Plz help before I kill myself : very easy

Discussion in 'The Projects Forum' started by simeonz11, Jul 5, 2009.

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  1. simeonz11

    Thread Starter Active Member

    Apr 29, 2008
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    Hi , I wish to do the following .

    I'm working with cmos btw , under 100 khz .

    I'm looking for some circuit or chip that can do this .

    Thx you
     
  2. hgmjr

    Moderator

    Jan 28, 2005
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    You are going to have to help us out by describing what "this" is.

    Nevermind, I see you have added an attachment.

    hgmjr
     
  3. hgmjr

    Moderator

    Jan 28, 2005
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    What is the frequency of the input signal and what is the pulse width of the pulse you need to form off of the positive going edge?

    hgmjr
     
  4. hgmjr

    Moderator

    Jan 28, 2005
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    It looks like what you may be needing is a one-shot. One IC that comes to mind immediately is an MM74HC123.

    hgmjr
     
  5. simeonz11

    Thread Starter Active Member

    Apr 29, 2008
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    well I would like to control the pulse width also , it should be very tiny but I fear this will be too tiny .

    The tiny little pulse should be terminated when my comparator reaches 0 volts , IE , another pulse .
     
  6. hgmjr

    Moderator

    Jan 28, 2005
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    What are you using the comparator to do?

    hgmjr
     
  7. millwood

    Guest

    hgmjr is right. 123 is the right part to use here.

    take a look at its datasheet and you will know how to control pulse width.
     
  8. hgmjr

    Moderator

    Jan 28, 2005
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    At this stage, we need to establish what voltage you were planning to use to power your design.

    hgmjr
     
  9. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    Can you have a delay and how much between the rising edge of the clock and the pulse you want to create?

    John
     
  10. simeonz11

    Thread Starter Active Member

    Apr 29, 2008
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    10 to 15 volts maximum

    regular cmos ranges

    basicy , that short pulse should end when my capacitor voltage is below 5 millivolts ( fast comparator pulse )

    this is too make sure my capacitor is empty

    the pulse controls a transistor wich "tries" to totally discharge a capacitor as fast as possible . ( pF range , dont worry )
     
  11. hgmjr

    Moderator

    Jan 28, 2005
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    You will not able to use the 74HC123. It is intended for 5V use. You will need to use the CD14538.

    hgmjr
     
  12. simeonz11

    Thread Starter Active Member

    Apr 29, 2008
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    well , that chip is much too rare , had trouble even finding the datasheet .

    There has to be an easier way .

    This is what I would like to achieve .
     
  13. hgmjr

    Moderator

    Jan 28, 2005
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    Digikey has them for 80 cents each.

    hgmjr
     
  14. simeonz11

    Thread Starter Active Member

    Apr 29, 2008
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    thx you hg

    but I need something more common , this IC is too <snip> .
     
    Last edited by a moderator: Jul 6, 2009
  15. BillB3857

    Senior Member

    Feb 28, 2009
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    Cap coupled input to RS FF???
     
  16. beenthere

    Retired Moderator

    Apr 20, 2004
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    Thread closed at request of OP.
     
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