need some help

Discussion in 'The Projects Forum' started by j taylor, Mar 2, 2013.

  1. j taylor

    Thread Starter New Member

    Nov 4, 2012
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    Hi everyone i would really apreciate some help with this circuit the problem i am having is that i carn't get the voltage i need from pin three on the 555 it will only go down to 0.52v when the pot is at min if i change the resistors then it goes to high at the other end over 1.24v has anyone any ideas on how to do this the scematic i have attached is the circuit i am using .
     
  2. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    Why would you have the wiper of the pot connected to the out-put? That's where the 0.52V is coming from. Rewire the pot correctly.
     
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  3. #12

    Expert

    Nov 30, 2010
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    D'oh! I didn't see that.
     
  4. tubeguy

    Well-Known Member

    Nov 3, 2012
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    Try this:
    PWM Controller-1.gif
     
  5. j taylor

    Thread Starter New Member

    Nov 4, 2012
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    Hi Shortbus and thanks for taking the time to look at my scematic i have the pot connected like this to vary the out put with the pot, the circuit works in part the only problem i am having is that it won't go down beond 0.52 and what i want it to do is go to 0.3v and up to 1.25v by varying the pot
     
  6. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    That won't happen.:) You will need a second pot on the out put. The pot you have is just for the timing of the 555.
     
  7. tubeguy

    Well-Known Member

    Nov 3, 2012
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    What are you using/needing the variable voltage for?
    Normally to provide a variable DC voltage using PWM you have a low-pass filter at the output.
    As-is the measurement will depend a bit on the meter you are using.
     
  8. j taylor

    Thread Starter New Member

    Nov 4, 2012
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    can you please explain how to inclued the second pot to achive what i require
     
  9. j taylor

    Thread Starter New Member

    Nov 4, 2012
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    Hi tubeguy i need the variable voltage to apply to a led driver control pin for dimming
     
  10. tubeguy

    Well-Known Member

    Nov 3, 2012
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    Besides the voltage, what is the current requirement or input resistance of the control pin.
    You might need a simpler circuit maybe even just a pot or a pot with a voltage follower - emitter follower transistor.
     
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