How do I detect telephone "off hook" condition?
James W. Lynch
Does anyone have plans for a simple off-hook detector? I need to know
when someone has lifted a receiver. A dial-tone detector circuit would
also work.
Any suggestions?
Thanks,
Jim.
ORONITI
integrated circuit. Basically you should detect a DC loop current.
Edward Lawrence
1. The phone company puts appx. 50 volts on your line, through the
equivalent of about 1000 ohms in series. If your phone is 'on hook', a normal
Voltmeter will read that 50 volts.
2. When you take your phone 'off hook', the voltage will drop. How
much? Depends on the phone. I have seen readings of 7 to 26 VDc. If another
extention phone goes 'off hook', the voltage will drop a bit lower.
A 28 VDC, 5 K ohm relay wired directlt across the telephone line will
normally drop out when a phone goes 'off hook', while not loading the line too
badly. I made a telephone recorder activation circuit this way, and added 200
or so turns arould the coil to pick off the audio. Worked first time!
Hope this helps.
Robb Hammack
comments, I thought I'd mention that in *most* residential lines,
the phone company's Central Office (CO) detects a Off-Hook
condition when a small current is drawn on the line. This is known
as "Loop Start". the alternative, mainly used in larger PBX's
(Private Branch Exchange, I.E. Company phone system) is "Ground
Start" where the CO equipment detects a momentary ground on the
line as an off-hook condition. SAM'S has an excellent book in
their "Understanding" Series (Title:Understanding Telephone
Electronics) that is well worth eading if you'd like to get into
telephone devices/accessories. Radio Shack also carries a book on
telephone electronics. Incidentally, the "On-Hook" voltage of a
standard phone line is 48VDC by spec, although this can vary
depending on distance from your local CO, Etc. a typical
Single-Line phone has an off-Hook resistance of Approx. 400 Ohms
this provides a 120ma draw on the 48V line, which is limited to
Typically 15ma. this is what causes the drop in voltage to approx
5-10VDC when a phone goes off-Hook (15ma * 400 Ohms = 6V) the
variance is caused by the fact that the CO has a lot of slop in
their current-Limiting :-) I seem to remember that they are
required by law to limit it to less than 150ma, which leaves them
a LOT of room.
I hope this proves helpfull
Robb Hammack
John Lundgren
: I don't know where this started, so I might be redundent with my comments.
That's way too much load on the line IHMO. The ampunt of current drawn by
the relay during the on hook is way too high: several milliamps! The CO
may measure this and determine that there is too much leakage current and
try to repair the lines. This could cost you the service call if they
find that the problem is in the CPE (customer provided eauipment). Or
the current could cause the switch to think that the phone was off hook
all the time, and then it would disable the line.
: Hope this helps.
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John Lundgren
: Hello! I missed the original post, but in addition to the other
The phone line should supply a minimum of 26 mA, according to the info in
"The Art Of Electronics". Anything below 20 mA, and the audio level will
be reduced. I've seen short circuit currents in the 30 to 60 or more mA
range, depending on the distance from the CO.
I don't understand how you can say "120 mA draw" (see above) if the
current is limited to something less.
Richard Steven Walz
John Lundgren <jlun...@delta1.deltanet.com> wrote:
>Edward Lawrence (eal) penned:
>: I don't know where this started, so I might be redundent with my comments.
>
>: 1. The phone company puts appx. 50 volts on your line, through the
>: equivalent of about 1000 ohms in series. If your phone is 'on hook', a normal
>: Voltmeter will read that 50 volts.
>: 2. When you take your phone 'off hook', the voltage will drop. How
>: much? Depends on the phone. I have seen readings of 7 to 26 VDc. If another
>: extention phone goes 'off hook', the voltage will drop a bit lower.
Some outdoor connection boxes have the new loopback tester modules
that permit remote operator performed loop-back testing, and free of
charge, then the operator can test your line to, but not including,
your home wiring. The voltage past these is 25 or 26 volts, so lately
the off-hook voltage is much lower, down in the 3 to 7 volt range
during dial and busy tones and for talking when connected.
-Steve
>: A 28 VDC, 5 K ohm relay wired directlt across the telephone line will
>: normally drop out when a phone goes 'off hook', while not loading the line too
>: badly. I made a telephone recorder activation circuit this way, and added 200
>: or so turns arould the coil to pick off the audio. Worked first time!
