Connecting HT9200A DTMF Encoder Output to Rotary Phone RJ11 Tip and Ring 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.

 

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.

Just thinking about it seem very hard already. Ive seen this video of "youtube("")":https://www.youtube.com/watch?v=ZLoQ5UA1tvc where he converted a rotary phone to dtmf. There is also a product from "dialgizmo("")":http://www.dialgizmo.com/, not sure you have heard about it.

Hi,
It depends on the country you are in - i assume that you are in the USA since you mention RJ11 plug ?.

In general, you must isolate the circuit from the line - as in the UK it is -50volts, and ringing voltage can be 75volts to 100volts, so how you connect the design to the telephone is important.

Do you have the schematic for the telephone ?. You will need to ensure that when the receiver is on hook (placed down on the phone) that your circuit is NOT connected to the incoming line - to stop the ringing voltage from affecting the circuit.

I think that without the telephone schematic, you may have some issues.

Regards,

Richard.

Hey Richard.

Yep, sorry, USA.

It's a Princess 702b. I attached the schematic below.

Hi Brad,

Without the circuit details of the letters referring to the terminals, it is difficult to know where to place the DTMF circuit.

So, the only guidance that can be offered is that your circuit needs to be capacitively coupled to the internal circuit of the telephone, and the ac path from your circuit to the outgoing line must NOT be present when the telephone handset is placed down (circuit to exchange is broken - no dial tone etc). That is, only when the handset is picked up can the path to your circuit be available to the exchange line. (exchange in UK is equivalent to US central office ?)

Usually, the bell/ringer circuit is also capacitively coupled - so you must ensure that you do NOT connect to any path (DC/AC) to the ringer circuit.

Hope this helps.

Regards,

Richard.

HiOnly just reading this as I'm new to the forum.Back in the 80's I'd a sort of similar requirement. I wanted the tones to control my circuit. Someone called, device answered then decoded the key tones being pressed at the far-end.  Simplest way was to use a 2w-4w convertor. Basically 600-Ohm transformers that were connected in such a way as to allow the 2wires from line to be turned into separate Tx and Rx paths (2w to 4w).Before "the internet" these convertors were everywhere as they were a key component for "private wires" or leased lines. The TRS would have them and the customer premises would also have them allowing Tx and Rx over one pair.You don't need any Rx so you could try a 600-Ohm 1:1 transformer wired in series with the line. Have this connected to the "telephone side" of the cradle switches and you should be good.Regards