piclist 2001\01\02\093506a >
Thread: telephone line interface
www.piclist.com/techref/pots/dtmf.htm?key=telephone
picon face BY : pang khong lin email (remove spam text)



Thanks for helping out David(s). So it seems i got all mess up. I did :-)
The transformer interface which i have seen aplenty on the net are meant for
receiving audio signals. All it does is providing the required 600 ohm
impedance so that the ac signals will be properly transfered. And as for
making the phone line off hook there should be a resistance of around 200
ohm across the line. ( or am i still wrong ?  :-(    )
Now my simple mind tells me that if i were to put the 200 ohm across the
line, the phone line should at least goes off hook. As for the 600 ohm
impedance, it will not be 600 ohm anymore. honestly i do not know how to
change the value on the transformer side. The datasheet for the transformer
gives a value of 6.8 uF and 430 ohm for the capacitor
and resistor. ( refer to the diagram below ). It will be helpful to know how
they got these values.

I did not managed to find any sites that offer some sort of method (
schematic ) to put a phone on hook and off hook. I have tried opening up
several house phones to see how they work, but it is very difficult to trace
the connection. Anyone has any schematic that shows how to bring the phone
off hook and on hook or maybe even the schematic
of a typical phone?
Thanks again and good day.


From: David Duffy <TakeThisOuTpiclistKILLspamspamTakeThisOuTAUDIOVISUALDEVICES.COM.AU>
Subject: Re: [EE]: telephone line interface
Date: Tue, 19 Dec 2000 15:26:20 +1000

pang khong lin wrote:
   hi and good afternoon. I am trying to interface to the telephone line
using an impedance matching transformer -  ETAL P3191. At the moment there
are only three part.
1. The transformer part that is suppose to bring the phone line to off
  hook
2. A ringing signal detector circuit
3. A double pole relay that is suppose to switch the telephone line to
  either Part 1 or    Part 2.

Based on the information i have, a typical telephone line will go off
hook when the impedance detected is around 600 ohm. Then the ringing signal
will be terminated. The problem is it does not goes off hook. I tried
changing different resistor on the secondary but no result. Any idea,
please? Below is the transformer circuit.

   Tip                0.68uF
   o----------------| |-------o--     --o------------------
                                S   S                    |
                                S   S                    |
                                S   S                 Resistor
                                S   S                    |
   o--------------------------o--     --o------------------
   Ring

I got this circuit from the datasheet. The original value of the cap is at
6.8uF. I replace it with 0.68uF because it is very expensive for a cap with
a rating 200Vac. at that value. Is that the reason for not being able to go
off hook?
Another question - the capacitor must be there to block dc and prevent
the transformer from being saturated. If there are no dc, how does it know
that the resistance is 600ohm?


You need to have a DC path on the telephone line side for the exchange to
detect the off-hook status. The circuit shown is for transferring audio to
and
from the line. The resistor goes across the line to generate off-hook
condition.
There should be plenty of telephone interface circuits on the net.
Regards...


From: David VanHorn <@spam@dvanhorn@spam@spamCEDAR.NET>
Subject:  Re: [EE]: telephone line interface
Date:    Tue, 19 Dec 2000 00:22:38 -0500

>1. The transformer part that is suppose to bring the phone line to off>
>hook

That series cap prevents you from going off hook, but beware, this
transformer may not be designed for a "wet" interface.

>2. A ringing signal detector circuit

Bridge rectify the line, on the output of the bridge, series a zener of
>60V, an optoisolator, and a current limiting resistor.

>3. A double pole relay that is suppose to switch the telephone line to >
>either Part 1 or    Part 2.

The ring detector can be left in circuit, it has no effect when the line is
looped. You do want a relay, or opto, to loop the line.
Current is at least 20mA looped, can be greater than 100mA >Based on the
information i have, a typical telephone line will go off hook
{Quote hidden}

Don't mess with that cap value, or you're messing with your frequency
response and return loss.

You'll also need a hybrid circuit, unless you are only talking OR receiving.

>Another question - the capacitor must be there to block dc and prevent the
> >transformer from being saturated. If there are no dc, how does it know
>that >the resistance is 600ohm?

Impedance, not resistance.

It's not as simple as it looks when done right.
:)


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