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'[EE] Good way to detect phoneline.'
2003\05\09@074144 by Eduardo Garcia

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Well, I am using now a simple circuit. I have connected to the tip and
ring
 a diodes bridge to fix the positive and negative references.
 For obtain the isolated signal corresponding to line presence have a 68K
 resistor in a diode of a opto-isolator. I need polarize the opto-isolator
 without to affect the telephone line. If I use low value resistors the
 telephone line is "answered" (dropped to a low voltage signal) and if I
use
 big value resistor I can't match the polarization of opto-isolator.


 How can I fix it?

 (sorry by bad english ;-) . I hope that you understood me.)



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2003\05\09@092300 by Dave VanHorn

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At 08:39 AM 5/9/2003 -0300, Eduardo Garcia wrote:
>  Well, I am using now a simple circuit. I have connected to the tip and
>ring
>   a diodes bridge to fix the positive and negative references.
>   For obtain the isolated signal corresponding to line presence have a 68K
>   resistor in a diode of a opto-isolator.

You need a larger resistor, like 10M, and a better opto with higher current
transfer ratio.

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2003\05\09@095217 by Eduardo Garcia

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Can you say me a good opto for this use?
Here I found 4n25, 4n35 and cny17-4.
I found the better results with 4n35.
Other detail remembered was that I need to use the same circuit to detect
the phone line in use too. This is a "cutted line circuit".
When there are many telephones attached on telephone line the +48V decrease
to +5V.
+5V with a 10M resistor give me 0,5uA!!!

How can I fix it now?

Thanks in advance,
Eduardo Garcia



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2003\05\09@110847 by Dave VanHorn

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At 10:38 AM 5/9/2003 -0300, Eduardo Garcia wrote:
>Can you say me a good opto for this use?
>Here I found 4n25, 4n35 and cny17-4.
>I found the better results with 4n35.

Check the current transfer ratio specs. 100 or even 200% is best.
On the output side, you need only a light pullup, say 10M

>Other detail remembered was that I need to use the same circuit to detect
>the phone line in use too. This is a "cutted line circuit".
>When there are many telephones attached on telephone line the +48V decrease
>to +5V.
>+5V with a 10M resistor give me 0,5uA!!!

That's correct. When one or more phones are off hook, the voltage will be
anywhere from 20V to a few volts.
Phones work on current, voltage isn't too relevant.


>How can I fix it now?

There's nothing to fix for this condition.
If you want to sense that condiiton, you need to be in series with the line.
In the US, that's called an RJ-31X connection.

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2003\05\10@025010 by Peter L. Peres

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Use a JFET constant current generator set at 1mA instead of the resistor
imho.

5V is out of spec for the line in any case. Many phones won't work that
low. Aim for 7-10V lowest loaded line voltage imho.

Peter

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2003\05\10@100757 by Dave VanHorn

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At 09:57 AM 5/10/2003 +0300, Peter L. Peres wrote:
>Use a JFET constant current generator set at 1mA instead of the resistor
>imho.

You can't draw that much at the on-hook condition, and pass part 68.
10 meg is what it takes.

>5V is out of spec for the line in any case. Many phones won't work that
>low. Aim for 7-10V lowest loaded line voltage imho.

There is no low voltage spec for phone lines, though I agree that line
powered devices will start to fail around there.
You get at least 20mA, and you're required to draw at least 20mA
Somewhat different in other countries, but phone systems worldwide work on
current.
The voltage is only relevant in that you need sufficient voltage to get the
current flowing.

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2003\05\10@101746 by Katinka Mills

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{Quote hidden}

Which is why line cards (with respect to the Telco Exchange) are speced for
loop resistance, low loop resistance cards can run from as low as 12Vdc,
high loop resistance ones work on 48Vdc.

Regards,

Kat.
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2003\05\10@105946 by Marcelo Puhl

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On 10 May 2003 at 9:07, Dave VanHorn wrote:

> At 09:57 AM 5/10/2003 +0300, Peter L. Peres wrote:
> >Use a JFET constant current generator set at 1mA instead of the resistor
> >imho.
>
> You can't draw that much at the on-hook condition, and pass part 68.
> 10 meg is what it takes.
>

       You can just load the line for say 1mS and measure the current to
know if the 48V is present. Did this with two optocouplers and a zener.

       Mark

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2003\05\10@110607 by Dave VanHorn

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>
>Which is why line cards (with respect to the Telco Exchange) are speced for
>loop resistance, low loop resistance cards can run from as low as 12Vdc,
>high loop resistance ones work on 48Vdc.

I've seen on-hook voltages here in the US as high as 58V, and as low as
12V, on CO lines.
I've seen off-hook voltages as high as 22V, which shows why sensing line
state by voltage is problematic.
At a minimum, you need two settable thresholds.

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2003\05\10@110613 by Dave VanHorn

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>
>
>         You can just load the line for say 1mS and measure the current to
>know if the 48V is present. Did this with two optocouplers and a zener.

Yes, but did you pass part 68 with it?
We sample for 1mS with 10 meg, and did pass part 68.

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2003\05\12@113723 by dr. Imre Bartfai

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Hello,

use a GOOD opto-coupler (I mean that with high CTRR, e. g. LTV817B), and
amplify the output.

Imre

On Fri, 9 May 2003, Eduardo Garcia wrote:

{Quote hidden}

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2003\05\12@115235 by Dave VanHorn

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At 05:39 PM 5/12/2003 +0200, dr. Imre Bartfai wrote:
>Hello,
>
>use a GOOD opto-coupler (I mean that with high CTRR, e. g. LTV817B), and
>amplify the output.

I don't remember the part number we used, but I think the CTRR was
something like 2000.
Probably a darlington.
NOT a fast part by any means, but we didn't need fast.

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2003\05\12@133715 by Eduardo Garcia

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Perfect!
Thanks all!

----- Original Message -----
From: "Dave VanHorn" <RemoveMEdvanhornTakeThisOuTspamCEDAR.NET>
To: <spamBeGonePICLISTspamBeGonespamMITVMA.MIT.EDU>
Sent: Monday, May 12, 2003 12:52 PM
Subject: Re: [EE] Good way to detect phoneline.


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