Path: utzoo!attcan!uunet!lll-winken!lll-tis!ames!killer!vector!nobody From: sleat@ardent.UUCP (Michael Sleator) Newsgroups: comp.dcom.telecom Subject: Re: sensitive relay wanted Message-ID: Date: 14 Oct 88 06:32:15 GMT References: Sender: chip@vector.UUCP Reply-To: sleat@ardent.UUCP (Michael Sleator) Organization: Ardent Computer Lines: 38 Approved: telecom-request@vector.uucp X-Submissions-To: telecom@bu-cs.bu.edu (TELECOM Digest Coordinator) X-Administrivia-To: telecom-request@vector.uucp (USENET Telecom Moderator) X-TELECOM-Digest: volume 8, issue 162, message 1 In article tedk@ihuxv.UUCP (55624-Kekatos,T.G.) writes: >My comments about using a single relay coil to detect loop-current, >that would not work because you can not draw enough current from >the phone-line, were if the >relay is connected in parallel with the phone line. I beg to differ. A long time ago, I wanted to record all activity on a phone line (never mind why! :-)). I didn't have much time to mess around, so I simply took what was known in the old days as a "plate relay", hooked it across the line (with a resistor in series, I think), and hooked the normally closed contacts up to the tape recorder. It worked fine. I don't remember the exact coil resistance, but the pull in current was certainly less than 2mA. I think that it might have been a 10kOhm coil, hence the resistor. Historical Note: A "plate relay" was so termed not because of any pecularities in the construction of the relay, but because it was intended to go in the plate (anode) circuit of a vacuum tube. Since tubes tend toward higher voltages and lower currents than transistors, a high resistance, low current relay was called for. >I never tried connecting a reply coil in series with a single phone. It ought to work just fine. In the off-hook state a phone draws somewhere around 30-40mA, which is plenty to trip a small relay. You might want to bypass the relay coil with a capacitor so that the inductance doesn't further roll off the high frequencies. On the other hand, in this case you want as low a coil resistance as you can get, so the inductance might not be significant at 3kHz or whatever the top end of the phone system is these days. I agree that active circuits are a much more desireable way to go, but I just wanted to point out that it is possible with a relay. Michael Sleator Ardent Computer ...!{decwrl | hplabs | ubvax | uunet}!ardent!sleat