RJ11 Electrical Specifications

Hi,

I'm looking for some pointers to information in an attempt to do something that may seem a bit impossible at first. I'm attempting to build an adapter that converts from 2.5mm audio to what is commonly known as RJ11 (the kind that would plug into a phone, like 4p2c or something similiar).

In order to do this, I need to track down the electrical specifications for rj11 jacks in the United States. All I've come across so far are some references to various off-hook, on-hook, and ring voltages. However, I haven't found anything detailed enough that would allow me to build such an adapter.

Before someone tells me that such a things cannot be done, Motorola used to sell something a while back call "The Cellular Connection Interface". One website mentioning it is this:

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Now, the device I'd like to built isn't exactly like this one (since it seems to use some proprietary way of getting a dial-tone out of the cell phone). However, it is similiar.

From my limited understanding so far, the adapter would, roughly,

connect the audio and mic part of the 2.5mm adapter to form the tip/ring signal and then the mutual ground of the 2.5mm becomes the negative line. All of this is somehow hooked up to a 9v battery and a transformer to get the right voltage on the line. Then, I plug one end of this contraption into my cellphone and the other end into a regular phone. I place a call on the cell phone, and then I should be able to pick up the regular phone's handset and speak. Viola. Any idea how I'd go about doing such a thing or where I can find information detailing the electrical requirements of the rj11 jack?

Any help is much appreciated. Also, if anyone knows of a more appropriate newsgroup on which to post this message, please let me know.

Reply to
ixarka
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You want something that will adapt the "headset" output on the cellphone to a "telset". Which is to say you want it to emulate a telephone line so that a regular telephone set can be plugged into it and function the same as would a headset.

First, would it make more sense to adapt the handset from a regular telephone? (I'm not sure that is any easier though, but it probably would be.)

Exactly. The fact that it uses an RJ-11 connector is insignificant, and just means that the plug needs to match, physically.

Nope. He wants to emulate one. The telephone set is already designed to connect to a loop start line! :-)

I had never seen that before, so I'm really glad you posted it, even if it won't help the OP, it's something I'm happy to have!

The essence of what the OP needs is:

1) A hybrid transformer, to convert the separate (4-wire) "mic" and "speaker" input/output lines to a 2-wire telephone line. 2) A source of loop current to drive the electronics in the telephone set. 3) It may or may not need amplification in either direction to provide suitable levels.

Note that the Motorola unit also includes "signaling", which allows the connected telset to indicate hook status to the cell phone. I'm not familiar with cell phones in specific, and don't know just what the available interface specifications are; hence I can't comment on how easy or difficult it might be to add hook supervision. And that also means I don't know what the levels or impedances are, and can't really comment specifically on the "local loop emulation" described above.

If the OP can come up with specifications for the interface on the cell phone, the 2-wire line side is pretty easy. It should have an impedance of something between 600 and 900 ohms. The level coming from the telephone set will be 0 dBm and the level sent to it should be between -6 and -9 dBm (those are testtone levels, actual measured voice will average approximately 10 dB lower).

The loop current should be at least 23 ma and no more than 120 ma at the most, and it can be assumed that the DC resistance of the telephone set will less than 350 Ohms (approximate value, I've forgotten exactly what the minimum is specified as, but most telsets will be 200-250 Ohms or so). Note that a 9 volt battery will provide about 26 ma across 350 ohms and about 45ma across 200 Ohms, and hence is just about the ideal voltage to use.

However... One might assume that no telset will ever actually be over 300 Ohms and that telsets will probably work with less than the minimum 23 ma... and add some resistance to keep the current lower and extend battery life. A 300 Ohm series resistance would provide 18 ma with a 200 Ohm telset, while allowing 15 ma with a 300 Ohm telset, which looks like a good choice.

For the OP, it he can find a URL to the specs for a cell phone's interface, I can probably explain what it means and perhaps provide a more specific design for a device to do what you need.

Reply to
Floyd L. Davidson

RJ-11 specifications are really mechanical in nature for a 6 contact,

2 wire modular plug that is the standard in the US for interfacing customer premises equipment to a single line. The only electrical thing in it is how to connect the tip and ring of the line.

I suspect what you are really after is the electrical specifications for connecting to a loop start telephone line. For that look at title

47 part 68 of the code of federal regulations, available at
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Eric Tappert
Reply to
Eric Tappert

specifications

phone).

