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DTMF to telephone line interfafce

E

eeh

Jan 1, 1970
0
Hi,

Could anyone show me an example circuit which demostrates how to
interface the DTMF signal from MT8880 or CM8880 to the telephone lines?
I have found in the internet but cannot find a match.

Thanks!
 
P

Pooh Bear

Jan 1, 1970
0
eeh said:
Hi,

Could anyone show me an example circuit which demostrates how to
interface the DTMF signal from MT8880 or CM8880 to the telephone lines?
I have found in the internet but cannot find a match.

Can I suggest you search for *application notes*.

Most semi manufacturers supply these to show how best to implement their
products.

Who makes the above for example ?

Graham
 
O

OBones

Jan 1, 1970
0
eeh said:
Hi,

Could anyone show me an example circuit which demostrates how to
interface the DTMF signal from MT8880 or CM8880 to the telephone lines?
I have found in the internet but cannot find a match.

just use a 1:1 600ohms transformer and that's it.
 
M

Michael Black

Jan 1, 1970
0
Pooh said:
Can I suggest you search for *application notes*.

Most semi manufacturers supply these to show how best to implement their
products.

Who makes the above for example ?

Graham
That's one of the things I suggested when he posted the same question
in another newsgroup in the past week.

The first suggestion was that he buy a cheap phone, and use that
intact. It's obviously got the interface in place, and you get the
touch-tone encoder too.

Note also that in the other newsgroup, people are still wondering what
the purpose of this project is. Because a solution obviously comes
from the situation. If it's something like he has a rotary phone but
needs touch-tones to work with automated systems at the other end,
there's the time honored scheme of feeding the touch-tones into
a small amplifier and then speaker, and then holding the speaker
up to the microphone of the telephone. Solutions are a dime a dozen,
but none of them may apply until he reveals exactly what he's trying
to do.

Michael
 
J

Jim Thompson

Jan 1, 1970
0
That's one of the things I suggested when he posted the same question
in another newsgroup in the past week.

The first suggestion was that he buy a cheap phone, and use that
intact. It's obviously got the interface in place, and you get the
touch-tone encoder too.

Note also that in the other newsgroup, people are still wondering what
the purpose of this project is. Because a solution obviously comes
from the situation. If it's something like he has a rotary phone but
needs touch-tones to work with automated systems at the other end,
there's the time honored scheme of feeding the touch-tones into
a small amplifier and then speaker, and then holding the speaker
up to the microphone of the telephone. Solutions are a dime a dozen,
but none of them may apply until he reveals exactly what he's trying
to do.

Michael

Years ago, like late '80's, early '90's, I was doing a project at
Sperry.

They had some kind of digital phone system that wouldn't send DTMF,
plus they forbade long distance calls (but not 800 number calls)
without going thru the company operator with a project number to
charge to. Thus I couldn't call other clients.

I subscribed to (IIRC) Sprint, which had an 800 number you called,
then keyed in the number you wanted to call.

I found a device at Radio Shack that fit in a shirt pocket. You held
it up to the phone and dialed with the buttons on the device.

I don't know if it's still made or not. I still have mine around here
somewhere... I recall seeing it during an enforced office cleaning ;-)

I'll try to find it and get the RS part number.

Found it!

33 Memory Pocket Tone Dialer, Model 43-141, length of a pack of
cigarettes, but narrower and thinner, 3 AAA cells.

...Jim Thompson
 
P

Pooh Bear

Jan 1, 1970
0
OBones said:
just use a 1:1 600ohms transformer and that's it.

You have to be careful with loading the line though.

Might also want to do an 'electronic hybrid' to stop the injected signal
passing back into the receiver.

Graham
 
J

Jim Thompson

Jan 1, 1970
0
Years ago, like late '80's, early '90's, I was doing a project at
Sperry.

They had some kind of digital phone system that wouldn't send DTMF,
plus they forbade long distance calls (but not 800 number calls)
without going thru the company operator with a project number to
charge to. Thus I couldn't call other clients.

I subscribed to (IIRC) Sprint, which had an 800 number you called,
then keyed in the number you wanted to call.

I found a device at Radio Shack that fit in a shirt pocket. You held
it up to the phone and dialed with the buttons on the device.

I don't know if it's still made or not. I still have mine around here
somewhere... I recall seeing it during an enforced office cleaning ;-)

I'll try to find it and get the RS part number.

Found it!

33 Memory Pocket Tone Dialer, Model 43-141, length of a pack of
cigarettes, but narrower and thinner, 3 AAA cells.

...Jim Thompson

http://www.phonelosers.org/redboxtonedial.html

...Jim Thompson
 
D

Don Bowey

Jan 1, 1970
0
Hi,

Could anyone show me an example circuit which demostrates how to
interface the DTMF signal from MT8880 or CM8880 to the telephone lines?
I have found in the internet but cannot find a match.

Thanks!

As I read the datasheet, the dtmf output of the IC is -2 dBm, but into a 10k
Ohm load, so you will need a matching transformer (10k:600-1300 or so). Or
(best) you can use some other transformer with an op-amp to get the right
level and a decent nominal impedance.

Exactly what else you need to connect the transformer to the Tel Line,
depends on what you are attempting to do.

I assume you have looked at the datasheet so you know how to build the final
circuit using the 8880, right?

I got 163 google hits shearching for "dtmf dialer using a MT8880."

Don
 
B

Bob

Jan 1, 1970
0
You have to be careful with loading the line though.

Might also want to do an 'electronic hybrid' to stop the injected signal
passing back into the receiver.

Graham

Yep. It's not as easy as "just use and that's it".

I suspect that 'eeh', the OP, is some sort of spy, terrorist, or alien
creature. That's why he/she won't tell us what he/she is doing.

