DTMF audio coupling

Discussion in 'The Projects Forum' started by Art, Dec 13, 2014.

  1. Art

    Thread Starter Distinguished Member

    Sep 10, 2007
    Hi Guys,
    I am working on DTMF input as a keypad interface for another project.
    The aim is to either have a hardware keypad connected with three wires (power and phono),
    and alternatively allow the use of a DTMF app on a smartphone audio coupled with a microphone.

    I got the first part working great some months ago when I breadboarded a DTMF chip,
    and left it there until last night when I did a microphone preamp with LM358.
    It's now these two circuits connected together at the audio.

    YouTube Video:



    The pot in the preamp circuit is replaced with a 1M2 fixed value resistor, and there is an extra 0.1uF
    cap in series with the DTMF audio input which means there are now two caps in series in the coupling
    between the two chips (4.7uF & 0.1uF).
    Basically I think the whole thing could be more sensitive and looking for suggestions.
    The DTMF chip datasheet specifies 0.01uF coupling at it's input which I didn't have at the time.
    I can tell you that if I omit the 0.1uF cap and just leave the 4.7uF, it's a lot less sensitive.

    The video looks perfect because it's a quiet room with a hi-fi stereo reproducing the sound from a PC.
    Results with a DTMF smartphone app are mixed. At some distances one digit is reliable while another doesn't register.
    Any other suggestions would be much appreciated.
    Last edited: Dec 13, 2014