555 Question, Inhibit output

Discussion in 'The Projects Forum' started by haxxx, Mar 3, 2009.

  1. haxxx

    Thread Starter Active Member

    Feb 14, 2009
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    Hey guys, quick question.
    In the attached circuit.
    After the initial Trigger, which pin could i take either high or low
    to prevent any future trigger from providing an output
    if normally closed is reopened. This event will occur during
    the initial output. and prevent future outputs.

    Thanx.

    Haxxx.
     
    Last edited: Mar 3, 2009
  2. Wendy

    Moderator

    Mar 24, 2008
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    2,536
    Pin4...........
     
  3. haxxx

    Thread Starter Active Member

    Feb 14, 2009
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    0
    I was trying to avoid pin three going low before it finished its initial output. Wouldn't dropping pin 4 low send pin 3 immediately low.

    Haxxx
     
  4. Bernard

    AAC Fanatic!

    Aug 7, 2008
    4,173
    397
    Reset will take pin3 low, then take pin 2 high after triggering.
     
  5. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    Pin 3 (output), is low as a default.
     
  6. haxxx

    Thread Starter Active Member

    Feb 14, 2009
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    0
    Maybe i'm not saying it clearly enough. You suggested pin 4. I asked wouldn't that immediately take my output low before the time designated by the the RC circuit which would be undesirable for my application. I want to apply a high or a low during the initial pin 3 output which would not affect that current output but inhibit any future output.
     
  7. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    Nope. What you have said is clear but your understanding of the 555 is not.

    You said you you want to "prevent 555 output" but in fact what you wanted is "keeping 555 with its output" once triggered.

    This is untested.

    "Pulling pin#2 low within the monostable timing period after triggering will keep 555 output high forever."
     
  8. haxxx

    Thread Starter Active Member

    Feb 14, 2009
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    Ok u told me what i want. Except what i want is not a perputual high (could get a smoke for that.....heh) but a simple monostable pulse
    that times out. Except after pin 3 goes low it stays low even when pin 2 is retriggered.


    Haxxx
     
  9. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    Adding a few component will do what you want.

    Open or high on point X will enable normal monostable operation. A LOW on point X will let the current timing cycle to finish but prevent any future triggering.

    If point X is pull LOW before any triggering, no triggering can occur.

    [​IMG]
     
  10. haxxx

    Thread Starter Active Member

    Feb 14, 2009
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    Thanks a lot I'll give it a shot.
    Haxxx
     
  11. italo

    New Member

    Nov 20, 2005
    205
    1
    Unless the lm555 was redesigned
    when I was not looking it is not by a tristate chip it seams to me just because there is no further transistion 0v and Vcc there is still an output. So now he must define an output. pin 4 will reset the output immidiately. and pin2 once it goes low any further lows on this chip will cause no difference not untill the tiimg has elapsed. YOU must add something a counter lm555 to reset this chip after your timing has expired .
     
  12. haxxx

    Thread Starter Active Member

    Feb 14, 2009
    35
    0
    I guess ur right
    I did try the fix posted by "EBLC1388", it had no effect on the output.
    Output expired and pin 2 was still able to retrigger. I still haven't been able to find a solution.
     
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