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Needing help with a 555 timer switch

Discussion in 'General Electronics Discussion' started by patsroom, Jun 30, 2016.

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  1. patsroom

    patsroom

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    Apr 25, 2013
    one time on switch.jpg
    What I am hoping to do is get a one shot non-retrigger-able timer switch of one or two seconds. One that no matter how many time you try to press the on switch after the first time it will not pay attention to the other pressing of the button.Thank You for reading..................Pat
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    A 555 is retriggerable.

    one option is to have one 555 set up as a bistable which is triggered by your input and its output triggers the second monostable 555.

    when the second minostable returns to its rest state it resets the first 555, allowing I to be triggered again.
     
  3. patsroom

    patsroom

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    Apr 25, 2013
    Thank you for your reply steve. I was think that if I did not connect pin 4 to the circuit it would not reset so that a new signal would not affect what it is doing until it's power was completely off to the 555 to re set the 555. And to control the time would be to use different resistor and or caps............Pat
     
  4. Herschel Peeler

    Herschel Peeler

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    Feb 21, 2016
    Both Reset and Trigger are active low inputs. They never go low in the circuit, If Reset and Trigger both go low at the same time will it ever trigger?
     
  5. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    How is the timer intended to be armed for another trigger? Will there be a reset button, will there be a timeout...?

    Steve's post #2 would arm the 1st 555 after the timeout of the 2nd 555.

    If you want a reset button, I suggest you use an RS flip flop which would be set by the trigger button, reset by a reset button. the output of the rs flip flop would trigger the 555 to genrate a single pulse when the flip flop is being set. Multiple pushes on the trigger button will not impress the rs flip flop unless it has been reset by the reset button.
     
    Chris01 likes this.
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    when referring to a monostable, "retriggerable" and "non-retriggerable" refer specifically to the behavior of the device if a trigger pulse arrives while the monostable is in its "unstable" state.

    If the OP has his own definition of "non-retriggerable" which differs from the conventional meaning, it would be best if it is defined for us. :)
     
  7. Harald Kapp

    Harald Kapp Moderator Moderator

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    From post #3 I take it that the op's intention is a real single-shot until reset by power off-on cycling. Not the conventional meaning of 'non-retriggerable'.
     
  8. Herschel Peeler

    Herschel Peeler

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    Feb 21, 2016
    Is the 555 retriggerable? I don't think so.
     
  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Yes. If you have a 555 set up as a monostable with a time period t and you send trigger pulses with < t between their falling edges, the 555 will remain triggered for ever.

    if it was non-retriggerable, the output would change state for time t then trigger again on receipt of the next falling edge.

    Actually its worse than that, the 555 is level sensitive, so while it is triggered on the falling edge, the time period does not start until after the rising edge. Typically you capacitively couple the trigger so this is not a huge issue.
     
  10. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    For the OP's case, the two 555 solution would work, removing the ability to reset the bistable would do the trick.

    note that it is important that the bistable starts in the right state. An appropriate power on reset will ensure this.
     
  11. Alec_t

    Alec_t

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    Jul 7, 2015
    Here's how you could do it with a CD4013 or other dual D-flip-flop :-
    SinglePulser.PNG
     
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