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Need help in connection of DTMF decoded with Telephone line

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Member level 3
Jul 29, 2011
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Hello All,

I am working on a project that is based on telephone. I want to make a circuit which detects the ring signal from the telephone line and the output of ring detector circuit is connected to the interrupt pin of microcontroller so if there is a high pulse detected on the interrupt pin, we have detection of ring of telephone. But before the phone rings, I need some process like character entering for entering password. So I need to use the DTMF decoder.

Now I do not know that how the telephone will be connected to the DTMF input? I want to use the telephone jack in which the telephone line wire will be connected. So I can easily detect the ring signal and easily have DTMF data from the caller in form of the password.

Can anyone help me to connect the telephone line to the DTMF decoder?


Not popular much anymore but this is done in SLIC chip.

e.g. **broken link removed**

Also look at MOT chips ( ON Semi)
and Agere SLIC chips ( Now Avago )

" But before the phone rings, I need some process ". I would use a high voltage capacitor to couple a small audio transformer to the incoming telephone line. This is because the telephone lines are balance to earth and are subject to lighting strikes. The output of the transformer must be fully protected against overvoltages then can feed a diode and a capacitor so when the 110V 20 HZ ringing tone comes in you get a voltage stored on your capacitor which can be used to trigger your micro. When it has done its stuff it can fire a relay to change over the line to the handset.

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