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Tone decoder debug

Discussion in 'Electronic Design' started by Ralph Olsen, Jan 1, 2012.

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  1. Ralph Olsen

    Ralph Olsen Guest

    I have been trying to get this circuit to work all morning. The LED's
    are supposed to turn on when a 19.2KHz tone appears on the LM567's

    Here is the circuit. Originally downloaded off the net (identical from
    2 sources) but I re-drew it in DipTrace to send here for clarity.
    Click the image to enlarge.

    Any suggestions where the problem might be? ... apart from me that

  2. Rich Webb

    Rich Webb Guest

    Yep. I dug up a TI data sheet (watermarked OBSOLETE on every page; not
    comforting ;-) that shows "R1 6.8 to 15K" as "typical" values. I could
    be wrong but 6.8 ohms is a bit of a reach for a low end value so I'd go
    with their intent being 6.8K to 15K typical and use a good ol' 10K for a

    I dug one of these out of the drawer (surprised I could find it, much
    less that I had one) and breadboarded up a quick test. With R1C1 at 10K
    and 4.4 nF (two 2.2 in parallel) the datasheet formula worked out to
    20.7 KHz, with the actual response at 16 KHz. So, it kind of works.
  3. Ralph Olsen

    Ralph Olsen Guest

    Thanks for the advice everyone. I changed C1 to a 472 and put a 50K
    pot in series with R1 as a "sensitivity" control. All is good. now.

    Happy New Year,

  4. Ralph Olsen

    Ralph Olsen Guest

    Sorry, I meant C4.
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