The cell phone signals are separated into mic in and earphone out with a common ground. The regular phone line (RJ-11) had both outgoing and incoming audio on a single pair that is balanced, without a ground. So you need a hybrid splitter or 'phone patch' to separate the balanced line into mic and earphone, and also you need, as you said, some kind of power source to make the phone set work. You just can't plug them together and expect them to work.

It would be a lot easier if you ran the four wires coming from just the phone handset to your cellphone. Forget the telephone set itself. But you will probably still need to furnish power to the handset microphone which nowadays is usually an electret microphone element.

Reply to
Watson A.Name - "Watt Sun, th

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Reply to
Watson A.Name - "Watt Sun, th

If the op wants to make a CO interface in his adaptor, I strongly suggest using a battery feed chip that is made for the job. They have the current limiter and enough op amps in the "office" side that the hybrid can be made of a few resistors and a cap or two. Some of these chips also contain the active portion of a voltage converter so that battery voltage (or ringing, for that matter) can be developed from a low voltage supply. Manufacturers have app notes for things like ISDN terminal adaptors or VoIP terminal adaptors that might be useful.

There are a few (OK, lots of them) "gotchas" with a line interface, but most chip manufacturers have app notes for them.

E. Tappert

Reply to
Eric Tappert

Accidentally posted this in a different thread: I have done this before, using a commercially-available adapter for connecting POTS line speakerphones to PBX (digital) phones through the handset jack. This is one example:

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alog/Product.jhtml?CATID=15303 &PRODID=1... You can find them cheaper elsewhere. I made a 2.5mm-to-RJ-9 adapter- the RJ-9 (or RJ-22) is the thinner version of the RJ-11, and requires a smaller crimp tool. You may find it easier to take a handset cord apart, but those wires are really thin to be really flexible- tough to solder and work with. But, the idea is to feed the headphone signal to pins 1+4, and the mic to pins 2+3. I found that the polarity is critical, so experiment before finalizing the solder job. In fact, the one I built worked with my cell phone would not work with my cordless phone because the polarity was opposite. Of course, there are other cell-to-phone adapters commercially available out there, for ~$130. What's your budget? Totally DIY, less then $50? or more? The least cost would need a telecom transformer made for this sort of current flow. You have to be able to inject DC along with the audio signal, which may saturate an incorrect xformer. Some kind of amplifier and hybrid would be needed as echo return would be a problem. It gets more complicated to build it correctly, but not too bad. A classic 2 op-amp hybrid would do the trick. Google it and you should find several examples. Balance it just right, and you should be fine. Hope this helped.

Reply to
TelcoMT

Sounds like a really interesting idea to me.

Nah, 4-wire! (That is more than just a wire count... it's an indication of there being two each paths, one for transmit and one for receive. Each path, if it were an individual circuit would have two wires. You just happen to be sharing the return signal ground in this case. Normally it is applied to balanced circuits where there is no part that can be shared.)

You will almost certainly have to provide loop current.

Heh heh. We'll make a knob twister out you yet though.

I figured that would be interesting... :-)

Well, unfortunately there could be a lot of variation. The mic, for example, could be anything from 50 Ohms to 20,000 or so, and the voltage produced would go down as the impedance goes up. The output to the receiver could be anything from 4 Ohms to 2000 Ohms or so, with the same range of voltages.

I suspect that these values are not standardized at all, and vary from one cell phone model to another. So what you actually need are the specs for your particular cell phone.

Reply to
Floyd L. Davidson

Yes, I bet it would be easier, but not nearly as useful ;) The idea is to be able to plug the cell-phone audio out into a normal phone's input line, and then be able to use that phone to talk. Now, the actual phone itself could be cordless so, in that case, there is no handset to plug into. One may ask why in the world I've want to plug a cell-phone into a cordless phone when the cell-phone itself is already cordless. Reasoning behind this is that I've already figured out how to control my cellphone from a computer (I can make and receive calls in software). All I need to do next is interface the cell-phone to a regular phone for use at home. There are many possbilities for this sort of scheme. One is having a "dockable" telephone system, so that I can plug my cellphone into my home's main telephone and use that when I'm home. If I have to head out, I just unplug the phone and take it with me. A truly portable telephone line. That's the idea anyway...part of this is just for the challenge. I can figure out applications later ;)

Well, technically 3-wire. mic, speaker, and their shared ground.

