Path: utzoo!utgpu!water!watmath!clyde!att!pacbell!ames!netsys!vector!nobody From: kaufman@polya.stanford.edu (Marc T. Kaufman) Newsgroups: comp.dcom.telecom Subject: Re: sensitive relay wanted Message-ID:Date: 28 Sep 88 15:07:41 GMT Sender: chip@vector.UUCP Reply-To: kaufman@polya.stanford.edu (Marc T. Kaufman) Lines: 16 Approved: telecom-request@vector.uucp (USENET Telecom Moderator) X-TELECOM-Digest: volume 8, issue 147, message 8 X-Submissions-To: telecom@xx.lcs.mit.edu (Mailing List Coordinator) 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 >a 200 ohm 4.5 mA relay. It didn't affect the phone operation too much, >but wouldn't quite pull in. If I pressed the armature in, it would >latch. Close, but no cigar. You want to use a voltage comparator IC to measure the voltage between the wires (through a full wave bridge rectifier, so you don't have to care which way you hook it up). Open circuit (on-hook) voltage is about 48, off-hook voltage is about 1-2 volts. I am not sure what "hold" voltage is, but it is higher than 2 volts. Simpler, for a single phone, is to see if the phone has a spare set of contacts on the hook switch (many do), and use them. Marc Kaufman (kaufman@polya.stanford.edu)