Connecting a Pot or Var. Resistor correctly..

Discussion in 'The Projects Forum' started by the kidson, Jul 26, 2011.

  1. the kidson

    Thread Starter Member

    May 15, 2011
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    hello everyone...i have a question about connecting a variable resistor or pot...i just built a "emergency" light type circuit with a 555 and 4017...i wanted to add the pot or vary resistor to control the flash rate...the problem is...i cant seem how to properly place it into the design..i did a google search and some people suggested just using two pins..others said all three..im not sure how to hook this up..it has 3 pins..2 at the ends on one side....and it has one in the middle on the other side...thanks for reading..
     
  2. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    Would be very helpful if you post a schematic and give more details about duty cycle, flash rate etc.

    If this is the standard 555 astable circuit you have 2 resistors through which the timing cap is charged and one through which it is discharged. One of those you replace with the potentiometer. both resistors enter into the frequency formula.

    You can connect the middle terminal of the potentiometer to one of the other two terminals, that will give you a variable resistor with two terminals.
    Depending on what adjusting accuracy you want you may also use a potentiometer in series with a resistor.
     
  3. wayneh

    Expert

    Sep 9, 2010
    12,156
    3,063
    A pot has 3 pins, two of which are the opposite ends of a resistor. The resistance between those two pins is fixed. The third pin is connected to a mechanical device which contacts the same resistor, but along its length and not at its ends. So any resistance from 0 to the max (same as the two pins mentioned above) is available to the 3rd pin.

    There are MANY ways to use a pot, so you'd need to post your circuit to get useful advice.

    Note that one useful but not obvious trick is to connect the middle pin to one of the ends, effectively giving a two-pin device. The advantage of this is those moments when the mechanical contact is disrupted. This technique sends the resistance up to the limit for the pot instead of wide open (infinite resistance). This can give much smoother results.
     
  4. the kidson

    Thread Starter Member

    May 15, 2011
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    thanks for the response's...i have included the drawing from which i used...R3 and R11 are actually 10K resistors..R11 is the one I want to use the pot for...i dont have C2 hooked up because i was not sure where to place this...but the circuit works..and i didnt use the transistors...thanks again

    http://imageshack.us/photo/my-images/94/strobes2.gif/
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    What value are you using for C1?

    What do you want the frequency range to be?
     
  6. the kidson

    Thread Starter Member

    May 15, 2011
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    C1 is a 2.2 uf capacitor...i didnt have the 1uf..by frequency range do you mean by how fast i want the leds to cycle back and forth?..sorry if im sounding slow...i originally had a 100K resistor from (+) to pin7 on the 555...i found that the rate was to slow..so thats when i added the 10K from (+) to pin 7...however..this rate is just a bit to fast..so i went a bought a 100K micro-pot in hopes of being able to adjust the rates...
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    Use a fixed resistor for R11, at least 100 Ohms per volt. Since you are showing a 6v supply, that would be at least 600 Ohms. You can use more, but not less. If 10k is all you have, that will work fine.

    Replace R3 with the pot. You should actually have at least a small resistor in series with the pot, just in case you turn the pot really low in resistance.
     
  8. the kidson

    Thread Starter Member

    May 15, 2011
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    0
    thanks...now when i connect the pot...how do i actually do it?...im not sure how to connect the pins...thats what im confused about...and what other resistor would i use in series with the pot?...and how would i connect that??...im kinda lost...
     
  9. wayneh

    Expert

    Sep 9, 2010
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  10. SgtWookie

    Expert

    Jul 17, 2007
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    For the resistor in series with the pot, you can use one of your 10k's you had used before. Basically, just about anything between 100 Ohms and 10k would work fine.
     
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