Telephone Connection Identification

[Hiran]

Hi everybody,

Background:
I am going to develp a device which can used to send a DTMF signal
from a remote Telephone.

Problem :
1)
How I can find out whether the telepone line has been connected to my
device?
Is there any voltage or impedence change? Send me a circuit diagram to
I dentify this situation.

2)
What is the good isolation transformer to connect the telephone cabel
to my device. How can I buy it?


Send me a mail : [email protected]

Thanks.

 

[Allen Windhorn]

Background:
I am going to develp a device which can used to send a DTMF signal
from a remote Telephone.

Problem :
1)
How I can find out whether the telepone line has been connected to
my device? Is there any voltage or impedence change? Send me a
circuit diagram to identify this situation.

This is not clear, but to detect that the line is connected to the
DTMF device AT the DTMF device, just look for DC voltage at the
terminals of the device -- it will be 48V or so "on-hook" and maybe
3-10V "off-hook". To detect remotely, you will have to have your
device send a signal of some kind to the other end.

2)
What is the good isolation transformer to connect the telephone
cabel to my device. How can I buy it?

Find a junk modem and use the one from that -- it's rated for
telephone usage. (If it's by Midcom, it's probably one I designed.)
The older, the better, since older ones were larger and had better
performance. However, bear in mind that there are two kinds of
isolation transformers for telephone lines. The one kind allows the
DC current from the line to flow through it (it will be about 2 cm
square or bigger). The other kind is smaller and requires a DC
current bypass to carry the phone line current (about 1 cm square).

If the DTMF device will only operate when a telephone on the line is
off the hook, then you can use the second type and provide a capacitor
to keep the DC out of it (about 2 uF 200V plastic is good).

Your device should not be connected to the line while it is ringing,
because this puts a high AC voltage on it.

Regards,
Allen