555 one-shot trigger switch

Discussion in 'General Electronics Chat' started by vinodquilon, Dec 21, 2012.

  1. vinodquilon

    Thread Starter Member

    Dec 24, 2009
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    What would be the output of 555 1 second monoshot if I press the triggering push switch for 5 second ?
     
  2. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Generally, it would stay on for 1 second after you release the trigger. There are some circuits that by-pass this requirement, but, in general, my statement is true. I would rather see your circuit to give you a proper answer. Alternatively, you could proto-type it and try it yourself to find the answer.
     
    Last edited: Dec 21, 2012
  3. Brownout

    Well-Known Member

    Jan 10, 2012
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    The timing capacitor would charge in 1 sec, and stay charged over the duration. When the input goes away, the output will switch off. That's how I think it works anyway.
     
  4. Dodgydave

    Distinguished Member

    Jun 22, 2012
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  5. vinodquilon

    Thread Starter Member

    Dec 24, 2009
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    Here is my circuit...
    Don't worry about OPAMP and transistor, now we discuss about output at 3rd pin of 555.
     
  6. Brownout

    Well-Known Member

    Jan 10, 2012
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    T=Time the switch is activated
    p=one-shot interval (1s in you example)

    For T<p, output active for p
    For T>p, output active for T.
     
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  7. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Assuming you want the 555 to tigger as soon as you push the button and trigger for the same interval no matter how long you hold the button, you could add a resistor in parallel with the switch (pin 2 to ground). The add a capacitor between the switch and pin 2. This will cause pin 2 to pull to ground for a brief period when the switch is pushed but, since it is a capacitor connecting to ground, the pulse is only a transient and pin 2 will quickly go high again (depending on the size of your capacitor). Start with a 0.1 uF then go bigger if necessary to trigger the 555.
     
  8. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    By my calculations, given a 1.2k resistor and a .82μF cap, your pulse length timing isn't 1 second; it's 1082.4 μS. So, unless you are very quick on the button, the pulse will be equal to the length of the button press.
     
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