Expression of esteem thanks....

Discussion in 'General Electronics Chat' started by vinodquilon, Feb 28, 2010.

  1. vinodquilon

    Thread Starter Member

    Dec 24, 2009
    Here I attach my final DTMF project schematic (report is not yet prepared).
    It has been taken around one month to reach at this final workable solution.
    In these days I have been asking a lot of doubts regarding my project in these forums,
    all about circuits

    I express my sincere thanks to all members, without whom I cannot realize my project.

    Project Aim-
    Home automation using Telephone.i.e; Control home appliances from a distant location through phone.

    Milestones in our project preparation-

    We have started with a fake project schematic and thanks for audioguru, he pointed out lot of errors.

    There is a restriction in our curriculum, mini project should be done only with discrete and digital electronic
    components. Use of micro controllers is not allowed. This was the first problem we have faced.

    Secondly, we choose Mitels' DTMF IC. But it was not reliable. We have purchased three ICs and only one gave 75%
    satisfactory performance.

    To get 100% performance we have added two extra circuits like PoR (Power on Reset) & 74123 miss-pulse detector.

    1:1 isolation transformer was another issue. After a lot of search we have found one from farnell.

    60 second delay was another issue, but we got accurate delay with 555 without any leakage problems.

    Audio fed back to telephone line was another issue, thanks for rogs he gave a bravo solution.


    When you are out of your home, just plug your telephone in device control mode.
    When you call to your home phone, ring detector circuit will activate DTMF IC & off-hook relay after detecting fifth ring.
    Off-hooking mode lasts only 60 sec. Thus saves our money.

    Here 4 devices are controlled through key presses 1 to 4. If anyone want to check the status of the device,
    just press the corresponding control key and you will hear some audio beeps indicating device is on.

    D flip flop is a main part in the output. It is configured in such a way that when a clock signal comes it will
    toggle from its previous state. Thus devices can be on/off by repetitive key events.

    To turn on a device press * (it will turn on buffer for 5 sec) followed by corresponding key event.


    No user authentication as in Micro controller employed systems.
    Takes too much space to do a small function.
    Long time timing accuracy is better for micro controllers than that of 555.
  2. rogs

    Active Member

    Aug 28, 2009
    Glad I was able to help with one or two suggestions. Two points from your new schematic:

    - The capacitors for the audio feedback bandpass filter should be 22nF, as on my original sketch, not 220nF as drawn on your new schematic.

    - Change the '74LS' ics for '74HC' versions. Interfacing 4000 series and 74 LS is not a good idea. It might work, on a good day! :)