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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

    214
    0
    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
    10
    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

    25,505
    2,852
    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

    214
    0
    Jun 18, 2012
    Awesome. Thanks.

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

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,505
    2,852
    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

    1,162
    109
    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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