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Phone and DTMF decoder

  1. Oct 31, 2008 #1


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    how does one connect a telephone line with a DTMF decoder(e.g CM8870). i.e which telephone wires(of the 2 pairs of the telephone cable) to i connect to the 8870?

    again if i use a cell/mobile phone how to i connect to the 8870?

    any help will be appreciated.
  2. jcsd
  3. Oct 31, 2008 #2


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    Science Advisor

    Well, if you look at the datasheet:

    ...and some background on telephone wiring:

    Then you see that you'd probably need some circuitry before the CM8870 to filter out the -48V bell ringing signal, and scale the actual dial tones so that they don't exceed 5V.

    For a cell phone, you'd need a microphone or an audio jack if the phone comes with handsfree capability.

    Also, a Google search for 'DTMF Decoder' brings this up:
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