I remember, back in the days when I, too, was an alien -- building blue
boxes and black boxes. *That's* how you learn about the phone system.
Ahhh....the memories...

Bob
 
B

Bob

Jan 1, 1970
0
As I read the datasheet, the dtmf output of the IC is -2 dBm, but into a
10k
Ohm load, so you will need a matching transformer (10k:600-1300 or so).
Or
(best) you can use some other transformer with an op-amp to get the right
level and a decent nominal impedance.


Specifying an output level into a given load does not imply its output
impedance. Also, if you used a 10K->600ohm transformer you would get an
EXTREMELY low level at the 600ohm side, because of the large--to-small turns
ratio of that transformer.

Exactly what else you need to connect the transformer to the Tel Line,
depends on what you are attempting to do.

I assume you have looked at the datasheet so you know how to build the
final
circuit using the 8880, right?

I got 163 google hits shearching for "dtmf dialer using a MT8880."


Now you're talkin'. However, I suspect that googling is beyond the OP's
capabilities.


Bob
 
P

Pooh Bear

Jan 1, 1970
0
Don said:
As I read the datasheet, the dtmf output of the IC is -2 dBm, but into a 10k
Ohm load, so you will need a matching transformer (10k:600-1300 or so).

Why ? You're doing that backward thinking thing about dBs again aren't you ?

The IC will work into any load that isn't below the rated value.

Graham
 
D

Don Bowey

Jan 1, 1970
0
Specifying an output level into a given load does not imply its output
impedance. Also, if you used a 10K->600ohm transformer you would get an
EXTREMELY low level at the 600ohm side, because of the large--to-small turns
ratio of that transformer.

When output is spec'd at 10kOhms I'm certain it is not going to be the same
output level at 1000 Ohms. What was your point?
 
D

Don Bowey

Jan 1, 1970
0
Why ? You're doing that backward thinking thing about dBs again aren't you ?

The IC will work into any load that isn't below the rated value.

Graham

It's output is rated at 10kOhms. The telephone line Z will probably be in
the range of 900 to 1300 Ohms.

So what's your point?
 
P

Pooh Bear

Jan 1, 1970
0
Don said:
It's output is rated at 10kOhms. The telephone line Z will probably be in
the range of 900 to 1300 Ohms.

So what's your point?

You buffer the output via an op-amp for a few pence you dimwit ! ( see my note
about the 'electronic hybrid' ). There are many op-amps that have no trouble
driving telephone line impedances.

If ppl stuck transformers everwhere you suggested we wouldn't be able to move for
iron !

Also - as was noted by others - a transformer will reduce the signal voltage !

Graham
 
B

Bob

Jan 1, 1970
0
Don Bowey said:
When output is spec'd at 10kOhms I'm certain it is not going to be the
same
output level at 1000 Ohms. What was your point?

If the output is spec'd into a 10k load then this is what the manufacturer
chose to rate its test load at. This DOES NOT mean that its output impedance
is 10kohms. It's important to make that distinction since they are separate
entities.

Typically, an output has a driving impedance of a few ohms. A manufacturer
will specify its minimum load impedance so as to limit the amount of power
that the output needs to, or can, drive. This is independent from its output
impedance.

You should experiment with devices similar to this. Any old opamp (with
appropriate hookup) will do. What you'll find is that whether you connect
its output to a 10k or 1000 or 100 ohm load, the output level (for any given
input level) will not change much. In fact, in terms of dB's, it will
probably decrease less than 0.1dB from a 10k to a 100ohm load. Try it.
You'll get a better feel for it.

Bob
 
D

Don Bowey

Jan 1, 1970
0
You buffer the output via an op-amp for a few pence you dimwit ! ( see my note
about the 'electronic hybrid' ). There are many op-amps that have no trouble
driving telephone line impedances.

1. You still did not make any new point that I can see - just your usual
"jump on a post and add confusion bullshit." I noted in my original post
that a best approach might be to use an op-amp and a transformer. However,
since we don't know what the OP is doing, one should not conclude that gain
IS NEEDED. I estimate the attenuation from the 10k to 1k connection would
be about 18dB, which leaves the signal within working range of the CO under
average conditions, though not within the Standards requirement. The time
to work on a real design is after all the OP's facts are known.

However, it would be good of you to deliver a good design to S.E.D so
everyone could see your transformerless approach.

If ppl stuck transformers everwhere you suggested we wouldn't be able to move
for
iron !

More BS nonsence on your part. The connection can ONLY be transformerless
if his gadget uses batteries and is isolated from earth ground. And why did
you recommend using a hybrid transformer? The OP said nothing about wanting
to receive dtmf signals.

Also - as was noted by others - a transformer will reduce the signal voltage !

Well, Duh! Of course it will, but unless the transformer is a piece of
junk, it's attenuation is not going to be a killer.
 
D

Don Bowey

Jan 1, 1970
0
If the output is spec'd into a 10k load then this is what the manufacturer
chose to rate its test load at. This DOES NOT mean that its output impedance
is 10kohms. It's important to make that distinction since they are separate
entities.

Typically, an output has a driving impedance of a few ohms. A manufacturer
will specify its minimum load impedance so as to limit the amount of power
that the output needs to, or can, drive. This is independent from its output
impedance.

I am well aware of this, but the device is not just an op-amp, and it would
be foolish to commit a design based on an assumption that it's output is
just like an op-amp.
You should experiment with devices similar to this. Any old opamp (with
appropriate hookup) will do. What you'll find is that whether you connect
its output to a 10k or 1000 or 100 ohm load, the output level (for any given
input level) will not change much. In fact, in terms of dB's, it will
probably decrease less than 0.1dB from a 10k to a 100ohm load. Try it.
You'll get a better feel for it.

Bob

You speculate too much.
 
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