>
>That's way too much load on the line IHMO. The ampunt of current drawn by
>the relay during the on hook is way too high: several milliamps! The CO
>may measure this and determine that there is too much leakage current and
>try to repair the lines. This could cost you the service call if they
>find that the problem is in the CPE (customer provided eauipment). Or
>the current could cause the switch to think that the phone was off hook
>all the time, and then it would disable the line.
>
>: Hope this helps.
>
>| John Lundgren - Elec Tech - Info Tech Svcs. | jlundgre@delta1 |
-----------------------------------------------------
If the person, whose net address was deleted, is in the USA or Canada,
the co will simply busy out your line and drop voltage to it till it
is repaired. Under our current (sic) service agreement it is the
user's responsiblity to arrange for private repair of their own lines
or report and schedule repairs done by The Phone Company. The
computers recheck the line periodically to see if the shorted
condition has been changed. Telephone techs regularly short the pair
of wires to busy them so they don't get bit by the 90V square wave
ring signal anyway without any harm to the system and without telling
the co.
They also do this in event of emergencies and excessive calling making
an overdemand on the exchange. You simply leave the phone off the
hook till your place in the queue comes up, and then you can make your
call when the dial tone pops on!
You do NOT incur any charge for having your wires shorted. Happens all
the time as a prime failure mode, especially in rainy weather, and in
many phones themselves. The reason the can do this is that the line is
protected by its high resistance from damage to the system from
shorting, just like the RS-232C interface in your computer. It is also
impedance limited so that you could strip all wires and twist them
together, and come back ten years later and fix it and the output
drivers wouldn't be harmed at all. This is typical of distance current
signaling design.
-Steve
--
-Steve Walz rst...@armory.com ftp://ftp.armory.com/pub/user/rstevew
mirrored: ftp://ieee.cas.uc.edu/pub/electronics/faqs/ftp.armory.com
and Europe: ftp://ftp.cised.unina.it/pub/electronics/ftp.armory.com
and Oz: ftp://ftp.peninsula.apana.org.au/pub/electronics/ftp.armory.com
DY HEINRICH
: In article <4gq4su$2...@news2.deltanet.com>,
: John Lundgren <jlun...@delta1.deltanet.com> wrote:
: >Edward Lawrence (eal) penned:
: >: I don't know where this started, so I might be redundent with my comments.
: >
: >: 1. The phone company puts appx. 50 volts on your line, through the
: >: equivalent of about 1000 ohms in series. If your phone is 'on hook', a normal
: >: Voltmeter will read that 50 volts.
: >: 2. When you take your phone 'off hook', the voltage will drop. How
: >: much? Depends on the phone. I have seen readings of 7 to 26 VDc. If another
: >: extention phone goes 'off hook', the voltage will drop a bit lower.
It seem to me like basically, when the phone is on-hook, it is taken
out of the loop, and when it is on-hook, it acts like a resistor across
the two wires???
So what happens when the phone is supposed to ring (ie. how does the
phone know when to ring and what kind of voltage is across the two
wires?).
Also, how much current can the phone line provide (thinking of using it
to power some transistor for switching etc).
Heinrich
92dy...@wave.scar.utoronto.ca
Colin Edwards
and lights an LED (or opto-isolator). This can then be used to drive
other circuitry.
It is self powered (from the line) and complies with the line loading
requirements, ie 50uA on the hook, 5mA off the hook.
Hundreds of these have been built. If you want a copy of the circuit, I
can mail it to you as text plus 2 off UUEncoded GIF files. The circuit
is only three transistors, five diodes and a few resistors.
--
Colin Edwards
Richard Steven Walz
>I don't know where this started, so I might be redundent with my comments.
>
> 1. The phone company puts appx. 50 volts on your line, through the
>equivalent of about 1000 ohms in series. If your phone is 'on hook', a normal
>Voltmeter will read that 50 volts.
> 2. When you take your phone 'off hook', the voltage will drop. How
>much? Depends on the phone. I have seen readings of 7 to 26 VDc. If another
>extention phone goes 'off hook', the voltage will drop a bit lower.
>
>normally drop out when a phone goes 'off hook', while not loading the line too
>badly. I made a telephone recorder activation circuit this way, and added 200
>or so turns arould the coil to pick off the audio. Worked first time!
