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DTMF tones over cell connection

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jmb4370

New Member
The DTMF dialer / tone generator I made using the TCM5089 chip works fine and will send the DTMF tones over a land line, or even when plugged into the headset jack of a wireless land line phone, but when the same output is plugged into the headset jack of a cell phone no tones are heard over the open connection. If the tone generator is output to a small speaker, and acoustical coupling is used by placing the wired headset mic next to the speaker, then the tones can be heard over the open cell connection. I would like to eliminate this inefficient accoustical coupling, and bypass the speaker to mic combination.

I think it has to do with the cell phones using true digital signals, and not analog, but how can a tone be sent to the cell phone and be heard as if you are physically pressing one of the cell phone buttons? This is not necessarily to do the initial dialing, but after the call is established to be able to send tones over the open phone line?

Any suggestions would be sincerely welcomed.
Michael
 

Gayan Soyza

Active Member
I think it has to do with the cell phones using true digital signals, and not analog, but how can a tone be sent to the cell phone and be heard as if you are physically pressing one of the cell phone buttons? This is not necessarily to do the initial dialing, but after the call is established to be able to send tones over the open phone line?

Actually cellphone dialing is the hardest part.

You can only play the receiving side of the cell phone & not the dialing part.For the dialing & make DTMF tones you need to study the communication protocol of the particular cell phone.Ex Nokia uses FBUS protocol you can send DTMF tones using it.The other thing is they don't provide latest protocol standards.You can find only very old versions of the protocol standards that is long obsolete nowadays.

I also gave off because of this & shifted to GSM modules.
 

ke5frf

New Member
What kind of headset jack is it?


"I think it has to do with the cell phones using true digital signals, and not analog, but how can a tone be sent to the cell phone and be heard as if you are physically pressing one of the cell phone buttons?"


The headset you are using, is it the kind that can be plugged into something else, general purpose so to speak? Or is it specially wired for your phone?

There is a transducer in that headet. That transducer is analog, probably electret. Now weather or not your cellphone accepts it directly at the input as analog, or if it is A/D-ed by the headset first, I don't know.

Regardless, even cellphones, at some point, have to convert sound from analog to digital. This is why DTMF can be triggered by a skilled "whistler" or "hummer" who can immitate the tones...via cellphone or analog set is immaterial.

If you have a spare headset, or if it is fairly cheapo, you should be able to take it apart, find the electret, remove it and wire your circuit in its place with the appropriate voltages, etc.
 

MacroTech

New Member
If I understand what you are doing, you need a cell phone with an external mic input.
All cell phones that I know of that use wired hands-free systems have a mic on them for talking into while you have the earpiece in your ear. I would suspect you just need to wire it up to the input correctly because then the tone would be played through the jack the same way your voice is.
 
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jmb4370

New Member
I did just what ke5frf and MacroTech were referring to. I took an old wired headset that indeed worked with this and most other cell phones, and wired the output from the tone generator directly in parallel with the electret mic from the headset, but I still couldn't hear the tone either in the earpiece that was part of the wired headset, or in the other phone that this first one had called.

If I will remove the electret mic from the circuit entirely, this is where I first started in connecting the output of the tone generator directly into the mic input of the cell phone... this didn't work. Is it possible that the mic needs to stay in place, and the output needs to be connected through an isolation transformer?

The output tone is actually amplified through a single transistor to increase the volume when I had it just playing through the small mini speaker.
 

flat5

Member
You may be grossly overdriving the mic input.
You don't say at what level you are feeding the signal to the phone.
 

jmb4370

New Member
I had to leave the headset earphone connected, and then disconnect the mic, and then I was able to get the signal to pass. In addition, I had to leave the small speaker connected to the output of the tone generator. It didn't matter whether I used the output directly from the tone generator chip, or a transistor amplified output.
 
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