NE555 extra trigger impulse not recognized...

Discussion in 'Digital Circuit Design' started by offroadflow, Sep 9, 2016.

  1. offroadflow

    Thread Starter New Member

    Sep 9, 2016
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    Hello all together!

    I´m fiddling with a tiny circuit - an NE555 which shall be triggered to hold for 5 minutes BUT shall prolong the time with every new trigger impulse.
    The normal timer curcuit does not care about any further trigger after the first and just keeps going until the time has passed after the first trigger.

    Is there any possibility to have it reset (but not restart) with every trigger after the initial and before the time ends..?

    Thanks a lot in advance!
    Flow
     
  2. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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  3. offroadflow

    Thread Starter New Member

    Sep 9, 2016
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    awesome - cheers!
    so simple and yet so vast... :D
     
  4. dl324

    Distinguished Member

    Mar 30, 2015
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    It would be helpful if you posted a schematic so we can see how your trigger is wired.

    In essence, you just need to pull the trigger input to less than 1/3VCC to retrigger, but the trigger input needs to rise above 1/3VCC before the one shot times out.
     
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  5. crutschow

    Expert

    Mar 14, 2008
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    Note that the output goes high at the start of the trigger pulse for those circuits but the timeout period is from the end of the trigger pulse.
    That's not a problem if the trigger pulse width is a negligible portion of the timeout period.
     
    Last edited: Sep 9, 2016
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  6. offroadflow

    Thread Starter New Member

    Sep 9, 2016
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    ...actually thats exactly what i needed...

    But now i´m facing another problem:
    It seems not possible to build a circuit with a 555 which waits a specific time before it triggers.
    Guess a simple RC before the trigger input should do the trick since it´s only a few seconds of delay needed...

    Or do you have a more PRO idea..? :)
     
  7. ian field

    Distinguished Member

    Oct 27, 2012
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    Its fairly common to feed the trig input via a CR differentiator circuit.

    With most of the circuits posted earlier - the capacitor would have to be big enough to make a "fat" spike to ensure fully dumping the 555 timing capacitor.
     
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  8. crutschow

    Expert

    Mar 14, 2008
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    Do you want the delay for all trigger pulses including the retrigger pulses, or only for the first trigger pulse?
     
    Last edited: Sep 9, 2016
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  9. offroadflow

    Thread Starter New Member

    Sep 9, 2016
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    in this case only for the first.
    this is further into the main circuit already...
     
  10. crutschow

    Expert

    Mar 14, 2008
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    Then that will require some added logic to bypass the delay when the one-shot output is high.
     
  11. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    hi,
    Post the circuit you have so far and tell us what is the need for a 5 minute delay, what is the application and its trigger source.
    E
     
  12. offroadflow

    Thread Starter New Member

    Sep 9, 2016
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    Thank you a lot for your effort!!!
    You really help me getting back on the track where i left about 20 years ago...

    It´s a row of 555s, the first has a delay OFF of about 5 seconds (5-10 sec adjustable), retriggerable.
    After that the delay ON (5-10 sec) before a 10 minute 555 (5-10 minutes).
    All in TTL (5V)

    It shall become a timer series to finally drive a relay for some time, but only if a few time parameters occure before.
     
    Last edited: Sep 10, 2016
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