@MIT-XX.ARPA ReSent-To: Telecom-Individual-Messages-List: ; ReSent-Message-ID: <12155366867.30.JSOL@MIT-XX.ARPA> Thanks to all who responded to my query, most of the designs I received would work, but careful evaluation and lenghty discussions with several electronics wizards led me to the following conclusion: What is needed is a self-powered line monitoring device which plugs in like any phone and does not steal power from the telco line, such devices can apparently be purchased (from DAK and others) for about $10. Some observations: Putting things in series with the telco line is not very nice because you need to get in at the house entry point which could be out of the way and the increased resistance in the line would cause some audio attenuation. None of the circuits described do a very good job of protecting the LED from ringing voltage (which is about 90V). The current during ringing can be large enough to zap an LED since it has low internal resistance and even clever zener diode designs may not provide adequate protection. Finally, I should say that no circuit was actually built, the analysis was done mostly on paper. The self powered "line monitor" just seems a much more elegant design, even if no telco power is stolen, oh well.... <OLE> <370> -------