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'[EE]: Telephone monitor'
2002\08\01@053101 by Russell McMahon

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>   I am thinking of making a simple on/off hook
> indicator. If the phone is "hung up", there are 42v on
> the line. If it is off hook, there are about 4v on the
> line. Would this work (in general)??
>
>   A bridge rectifier to a 12v reg. to a 5v reg to an
> I/O line on a pic (or an LED, for that matter).

May work but the telephone company will not be your friend.
Any load placed across the phone line MUST have a very high effective
resistance.
Most regulators will draw substantial current in the above scenario. Also,
the line will be unbalanced and this will cause hum due to noise being
coupled into the line. The arrangement MAY also stop the line releasing
after a call. There are also regulatory/permission issues about connecting
things to phone
liones.

A simple idea to play with is to use a zener diode of say 10 to 20 volts in
series with a large resistor joined to a PIC pin with another resistor
pulling the same PIC pin to ground. When the line voltage is greater than
the zener voltage
the PIC pin is pulled high. When lower than zener voltage the PIC pin is
low. This is just the basic principle - have a play and see if you can work
out a practical circuit.

Use of a simple differential amplifier is liable to more easily produce a
more "real world" result.




       Russell McMahon




{Quote hidden}

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2002\08\01@085401 by sbryden

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Also, be careful about ringig voltage. Apart from anything else, your circuit
will probably see this as lots of on/off hook transitions. I used a similar
circuit to what Russell described with a transistor and a large electrolytic
to smooth the ring and it worked fairly well.

Also, there are a lot of telephone circuits out there. Spend some time
searching and you'll find lots of onn/off hook indicator examples.

Simon.
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On Thu, Aug 01, 2002 at 09:31:00PM +1200, Russell McMahon wrote:
{Quote hidden}

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2002\08\01@092552 by Jim

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... and *I* would be interested in reports
on any and all circuits and their longevity
in light of real-world experience and all
that mother nature (in the way of secondary
induced secondary lightning transients as
well as man-made transients in industrial
and office environments) literally THROWS
at them ...

Doesn't anybody think safety and or "isolatin
of the line" from possible human contact anymore?
That used to be a consideration (BUT with the
declining cost of technology I fear safety has
taken a back seat to features and bottom-line
cost).


RF Jim


{Original Message removed}

2002\08\01@103700 by 4HAZ

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The US thresholds are:
When on hook, greater than 32 volts (typically 48)
When off hook less than 30 volts
Current loop 20-30ma
So if you used a bridge rectifier to keep from worrying about line polarity
and connect a 30 volt zener in series with the line rectified line,
and a resistor of say 1meg to the other side of the line,
you could use a differential comparator with series resistors of at least
100K to the inverting and non-inverting inputs to read the voltage across
the 1M resistor. You probably should also include a pair of 12v zeners
reverse series as snubbers directly across the comparators input pins to
limit the differential voltage across the comparators inputs during ring
cycles.
Most such comparators have a small offset voltage so simply reverse the
inverting and non-inverting lines if it will not respond.

Alternately you could just pick up a device from Radio Shack that is
designed to monitor line-in-use. But where's the fun in that.

Lonnie - KF4HAZ

----- From: "Russell McMahon" <apptech@
{Quote hidden}

in
> series with a large resistor joined to a PIC pin with another resistor
> pulling the same PIC pin to ground. When the line voltage is greater than
> the zener voltage
> the PIC pin is pulled high. When lower than zener voltage the PIC pin is
> low. This is just the basic principle - have a play and see if you can
work
{Quote hidden}

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2002\08\01@142457 by Bill & Pookie

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And maybe put a opto isolator in the circuit with
a reverse polarity diode (to protect the opto
isolator when things go reverse polarity as in
ringing) across the opto isolator's diode.  then
connect the other side of the opt to a pic pin.

Bill

{Original Message removed}

2002\08\01@182048 by Russell McMahon

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> Doesn't anybody think safety and or "isolatin
> of the line" from possible human contact anymore?
> That used to be a consideration (BUT with the
> declining cost of technology I fear safety has
> taken a back seat to features and bottom-line
> cost).

Rest assured that this is still required just as much as ever - at least in
this country where there are regulatory standards to meet, as I noted.
I assume that most other countries also haven't thrown away the rule book in
the face of the declining cost of technology - certainly the Telcos won't
have as the purpose of isolation is to protect both people from lines and
lines from people. The  latter is more important to the Telco's Ops manager
and the former to the Telco's lawyers.

As I noted, lest the point has somehow been missed, this was just a
discussion of the principles concerned - a starting point for someone to
learn something instead of just going down the road and buying an existing
technologically encheapend one for a fraction of the price of a
roll-your-own.

None of the techniques mentioned so far by any respondent actually *demands*
a
lack of 'isolatin of the line' - what one does in the privacy of one's own
breadboard and how this is implemented in a product are different. Safety
during development is, of course,  important but the final circuitry can
float in isolation from the real world if needs be.


       RM

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2002\08\01@205857 by Kevinhoward

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In a message dated 01/08/2002 10:32:35 GMT Daylight Time,
spam_OUTapptechTakeThisOuTspamPARADISE.NET.NZ writes:


{Quote hidden}

Keep the impedance high so as not to upset the Telecommunication company line
tests and stop any "hang up" problems!

For experimenting -Forget trying to power regulators from the line- it will
drop the voltage!

Try using an opto-isolator via a bridge rectifier via 150R approx. resistors
to the line. Adjust the bias to suit your on hook voltage (about 50v UK) and
off hook voltage.
The transistor side of the opto can be powered by whatever without any
inteference to the Telephone line- common emitter and open collector would
not require another power source apart from the attached gadget.

If you are using a PIC then the collector of the opto could go direct to an
input with a pullup resistor to + , as soon as the line is answered the
output would move from low to high (based on npn type opto)
Remember to connect the transistor emitter to -ve of your circuit!

Hope this helps,

Let me know, I will mail you diagrams ( off list) if you get stuck!

regards

Kev Howard

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2002\08\02@125509 by Jim

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I think you missed my point Russel. (This seems
to happen a lot. Wonder why ...)

Safety and isolation should be a design goal
in *real* product from the beginning - not an
after thought ... even if the 'product' never
sees the light of day in production the daily
use of something built by an by an experimenter
could one day prove fatal ...

That's my point.

NEVER underestimate what Mother Nature, physics
and other people (perhaps making mistakes while
doing high-voltage line work on a pole somewhere!)
can throw at at you. Never underestimate ...

When you're connecting to a "wire" that runs 15,000
some feet to a telco 'switch' via both underground and
above ground paths - there is plenty of potential
('scuze the pun) for trouble when least expected.

It's *one* thing to fool around with wiring that resides
*solely* on one's own premises - it's something else
quite again when a twisted pair runs onto your property
and into one's lab from afar ...

RF Jim


{Original Message removed}

2002\08\02@151418 by Peter L. Peres

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On Fri, 2 Aug 2002, Russell McMahon wrote:

>> Doesn't anybody think safety and or "isolatin
>> of the line" from possible human contact anymore?
>> That used to be a consideration (BUT with the
>> declining cost of technology I fear safety has
>> taken a back seat to features and bottom-line
>> cost).
>
>Rest assured that this is still required just as much as ever - at least in
>this country where there are regulatory standards to meet, as I noted.

And where no imports from certain countries make their way to , ever,
right ?

Otherwise I aggree with your opinion.

Peter

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