>
----------------------------------------------
Check your dialtone DC component, because the loop-back modules in
some people's boxes knock it down to 24-26 volts during on-hook! It
would be best to use a relay for 12 volts anyway, I believe, as most
phones suck it down to 3 - 7 volts during off-hook anyway. You may
have to tailor this to your phone system.
Stig Andersson
systems, measuring the voltage is not sufficient according to
specifications. This is because modern telephone switchers can have as
low voltage as 7-15 volt when on-hook and nearly zero volt when
on-hook.The only safe way is to measure the current as the
specification tells that a absolute minimum of 9ma will always float
when off-hook and a maximum of 25ua when on-hook.
Of course there is still problem measuring current...
The maximum serial resistance added should be 75 ohms.
This gives a pretty low voltage fall over the resistor...
Stig
John Lundgren
: I don´t know about the US tele systems but with the scandinavian
: Stig
If most phones only had 9 mA of current, the sound would be too weak to
be heard. I think Bell standards require 3 times that much.
If the telephone instrument is powered by the line, then there has to be
enough power to get the job done. The subscriber loop can have more than
a thousand ohms resistance, so there has to be enough voltage to overcome
this and give the needed current. That's why the on-hook voltage is 48
volts in the U.S., and off hook is may drop to under 10 volts.
Whatever method is used to sense the voltage, it should be isolated from
the high voltages and currents that can be on the phone line when
lightning storms are nearby.
--
#======P=G=P==k=e=y==a=v=a=i=l=a=b=l=e==u=p=o=n==r=e=q=u=e=s=t======#
| John Lundgren - Elec Tech - Info Tech Svcs. | jlundgre@delta1 |
Stig Andersson
Here愀 a simple and working solution. Use two optocouplers with the
diode side in serial with the line and pointing one in each direction.
Connect outputs in paralell and use as "OC".
I have used two PC815 which have sufficiient CTR to work at currents
as low as 5ma. If line currents exceeds 50ma you may connect a pair
of diodes in series, paralell with the opto-diode, pointing each way.
Stig
John Lundgren
: On 21 Feb 1996 08:42:47 -0500, oro...@aol.com (ORONITI) wrote:
: Here´s a simple and working solution. Use two optocouplers with the
: diode side in serial with the line and pointing one in each direction.
: Connect outputs in paralell and use as "OC".
That's kind of a waste of opto's. Why not use a diode in series to make
sure it will not work if connected backwards.
: I have used two PC815 which have sufficiient CTR to work at currents
: as low as 5ma. If line currents exceeds 50ma you may connect a pair
: of diodes in series, paralell with the opto-diode, pointing each way.
: Stig
CTR = Current Transfer Ratio?? I'm not sure.
MCT-2 opto's seem to be the popular choice for this app in the schems
I've seen. I thought about using this circuit for a long time, but I
abandoned it because of a couple reasons.
It has to be put in series with the line at the point where the line
enters the dwelling if it is going to sense all the phones. So that
means that another pair of wires has to be run to all the locations where
the off-hook condition is to be used/sensed. That can add up to a lot of
wire.
It can be damaged be high fault currents that might be caused by
lightning or other high voltages. The LED of the opto should have a low
value resistor across it, such as 100 ohms or maybe less. Or if the
rectifiers are placed across it, then they should be high current ones.
The resistor wil reduce the current from the line at all times, so it has
to be selected for the particular line, which may have a large difference
in current depending on the distance from the CO. This is another
problem.
The voltage sensor that runs off batteries is the best solution. The one
I built and works is the one in the group of schematics that uses three
volts from two D cells and has two transistors. The header says that it
was posted by a guy from Canada whose last name is Boisvert.
The schematics can be found on URL http://www.paranoia.com/~filipg or
www.access.digex.net/~cps.
Never use any of those circuits that have one side of the phone line
connected to the circuit or to ground. That's BAD. The phone line will
become unbalanced and it could be hazardous.
robbie...@gmail.com
Andrew Smallshaw
initially posting.
--
Andrew Smallshaw
and...@sdf.lonestar.org
Don Bruder
G. Morgan <seal...@osama-is-dead.net> wrote:
> Andrew Smallshaw wrote:
>
> >On 2012-11-10, robbie...@gmail.com <robbie...@gmail.com> wrote:
> >> On Tuesday, February 13, 1996 3:00:00 AM UTC-5, James W. Lynch wrote:
> >>>
> >>> Thanks,
> >>> Jim.
> >>
> >> was there a receiver off hook tone called a queue tone?
