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555 timer problem

Discussion in 'General Electronics Discussion' started by robertbarron, Apr 8, 2016.

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  1. robertbarron

    robertbarron

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    Jan 17, 2016
    The 555 monostable requires a low going edge to "trigger" it. If this signal remains low it will affect the timing since the "threshold" signal is ignored whilst the trigger signal remains low.
    I need any simple way to take a low going trigger signal and convert it to a quick down up signal irrespective of how long the initial signal remains low. I have been playing around with simple RC circuits but without an oscilloscope I cannot get one to work.
     
  2. Alec_t

    Alec_t

    2,883
    770
    Jul 7, 2015
    Use a capacitor on the Trig input, like this:
    Short-trigger.PNG
     
  3. AnalogKid

    AnalogKid

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    664
    Jun 10, 2015
    The internal trip point for the Trigger input is Vcc/3. The time for the capacitor to charge up to this is approx. 0.4 x R x C. As long as this is shorter than the output pulse (I'd say 25% for margin), the output timing will not be affected.

    ak
     
  4. robertbarron

    robertbarron

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    Jan 17, 2016
     
  5. robertbarron

    robertbarron

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    Jan 17, 2016
    Thanks for trying to sort this out - but, as I said in the question, my trigger pulse IS longer than the monostable timed period and I was wanting the high to low transition to change to a high-low-high pulse so the monostable works properly even though the "switch" used kept a low output.
     
  6. robertbarron

    robertbarron

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    Jan 17, 2016
    Thanks a lot Alec. I have tried all versions of your circuit but this one got by me. Again many thanks for setting me straight.
     
  7. AnalogKid

    AnalogKid

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    Jun 10, 2015
    Although my post is brilliant, it is too succinct for its own good.

    Alec's post shows a differentiating network capacitor C1 and resistor R2, to shorten a long input pulse into a short triggering pulse. My post describes how to calculate C1 and R2 based on the output pulse width you want to be shorter than.

    ak
     
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