Path: utzoo!attcan!uunet!lll-winken!netsys!vector!nobody From: westmark!dave@rutgers.edu (Dave Levenson) Newsgroups: comp.dcom.telecom Subject: Re: sensitive relay wanted Message-ID: Date: 10 Oct 88 01:11:54 GMT Sender: chip@vector.UUCP Lines: 27 Approved: telecom-request@vector.uucp (USENET Telecom Moderator) X-TELECOM-Digest: volume 8, issue 156, message 3 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.ATT.COM (Kekatos) writes: > 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 > | a 200 ohm 4.5 mA relay... > > Sorry, This can not be done with a ONLY a relay. You just can not > draw enough current from the phone line. Yes, this _can_ be done with a relay. 1A, 1A1, and 1A2 key telephone systems all operate with relay coils in series with the telephone loop. You need a relay with less than 100 ohms, and one which will operate on less than 10 mA for reliable operation. To do it right, it should have two coils, of ~50 ohms each, and you should put one in series with Tip, and the other in series with Ring, to preserve line balance. To debug your existing circuit, place a D.C. milliampmeter in series with your loop and measure the loop current with your telephone set offhook. Try it with your relay in the circuit. The loop current should be 15 - 25 mA or so. The exact amount, of course, depends upon the kind of telephone set you use, and the length of the loop (miles to your central office). Get a relay that will operate on the current you measured. -- Dave Levenson Westmark, Inc. The Man in the Mooney Warren, NJ USA {rutgers | att}!westmark!dave