wheels@mks.UUCP (Gerry Wheeler) (09/23/88)
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.
Perhaps I should describe what I'm up to. I want to share a phone line
between a fax machine and a phone or modem. Only the fax will receive
incoming calls. I want to put a relay coil in series with each device.
Its normally closed contacts will be in series with the *other* device.
Thus, when one is in use, the other is excluded.
So, what type of relay should I be looking for (impedance/current). Is
such a thing commonly available? Will this scheme work? Does ringing
current cause any problems?
Our site doesn't receive this group, so direct email would be
appreciated.
--
Gerry Wheeler Phone: (519)884-2251
Mortice Kern Systems Inc. UUCP: uunet!watmath!mks!wheels
35 King St. North BIX: join mks
Waterloo, Ontario N2J 2W9 CompuServe: 73260,1043kaufman@polya.stanford.edu (Marc T. Kaufman) (09/28/88)
In article <telecom-v08i0146m04@vector.UUCP> 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)
kirk@tikal.TELTONE.COM (Kirk Bellar) (09/30/88)
Teltone sells a relay for this purpose, called the M-949. It closes a 1 form A (SPST) contact if loop current is 20 mA or better. You can contact Teltone Ltd. in Markham for particulars: Teltone Ltd. 183 Amber Street, Markham, Ont. L3R 3B4 (416) 475-0837 Good luck with your project! --- Kirk Bellar KE7TN Teltone Corp. (Tones `R' Us) uw-beaver!tikal!kirk Kirkland, Wa. USA (206) 827-9626 "Oh what a tangled web we weave, when first we practice to conceive..."
wheels@mks.uucp (Gerry Wheeler) (09/30/88)
I originally asked about using a sensitive relay to detect which of two devices (phones, faxes, whatever) was offhook. (I didn't phrase it that way, but that was the gist of it.) In my situation, one will be a modem, and the other a fax. In article <telecom-v08i0147m08@vector.UUCP>, kaufman@polya.stanford.edu (Marc T. Kaufman) writes: > You want to use a voltage comparator IC to measure the voltage between the > wires. I thought about that originally, but it won't work. It will tell me if either device is offhook, but not which one because they're wired in parallel. > 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. Agreed. I think some Hayes type modems include an extra pair of contacts to be used on the A-A1 lines of a key system. However, it's not so easy to do with a fax. Unfortunately. -- Gerry Wheeler Phone: (519)884-2251 Mortice Kern Systems Inc. UUCP: uunet!watmath!mks!wheels 35 King St. North BIX: join mks Waterloo, Ontario N2J 2W9 CompuServe: 73260,1043
brian@umbc3.UMD.EDU (Brian Cuthie) (10/05/88)
In article <telecom-v08i0146m04@vector.UUCP> wheels@mks.UUCP (Gerry Wheeler) writes: >X-Administrivia-To: telecom-request@vector.uucp (USENET Telecom Moderator) > >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. > All you really need to do is use an Opto-Isolator. It is a good idea to either use an AC opto-isolator, or to use a full wave bridge rectifier in front of it.,Also, bypass the diode with a 470 ohm 1/2 watt resistor. This technique works quite well. If you need more info just call me. I hate trying to write a novel. -brian (301) 381 - 1718
tedk@ihuxv.ATT.COM (Kekatos) (10/05/88)
{{{ I have edited down the quoted material in this message. -chip }}}
In article <telecom-v08i0146m04@vector.UUCP> 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.
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 have several phone extensions in my home, and several other devices
that use the phone lines (modems). I wanted to build a device that
detects off-hook and lights a LED on each phone. (The modem are smart
enough to check for dial-tone).
The only circuit that worked (and worked quite well) is a circuit
with several transistors and a battery to light the LEDs. I use one of
those self-blinking LEDs from Radio shack.
I never tried connecting a reply coil in series with a single phone.
------
Ted G. Kekatos
UUCP: ..!att!ihuxv!tedk (312) 979-0804
AT&T Bell Laboratories, Indian Hill South, IX-1F-460
Naperville & Wheaton Roads - Naperville, Illinois. 60566 USAdave@rutgers.edu (Dave Levenson) (10/10/88)
In article <telecom-v08i0150m10@vector.UUCP>, tedk@ihuxv.ATT.COM (Kekatos) writes: > In article <telecom-v08i0146m04@vector.UUCP> 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
wheels@mks.UUCP (Gerry Wheeler) (10/12/88)
In article <telecom-v08i0150m10@vector.UUCP>, tedk@ihuxv.ATT.COM (Kekatos) writes: > Sorry, This can not be done with a ONLY a relay. You just can not > draw enough current from the phone line. > > Ted G. Kekatos > AT&T Bell Laboratories, Indian Hill South, IX-1F-460 I have to disagree with this. I've seen several items which draw enough line power to run a relay. One was a Radio Shack box which sensed line current to close a relay which could be wired to the A-A1 control leads of a key system. It allowed an answering machine to be used and to give an indication on all the sets. (In fact, if Radio Shack still sold this one, I would get it and use the relay. Sigh.) I received a mail msg the other day on this, as follows: > From: kirk@tikal.TELTONE.COM (Kirk Bellar) > Subject: Re: sensitive relay wanted > Date: 29 Sep 88 21:44:28 GMT > > Teltone sells a relay for this purpose, called the M-949. It closes a 1 > form A (SPST) contact if loop current is 20 mA or better. > --- > Kirk Bellar KE7TN Teltone Corp. (Tones `R' Us) > uw-beaver!tikal!kirk Kirkland, Wa. USA (206) 827-9626 I called my closest Teltone rep, and they are going to send me one as soon as they can get it from the office in Kirkland. (They kindly offered to waive the usual $100 minimum order.) So, I'll let you know what happens. Thanks for all the replies. -- Gerry Wheeler Phone: (519)884-2251 Mortice Kern Systems Inc. UUCP: uunet!watmath!mks!wheels 35 King St. North BIX: join mks Waterloo, Ontario N2J 2W9 CompuServe: 73260,1043
brian@umbc3.UMD.EDU (Brian Cuthie) (10/12/88)
In article <telecom-v08i0150m10@vector.UUCP> tedk@ihuxv.UUCP (55624-Kekatos,T.G.) writes: >X-Administrivia-To: telecom-request@vector.uucp (USENET Telecom Moderator) > >In article <telecom-v08i0146m04@vector.UUCP> 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
sleat@ardent.UUCP (Michael Sleator) (10/14/88)
In article <telecom-v08i0153m05@vector.UUCP> 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
kevin@calvin.ee.cornell.edu (Kevin Tubbs) (11/02/88)
Radio Shack used to sell a $4.95 adapter to control A and A1 leads for 1A2
key systems - all it was was a relay coil in series with the phone line.
Worked very well, except when trying to pick up a line that was on hold,
because the hold circuit in the KSU diverted enough current to prevent the
adapter relay from picking.
---
Kevin Tubbs, 5152 Upson, Cornell University, Ithaca NY, 14853 (607) 255-8703
kevin@calvin.ee.cornell.edu {uunet,rochester}!cornell!calvin!kevin