Rotary Phone Interfacing with Modern Phone Lines


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Old 06-08-17, 12:54 PM
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Question Rotary Phone Interfacing with Modern Phone Lines

Hello everyone.

A friend of mine recently purchased a rotary phone, and after plugging it into the wall, discovered his provider doesn't support pulse dialing. This seemed like a fun project, so I offered to help him out.
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I purchased a couple things for the project: an atTiny104 (xplained) uC and an HT9200A DTMF encoder IC. The uC counts the number of pulses, not hard at all. This information gets pumped serially to the DTMF encoder, which then produces a tone matching the provided number. Overall, this process was fairly simple, although I'm still working on the serial encoding (which will be done very soon).
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I have a new problem, however. Now that I'm this far in, I've started to think about where the DTMF signal will actually go, and that's where I start to get lost. A standard RJ11 plug is supposed to have 2 pairs of 2 wires each, individually referred to as a 'tip' and a 'ring'. The jack in my rotary phone seems to use all four wires, even though they're duplicates, and I'm not sure why.
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But, it gets more confusing. Interfacing with the tip and ring seems to be rather difficult, requiring either an octocoupler or a 1:1 600 ohm transformer, both requiring transistors, diodes, and more. I was hoping I could just pump the DTMF wave into the wall and be done with it, but nothing is ever that easy. I really need some help with this interface -- does anyone have an idea of what to do?
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I'd be happy to provide block diagrams, photos, or any more information that is necessary. I've read through dozens of forum posts related to this concept already, and most of the links provided were dead, so I couldn't find much.
 
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Old 06-08-17, 01:59 PM
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The phone company is very particular about what is hooked to your phone line.

You need a DAA (Data Access Arrangement). They are available as modules or ICs.

Old POTS really only used red (ring) and green (tip). Black and yellow were sometimes used for accessories.
 
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Old 06-08-17, 02:05 PM
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Sometimes it's easier to just buy the adapter...
Ebay/convertor

That being said.... don't interface to the line..... interface to the microphone.
You should be able to use high value caps to tie into the mic at R and B on the coupler unit.

Rotary Dial Phone Electronic Circuit Schematic
 
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Old 06-08-17, 04:36 PM
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Carbide: Man, I was worried about that. Didn't want to stoop to buying a DAA, but that might be my only choice, dunno. Good to know about the colors, thanks! I'm going to see if there's any way I can do it without the module / IC, but I'll keep that as a last resort.

Pjmax: Haha, that's what my friend has suggested, but I'm so close at this point I'd like to finish it. That is SUCH a helpful diagram!! Thanks for digging that up, I don't know how I haven't seen it before. You're saying I should tie into the black & white 'TX' lines from the handset, right?
 
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Old 06-08-17, 05:03 PM
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Yes... use caps to keep the voltage from your DTMF generator.
 
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Old 06-09-17, 02:48 PM
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That's really interesting. I found a lot of people recommending transformers, I'm intrigued at the idea of just using capacitors. How would you recommend wiring it? I can't visualize it, for some reason.
 
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Old 06-10-17, 02:19 PM
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It depends on how you are powering the tone generator. Any ground from the power supply will cause hum in the phone.

The mic has bias on it so I'd try coupling with two .01ufd caps. It will be experimental to get the best tone volume and least amount of interaction with the mic.

One cap from tone board ground to B and one from audio out to R.
 
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Old 07-02-17, 10:08 AM
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Interesting a dial pulse to DTMF converter! Years ago the opposite interface was common. Good tip re: isolating the connection to the transmit pair, as the 500 type network does have voltage for the carbon mic. I'd expect the level to be kind of loud, a 42 OPG or any other touch tone pad has a pair to the receiver which reduces the receive level when any digit is pressed.
 
 

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