> >
> >I think it is unlikely Jim is still looking at this 16 years after
> >initially posting.
>
Look! It's the amazing Google NecroThread<TM>
This just might be the oldest one I've seen to date. About 3 weeks ago
in another group I haunt, one came to life (obviously another google
groups inspired incident) after 7 years dead.
--
If the door is baroque don't be Hayden. Come around Bach and jiggle the Handel
Michael Black
(and the one where Andrew Tannenbaum argues that Minix is better, if that
is a separate thread) has been vandalized in the minix newsgroup, and the
announcement is 1991.
In that case, google brought the vandalism on itself. At one point years
back they put up a timeline of Usenet, pointing to some "key" threads,
which included that Linux announcement. The last time they changed the
interface, they "forgot" to keep the ability to reply limited to 30 days
(which had always been the case previously), so it allowed the vandalism,
and people wanted to leave their mark on important threads. I never did
check at google to see if they removed the recent replies or not, I know I
complained a bunch of times at the time, including about replies to that
thread. Soon/eventually, they fixed that "bug".
But then it happened again when they changed the interface recently, and
more vandalism of that thread. INdeed, about the only traffic in the
minix newsgroup right now is people replying to that thread, they don't
even seem aware of what they are doing.
It's easy to notice these replies to old threads. Suddenly someone is
replying to a message you've never seen. Sometimes they even quote the
date, other times you just know since it's already happening. Yes, I know
messages can go astray, but that happens less and less, unlike the old
days. I remember when I was reading some Usenet newsgroups via a local
BBS starting in late 1994, and it was relativley common for messages to
not make it there, or for a reply to appear before the original post.
The only other reason for seeing a post you haven't seen before is if
someone is reading another newsgroup and feels the need to add another one
to their reply. It seems to happen a lot less than in the old days.
I think I've seen people reply via google to messages older than 1991, but
I'm not certain.
I still can't figure out what someone is replying to an old message.
They'd have to do searching going quite far back to find the old message,
which then can't be accidental.
Michael
robbie...@gmail.com
> when someone has lifted a receiver. A dial-tone detector circuit would
> also work.
>
> Any suggestions?
>
> Thanks,
> Jim.
Josef Moellers
> On Tuesday, February 13, 1996 3:00:00 AM UTC-5, James W. Lynch wrote:
>> Does anyone have plans for a simple off-hook detector? I need to know
>> when someone has lifted a receiver. A dial-tone detector circuit would
>> also work.
consisted of two relays with each relay's coil put in series with a
phone and a capacitor in parallel to the relay: if you take the receiver
off-hook, a direct current will flow which will cause the relay to
switch and disconnect the other phone. The capacitor was there to allow
the alternating current which carries the voice to pass through.
IIRC the relays' voltage was matching the DC voltage (24V?) and the
capacitors were some very large unpolar (i.e. no tantals or electrolytic).
Please don't ask me to dig the circuit up, it's very deeply buried ;-)
Josef
robbie...@gmail.com
> On 11/10/2012 06:09 AM, robbie...@gmail.com wrote:
>
> > On Tuesday, February 13, 1996 3:00:00 AM UTC-5, James W. Lynch wrote:
>
> >> Does anyone have plans for a simple off-hook detector? I need to know
>
> >> when someone has lifted a receiver. A dial-tone detector circuit would
>
> >> also work.
robbie...@gmail.com
> Does anyone have plans for a simple off-hook detector? I need to know
> when someone has lifted a receiver. A dial-tone detector circuit would
> also work.
>
>
> Thanks,
> Jim.
What was the telephone receiver off hook tone for the Concord Telephone Exchange Inc. Company in Farragit Tn.
robbie...@gmail.com
> Does anyone have plans for a simple off-hook detector? I need to know
> when someone has lifted a receiver. A dial-tone detector circuit would
> also work.
>
> Any suggestions?
>
> Thanks,
> Jim.
Jasen Betts
taken off hook for any reason (eg to make or answer a call) nothing
has changed since then.
--
⚂⚃ 100% natural
--- news://freenews.netfront.net/ - complaints: ne...@netfront.net ---
robbie...@gmail.com
robbie...@gmail.com
> Does anyone have plans for a simple off-hook detector? I need to know
> when someone has lifted a receiver. A dial-tone detector circuit would
> also work.
>
> Any suggestions?
>
> Thanks,
> Jim.