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Audio Power to Activate a sensitive Relay

Discussion in 'Electronic Basics' started by GerryO, Mar 25, 2005.

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  1. GerryO

    GerryO Guest

    Hi Folks:
    I would like some assistance with the following:
    Could someone direct me to a simple schematic that will perform this
    function: Take the low voltage audio call tone from a (family radio service
    handset) that is heard in the headset and amplify the signal enough to
    activate a sensitive a small relay of 9-12 vdc coil drawing 25ma ( the
    type normally found on printed circuit boards). What would a circuit that
    performs this task be called, so I could do a google search?

    Thanks for your help...

    Gerry
     
  2. Lord Garth

    Lord Garth Guest

    You are looking for a tone decoder...

    You will need to know the frequency of the received tone in order to build
    a decoder. Although the LM567 IC is quite old and, more than likely, out of
    production, they will be available for quite a long time. Look at the data
    sheets and we'll see.
     
  3. Building on Lord Garth's suggestion, here's a practical starting point
    from which you can adapt:
    http://www.terrypin.dial.pipex.com/Images/567ToneDecoder.gif
     
  4. One point about 567s is that I've found they are often difficult to trim
    for an exact match of frequency. You need to use a pot, and things can
    subsequently drift around. The datasheet is fairly cryptic, as I recall,
    giving the values required in a couple of crude graphs, which are
    labeled badly.

    --------
    Regards,
    Robert Monsen

    "Your Highness, I have no need of this hypothesis."
    - Pierre Laplace (1749-1827), to Napoleon,
    on why his works on celestial mechanics make no mention of God.
     
  5. I've recently seen a bandpass filter -> signal using opamps on Bill
    Bowden's site:

    http://ourworld.compuserve.com/homepages/Bill_Bowden/page4.htm#whistle.gif

    It looks to be a bandpass filter into a level detector, and from there
    into a solid state relay. This one flipflops (first whistle on, second
    whistle off) so you may need to mess with it for your application.

    Also, You'll have to fiddle with the filter values to get it to
    recognize your tone, unless you are lucky. The computation for the
    resistor values required is shown.

    --

    --
    Regards,
    Robert Monsen

    "Your Highness, I have no need of this hypothesis."
    - Pierre Laplace (1749-1827), to Napoleon,
    on why his works on celestial mechanics make no mention of God.
     
  6. GerryO

    GerryO Guest

    25 March 2005 6:08 PM

    Thanks a lot for your help guys , I will take your tips into consideration.
    I will use the 567 as I have several and my tone is very close to 2 khz.

    Cheers...Gerry
     
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