Monostable 555

Discussion in 'General Electronics Chat' started by stevezila, Apr 13, 2010.

  1. stevezila

    Thread Starter New Member

    Dec 19, 2008
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    Can anyone tell me hoe to get a 555 to time out after the imput signal train stops. Currently I have a 555 configured as a one shot which works great except for the fact that it starts timing at the first toggle of the input. What I need is for it to start timing when the input stops toggling. What I mean is that as long as I continue to get an input the timer will be constantly resetting until the input stops and then the timer will time out. I'm using it for a occupency sensor so as long as I detect movement I want to stay on and when I no longer detect movement it will then time out and shut off. Any suggestions are appreaciated. Thanks
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    Well, when the input toggles, guess what - the timer has to start timing out from there!

    Otherwise, how are you going to recognize when the input stops toggling?
     
  3. stevezila

    Thread Starter New Member

    Dec 19, 2008
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    Yes I relize that but how can I get it keep resetting and timing each time I get a new pulse so that it will time out after last pulse?
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
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  5. SgtWookie

    Expert

    Jul 17, 2007
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    Why don't you post your circuit as it is now. Use the "Go Advanced" then "Manage Attachments" buttons to attach an image of your circuit. .PNG format images are preferred, as they are compact, not "lossy" like .jpg format, and you don't need any software to view them.

    Then explain how long you would like for it to take for the monostable to time out after receiving a pulse.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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  7. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    A slightly more simple retriggerable 555 monostable that Bill Marsden came up with for me.

    Ken
     
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