Telephone Interfacing

[Astable Schmitt]

Hi there,

I am just wondering if somebody can help me.
I am trying to decode DTMF tones on the phone line. I am using a DTMF
decoder IC. The telephone lines supply a approx 50V DC positive earth
supply which I do not need. I have a capacitor blocking the +ve dc
supply. The problem I am having is that the circuit works fine when I
find out the polarity of the incoming wires. However the circuit needs to
be able to function with either polarity as the phone system is not
polarity dependant and installers connect the wires both ways.

So basically what I am saying is that when the polarity is reversed my
circuit falls to pieces.
So if anyone has any ideas how I can fix this please let me know.

Is it as simple as putting another capacitor to ground on the other wire
or is there more to it.

Cheers

 

[Peter Jetson]

Lord Garth wrote:

Is it as simple as putting another capacitor to ground on the other wire
or is there more to it.

Could it be that the OP is only connecting his circuit to one side of
the phone line?

Peter

 

[Whytech]

A diode full bridge will allow polarity independence. While loaded by your
line holding cct the bridges will pass low frequency AC with little
distortion

-Andrew Metcalfe


Whytech
MSP430 / PIC / AVR tools, SMD prototyping adaptors
Imagecraft MSP430 C compilers
http://www.whytech.com.au


"Astable Schmitt" <sick@of.spam> wrote in message
news:Xns93D4AEAEA678sickofspam@202.89.128.20...

Hi there,

I am just wondering if somebody can help me.
I am trying to decode DTMF tones on the phone line. I am using a DTMF
decoder IC. The telephone lines supply a approx 50V DC positive earth
supply which I do not need. I have a capacitor blocking the +ve dc
supply. The problem I am having is that the circuit works fine when I
find out the polarity of the incoming wires. However the circuit needs to
be able to function with either polarity as the phone system is not
polarity dependant and installers connect the wires both ways.

So basically what I am saying is that when the polarity is reversed my
circuit falls to pieces.
So if anyone has any ideas how I can fix this please let me know.

Is it as simple as putting another capacitor to ground on the other wire
or is there more to it.

Cheers