Path: utzoo!attcan!uunet!lll-winken!netsys!vector!nobody From: brian@umbc3.UMD.EDU (Brian Cuthie) Newsgroups: comp.dcom.telecom Subject: Re: sensitive relay wanted Message-ID: Date: 12 Oct 88 03:32:48 GMT Sender: chip@vector.UUCP Lines: 27 Approved: telecom-request@vector.uucp (USENET Telecom Moderator) X-TELECOM-Digest: volume 8, issue 161, message 6 X-Submissions-To: telecom@bu-cs.bu.edu (TELECOM Digest Coordinator) X-Administrivia-To: telecom-request@vector.uucp (USENET Telecom Moderator) In article tedk@ihuxv.UUCP (55624-Kekatos,T.G.) writes: >X-Administrivia-To: telecom-request@vector.uucp (USENET Telecom Moderator) > >In article wheels@mks.UUCP (Gerry Wheeler) writes: >| I'm looking for a relay to place in series with a telephone which will >| open (or close) when the set is offhook. The closest I've come is with > >Sorry, This can not be done with a ONLY a relay. You just can not >draw enough current from the phone line. You must use a >circuit that has a few transistors to SENSE the activity on the >phone line, and another transistor and 9volt battery to drive a >relay, if you still want relay contacts. This is really the RIGHT >way to do it. There ARE products on the market that do this >kind of thing. You will have to shop around. > Well, while I agree with you in principal, I must comment that AT&T then has been doing it incorrectly for *many* years. They have been using relays to sense loop current in 1A2 key systems for as long as I can remember. It is also not too uncommon to see C.O. equipment which relies on a relay of sorts. One point that noone has mentioned yet is that of longitudinal imbalance. You should always keep the line balanced. This means you should always do the same thing to *both* sides of the line. This is why the teltone relay is a four wire device. regards, brian