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Can You Fake a Large Pot?

Discussion in 'General Electronics Discussion' started by Yoa01, Dec 29, 2012.

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  1. Yoa01

    Yoa01

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    Jun 18, 2012
    Hi all,

    Is it possible to make a potentiometer act like it has more resistance? I'm trying to create a very wide range 555-based oscillator. I would like for it to be able to run from 1Hz to 1000Hz, but the only way I can see to do that (using my very limited knowledge) is to use a 100M pot (which, if that exists, I'm scared). Surely there's another way to do this. I'm open to almost all suggestions.

    Thanks.
     
  2. Raven Luni

    Raven Luni

    798
    8
    Oct 15, 2011
    There is no way to increase the resistance of pot. All you can do is put it in series with another resistor and only use a fraction of the total range equal to the original value of the pot. You could use a multi gang pot or have a whole bunch of single ones synchronised mechanically, but for your purposes, you really want to consider reducing the value required.

    The timing of a 555 is based on the RC constant. So instead of looking for a bigger R, why not use a bigger C which will allow you to use a smaller R and still get the same output range?
     
  3. Yoa01

    Yoa01

    214
    0
    Jun 18, 2012
    Yeah, I'm a noob. I should have guessed that increasing the cap size would help but never thought to try.

    WOW! I just simulated using 1u caps (I've been using 10n) and, with a 1M pot, it can go from .5Hz to 1500Hz - perfect!! At 1500Hz the pulse width is pretty wide, but that shouldn't cause much of a problem since I doubt I will ever use it at that kind of speed. It's being used to clock a 4017 being used as a sequencer (as posted here: http://bit.ly/YpFKJl), if you were curious.

    Thank you so much!
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    The mark/space ratio should not matter at all if you're using it to clock a 4017

    (OK, it *might* if your fastest speed was in the order of several MHz, but you're not going to get anywhere near that with a 555)
     
  5. Yoa01

    Yoa01

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    0
    Jun 18, 2012
    Awesome. Thanks.

    I think only crystals can go that high, right?
     
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    No, you can make lots of things oscillate faster than a 555. It's just that it's a relatively slow device, it's not designed with high frequencies in mind.
     
  7. cjdelphi

    cjdelphi

    1,096
    104
    Oct 26, 2011
    You can fake a large pot, use a piece of cardboard, paint a pot, and then set it near a wall, nobody will notice a difference :)
     
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