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.

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).

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.

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?

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.

 

DTMF was designed to be transmitted in the audible band. The signal is fed in-band exactly like voice signal from microphone.

Modern exchanges are not polarity sensitive anymore with regards to tip and ring.

 

are you familiar with phone lines (voltage levels etc, for example when receiving call)?
btw. your phone provider will not be happy that you are adding something to their circuit.

 

jterblanche: So, does that mean I could hook it up to the microphone wires themselves? With some capacitors in between, or something. I think I saw a diagram kind of like that.

panic mode: Yes, somewhat -- not as well as I'd like. I've struggled to find literature on it. I know the line is usually idle around 50V, dropping below 10V when it's picked off the hook. The ringer runs at 90V AC (20hz) but I don't think it does that anymore. I know they won't be happy, though, haha. But I see people sell in-line DTMF converters, so if it already exists, I don't feel too bad about it. Besides, it's still a phone

 

Suitably coupled and scaled yes. In-band superimposition is the raison d'être for DTMF - that's why it appears on sidetone during IVR interaction.

 

POTS (plain old telephone service) is a current loop with a 600Ω nominal impedance.
You may have to use a 600Ω telephone transformer (output in series with the line, not in parallel) to couple in the DTMF signal.
Here's a discussion on POTS service.
It states the DTMF signal should be about -7dBm. That's ≈0.45mW into 600Ω or 500mVrms into the 600Ω transformer primary.

So you would need an interface between the DTMF output and the transformer that can generate 500mVrms into 600Ω.

 

jterblanche: So, does that mean I could hook it up to the microphone wires themselves? With some capacitors in between, or something. I think I saw a diagram kind of like that.

panic mode: Yes, somewhat -- not as well as I'd like. I've struggled to find literature on it. I know the line is usually idle around 50V, dropping below 10V when it's picked off the hook. The ringer runs at 90V AC (20hz) but I don't think it does that anymore. I know they won't be happy, though, haha. But I see people sell in-line DTMF converters, so if it already exists, I don't feel too bad about it. Besides, it's still a phone

Theoretically, you can just play an audible tone into the microphone (through the air) to initiate the dialing. There used to be commercials (for a short time) telling kids to hold the phone up to the tv to dial the store.

 

Theoretically, you can just play an audible tone into the microphone (through the air) to initiate the dialing. There used to be commercials (for a short time) telling kids to hold the phone up to the tv to dial the store.

Yes, many years ago, I had a hand-held auto dialer (about the size of a flip-phone) which I could program with phone numbers (before phones with number memory were common).
You just held the small speaker against the phone mic and pushed the button for the desired number.
It would output the DTMF sound sequence and the selected number would ring.

 

That's awesome, really cool!! I know back in the day of rotary phones you could tap the switchhook to dial, too. Interesting that there's always been a way to cheat the system.

If I purchased something like this, what else would I have to do? I guess I'd have to take out a multimeter / scope and figure out what my DTMF output level is, then try to lower it to the 0.45 mW level? One person somewhere recommended twin capacitors in lieu of the transformer, which I thought was interesting. I wonder if there's a way to do that.

Thanks for all the help, guys.