I don't actually plan on plugging this device into an "old-fashioned" phone that derives its power from the phone line. This is going to get plugged into phones that are already powered seperately, so I'm hoping I won't have to pump too much current down the wire.

Like I mentioned above, I've already figured out how to control the phone via a computer, so I don't need to worry about dial-tone, ring simulation, or hook status with this adapter. I'm much better at software than hardware.

Fantastic! Thank you very much. Those are exactly the kind of specs I've been looking for.

As for the electrical interface to the cell-phone, I'm afraid I don't have any details on that. I assume (wrongly, perhaps) that all 2.5mm or 3.5mm jacks have roughly the same electrical requirements, since they're just carrying audio over the line, with different amounts of current for different volumes.

Thanks a lot for your help!

Reply to
ixarka

Would you happen to have any pointers for manufacturers (or mayb even model numbers) of these chips? Are they called "battery feed chips" or do they have some more specific name? I would much rather use a ready made chip that does all of this for me rather than doing it from scratch.

Reply to
ixarka

This looks like more how to go about connecting a cell-phone to a headset from a telelphone, not the actual telephone itself. The phone only has 2 wires, whereas the scheme above would involve 4 wires (headphone +/- and mic +/-). I need a way to combine the speaker, mic, and ground into 2 wires that I can plug into a regular phone.

This is pretty much a DIY project. I also saw some adapters in that price range. Parts shouldn't be much of an issue for me, though, because I am a student and have access to either free of very cheap electronic parts.

Reply to
ixarka

They are made by all the major telecom chip folks that have a 60-100 volt analog technology. Agere used to have a big business in these devices and they sold the business to Legerity (Austin, TX).

The devices are called "battery feed circuits".

E. Tappert

Reply to
Eric Tappert

What I meant was that adapter I mentioned needs a handset connection to work, and then all I had to do was make the appropriate plug adapter. It works fine. But, yes, it also worked to enable the use of a regular handset to talk on the cell phone- looks funny driving a car and using a 2600-type handset, but that's not what you are looking for. The transformer and hybrid are what I think you are looking for. Have you found these on the `net? Here is a suitable xformer:

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is a classic hybrid schematic:
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just a quick Google search. You may be able to find better.

Reply to
TelcoMT

If you are looking for a challenge, what about this scenario: make a converter cable from cellphone to two 3.5mm plugs, and plug that into the sound card (line in/line out) of a PC. This might need a bit of impedance matching, as the cellphone expects a microphone, and not something line level, but a few resistors should do the trick. Now take a voice-capable modem (like an old USR Sportster Voice), and hook that up to the cordless phone with a battery to provide some line current.

What's left as an exercise to the reader is the software to hook the two together ;-)

J.

Reply to
Jack Masters

It doesn't make any difference. You _still_ have to "pump" the same amount of current down the line! That's because it's a _standard_ POTS line! If you don't have enough current, then the line will 'hang up'!

see below. [snip]

Then you do _not_ need to have the base unit of the telephone set itself, all you need is just the handset! Just use a second pair of wires.

[snip]

You assume wrong, they do not. Each is proprietary to the cell phone. Some require DC current for an electret microphone element, some do not. Different earphones require different electrical levels to get the same audio level.

Reply to
Watson A.Name - "Watt Sun, th

interface,

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Reply to
Watson A.Name - "Watt Sun, th

No, it is *not* POTS, and is not a line. The device he is building will be emulating the line, and *it* sure isn't going to hang up!

The telset either needs current to function, or not. If it doesn't, he need not provide it.

Reply to
Floyd L. Davidson

Yup!

I think he may be in for a surpise as to whether it needs line current or not. I'll bet it won't work without it, plugged in or not.

Reply to
Floyd L. Davidson

"old-fashioned"

My error. The line will not hang up. However the usual telephone set is powered by the line current, so if it is a regular (unpowered) set, then it will need the few tens of milliamps of line current. The OP did state that he will be using a cordless powered by a wall wart. In these types of sets usually there is an optoisolator that monitors the line for current. So it may need to have the usual DC line current in order to operate correctly.

Reply to
Watson A.Name - "Watt Sun, th

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