Path: utzoo!attcan!uunet!lll-winken!netsys!vector!nobody From: kitty!larry@cs.buffalo.edu Newsgroups: comp.dcom.telecom Subject: Submission for comp.dcom.telecom Message-ID: Date: 9 Oct 88 16:12:30 GMT Sender: chip@vector.UUCP Lines: 50 Approved: telecom-request@vector.uucp (USENET Telecom Moderator) X-TELECOM-Digest: volume 8, issue 157, message 5 X-Submissions-To: telecom@bu-cs.bu.edu (TELECOM Digest Coordinator) X-Administrivia-To: telecom-request@vector.uucp (USENET Telecom Moderator) In article MCHARRY%BNR.CA@MITVMA.MIT.EDU (J.) writes: > Some years ago I saw a nifty way of putting a 'busy light' > on a phone. Normal phone systems have the tip wire grounded > and the ring hooked to -48v. (Some systems fail to provide a > ground reference.) ALL telephone systems are designed to provide a "ground reference", and for certain, ALL central offices would be installed in this manner. The only exception to the above is a PABX which was incorrectly installed (it certainly wasn't designed that way!) without a ground connection. > The trick is that both -48 and ground are > applied through about 200 ohms. When a phone on the loop is > off hook, tip and ring are nearly shorted together, and appear > at about -25v to ground. An led hooked through a resistor to > the tip side will come on if anyone is off hook. Of course, > there is a catch or two: 1) You have to make sure the ground > wire is connected and not just tip and ring. You may have to > hook it up at the building entrance block. 2) This will > likely introduce a large ground loop and a lot of hum. A loud hum, eh? What we have here is, ahem, a classic example of what is properly called "longitudinal imbalance". You won't get FCC Part 68 certification with that design! :-) > I haven't > tried this, but old ringers are usually rigged with two windings > having a blocking cap between them. Going from the tip side > winding to ground might stifle the hum. Alternatively, a good > sized choke in series with the led ought to kill it also. The > resistor is 25k / (number of mils to light the led). I would suggest not trying ANY variation of the above scheme. There is NO WAY to design an acceptable circuit based upon the above technique. There is never any justification to sense line status (i.e., on-hook, off-hook) by means of any external ground reference, regardless of how high a bridging impedance is placed across tip and ring. The only exception to this statement involves ground-start PABX trunks or coin telephone lines - which is not exactly what we are talking about. There are numerous ways to sense line status by means of ISOLATED sensing of series loop current, or by high-impedance (> 100,000 ohms) briding across tip and ring to sense loop voltage. No ground reference is needed in either of these techniques. <> Larry Lippman @ Recognition Research Corp., Clarence, New York <> UUCP: {allegra|ames|boulder|decvax|rutgers|watmath}!sunybcs!kitty!larry <> VOICE: 716/688-1231 {att|hplabs|mtune|utzoo|uunet}!/ <> FAX: 716/741-9635 {G1,G2,G3 modes} "Have you hugged your cat today?"