DTMF to telephone line interfafce

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

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

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

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?

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?

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

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.

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.

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.

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

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

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 speculate too much.

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

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.

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

Bob

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

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

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