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Is there any Multi Turn LOG/AUDIO pots?? If not, how can I make one with a LIN pot?

Discussion in 'Electronic Basics' started by Mike Beauchamp, Jan 5, 2004.

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  1. Hey all..
    I'm working on something where I need a highly precise Potentiometer but it
    has to be LOG. I want to use a multiple turn pot (for the resolution), but I
    can't find any that are listed as LOG, only LINEAR? Does such a thing
    exist?

    I want the Potentimeter to be 100K.. and according to a few sites I can make
    it close to LOG by adding a 35K resistor.

    But, on the "beginners guide to pots", it shows a graph:
    http://sound.westhost.com/pots.htm

    and it ends up being more S shaped than log, but lower down, it shows a dual
    ganged volume balance control using a linear pot and the resistor, and the
    curve definately looks more log to me.

    Mike
    http://mikebeauchamp.com
     
  2. Just to clarify more, because I seemed to have confused myself looking at
    thsoe diagrams since they were in a Voltage Divider situation..

    My situation is this.. between two contacts, I need to vary the resistance
    in in a Log function, compared to the Linear turning of the potentiometer.
    Just hooking one edge of the pot up to one contact and the other contact to
    the wiper of a LOG pot would be absolutely perfect for me. BUT, regular log
    potentiometers don't seem to offere the resolution and precision that I
    need..

    Mike
    http://mikebeauchamp.com
     
  3. John Fields

    John Fields Guest

     
  4. How about using a gear reduction drive with a standard 270° log pot?

    --
    We now return you to our normally scheduled programming.

    Take a look at this! ;-)
    http://home.earthlink.net/~mike.terrell/photos.html

    Michael A. Terrell
    Central Florida
     
  5. Louis Bybee

    Louis Bybee Guest

    It would still be linear, but with a different slope.

    Possibly a large off center pulley where the rate would change depending on
    the position of rotation similar to some of the exercise machines
    (nautilus)?

    Louis--
    *********************************************
    Remove the two fish in address to respond
     
  6. Louis Bybee

    Louis Bybee Guest

    This solution would take a higher degree of ability to implement, but is
    spot on, and could result in a very precise result depending on the care in
    its implementation. Another positive aspect of this method would be the
    ability to tune the output for any anomalies in the surrounding process.

    Louis--
    *********************************************
    Remove the two fish in address to respond
     
  7. John Fields

    John Fields Guest

     
  8. Louis, I stated that you need a log pot, not a linear one. Also, you
    should put some mechanical stops like a notched wheel and a heavy steel
    pin or block to prevent damaging the pot. The gear reduction drive will
    have a lot more toque than a straight shaft.

    --
    We now return you to our normally scheduled programming.

    Take a look at this little cutie! ;-)
    http://home.earthlink.net/~mike.terrell/photos.html

    Michael A. Terrell
    Central Florida
     
  9. If I needed that degree of control, I'd skip the pot and ADC and use a
    shaft encoder to set the address into the lookup table. Could also
    have a digital readout and would not have any discontinuities (pots
    get noisy eventually). The stability would be limited to the
    performance of the voltage ref into the DAC.
    GG
     
  10. John Fields

    John Fields Guest

    ---
    Excellent idea!

    Plus, if there's no need for an analog output he could make the output
    of the lookup table whatever he wants and be done with the DAC and
    reference.
     
  11. Steve

    Steve Guest


    Louis, with a sentence like that you should consider going into politics!

    niftydog
     
  12. JeffM

    JeffM Guest

    I need a highly precise Potentiometer but it has to be LOG.
    Yup. I've got one of these DIY-shunts on my Grandpa's old portable radio.
    The results are useable. It has an exponential response.
    What the exact curve is isn't critical.

    You'll notice that the referenced page is about AUDIO.
    What's that they say about good enough for rock'n'roll?
     
  13. John Fields

    John Fields Guest

     
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