2.5 mm jack to rj-11 adaptor

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 Question by Chris Thompson posted 24 Jul 2005
 2.5 mm jack to rj-11 adaptor
Let me begin by saying that I am a fairly new electronics hobbyist and I have done no work with phones, so my thinking here may somehow be completely flawed. I hope for someone both to confirm the plausibility of my idea and to help me determine how to correctly wire my adaptor. I am attempting to use my cell phone as a modem with both my PDA and laptop, but I don’t want to use regular data cables because I would have to use at least two adaptors to change it from USB to the serial connection on my PDA, and I would have to get a new (and expensive) data cable when I get a new phone. Infrared is a possibility, but phones with infrared ports are rare and expensive. So, I had the idea of making an adaptor to go from the 2.5mm ear bud jack on a cell phone to a standard rj-11 telephone cable. You would then plug in the end of the rj-11 cable to the modem on the laptop or a modem module on the PDA. That would complete the physical connection between the cell phone and the PDA/laptop. Cell phones do not have dial tones, so in order to make a call you would have to configure your laptop/PDA to dial without listening for a dial tone. While the PDA/laptop is “dialing” (it will dial the tones for the number you set to call, but the cell phone will not respond to that), you will then make the cell phone dial the number to which you want to connect, just like a normal call. Within a few seconds the modem on the other end should begin squealing, and your modem will hear it and initiate a connection. This would effectively fool your modem into thinking you are calling by a regular landline, and it would fool your cell phone into thinking that you are making a regular voice call. Because it is a regular voice call to the cell phone, you may get away with a full 56k connection instead of the 14.4k connection that data calls are usually limited to on cell phones, plus you will avoid any charges that your cell phone provider might levy for data calls! I noticed that there is already a question/answer about an adaptor very similar to this one, but that question was rather vague as to the application of the adaptor and therefore the answer was unclear and not very helpful (to me, anyway). Thank you for your time.
 Answer by Joseph Golan posted 25 Jul 2005
Dear Chris,

Sorry to say but your theory is making a lot of assumptions that are just not factual. First of all more then likely your 2.5mm ear jack is more like the handset circuitry of a telephone then the part that connects to the network and would not have the proper voltages or signaling to support a modem. This wood end up being more like an acoustical type modem back from the 60's-70's and typically they were only 300 baud.

Some cell phone today have a data adapter port for this purpose and the devices you are connecting must have the software to work with it.

I personally think that the time and money it would take to build this device could be better spent on a new cell phone that has the feature you desire.

Joseph Golan, RCDD

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