I need a simple pulse activated circuit.

Discussion in 'General Electronics Chat' started by illusive, Jul 21, 2015.

  1. illusive

    Thread Starter Member

    Jul 9, 2015
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    Hello everyone!
    I need a simple pulse activated circuit. I have set up an op amp comparator which output goes high just for a split second. I need a circuit that will hold that high output level for some time (minute or more). I thing something with a 555 timer will do the job, but i'm not very familiar with that IC so i can't choose the right circuit.
    How about this circuit ?
    monostable_image.jpg
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    If your trigger input is a negative-going one (e.g. 5v dropping to 0V then back to 5V) you can apply it directly to pin2 0f the 555. If not, you will need a transistor stage to invert the trigger first.
     
  3. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Why not reverse the input connections to the comparator so that it outputs a negative going pulse? DUH!
     
  4. illusive

    Thread Starter Member

    Jul 9, 2015
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    OK, so do pulse to pin2 must be going from positive to negative voltage. MikeML, this would be the simplest solution.
    Is that all? So pin 5 is left unconnected and i just put whatever capacitor and resistor pair for the time i want the output of the 555 to be high?
    Also, lets say i want to interrupt the positive output, will a button that shorts the resistor do it?
     
    Last edited: Jul 21, 2015
  5. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Can you follow this:?

    10.gif

    does it do what you want? Look at the polarity of the inputs?

    Obvious improvement: add hysteresis to the comparator.

    Does the comparator need a pull-up resistor?

    Did I forget the RESET?
     
    Last edited: Jul 21, 2015
    absf likes this.
  6. illusive

    Thread Starter Member

    Jul 9, 2015
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    Yes i can, thanks!
     
  7. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    For minute-long timing periods, use the CMOS LMC555 version. It handles large timing capacitors better.

    ak
     
  8. illusive

    Thread Starter Member

    Jul 9, 2015
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    0
    In inverting comparator if i want to add a 1Mohm resistor for a hysteresis where do i connect it? Between the output and the positive input or between the output and the negative input ?
     
  9. crutschow

    Expert

    Mar 14, 2008
    13,013
    3,233
    Positive.
     
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