Reduce input signal by 500ms

Discussion in 'General Electronics Chat' started by XtinctElise, Jan 21, 2012.

  1. XtinctElise

    Thread Starter New Member

    Jan 21, 2012
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    I need a circuit to reduce the amount of time that a 12V relay is turned on by 0.5 seconds.

    In this circuit if I have a 1 second input pulse, i need an output pulse of 0.5 seconds and if it is a 5 second input pulse, then i need a 4.5 second output pulse.

    I would think this would be a rather simple circuit, but i have been away from circuit design a bit too long and need some help kicking my mind back in gear.

    Can anyone supply a schematic or information on the design of such a circuit? Thanks in advance!
     
  2. Brownout

    Well-Known Member

    Jan 10, 2012
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    Sure. A simple timer set to .5s, and then logically inverted/anded with the original pulse. What happens then is nothing will happen for the first .5s, then the relay will activate until the pulse is gone. A 555 can be used for the timer. You'll have a bit of a race condition between the original pulse and the timer pulse, so you'll need a little delay, R*C, at the point where the signals hit the AND logic.
     
  3. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    As I'm kinda sure you don't want to do anything silly like violate causality then all you need is a 1/2 second delay. AND that with your turn on signal and drive the relay with that signal.

    Basically it just subtracts the first 1/2 second from any pulse.
     
  4. Brownout

    Well-Known Member

    Jan 10, 2012
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    ^-great minds think alike :)
     
  5. XtinctElise

    Thread Starter New Member

    Jan 21, 2012
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    Do you have a schematic for this layout? How much current will the 555 draw when it is not in use?
     
  6. Brownout

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    Jan 10, 2012
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    Hi, no I don't have a schematic. But you'll want a one-shot, and you'll find many schematics for that with a simple search. Failing that, I can cobble someting up. But my time is limited, and you might wait for awhile. Some other members might have a diagram to post.
     
  7. Brownout

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    Jan 10, 2012
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    Here is a page with some info. If you scroll halfway down, you'll find information on a monostable multivibrator ( one-shot ) I can help you understand the operation and how to build one, if you have any questions. http://www.kpsec.freeuk.com/555timer.htm
     
  8. Adjuster

    Well-Known Member

    Dec 26, 2010
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  9. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    Delaying the "pull-in" of the relay by 0.5 second assumes that there is no requirement that the start of the input pulse and the relay pull-in be coincidental.

    XtinctElise,
    Do you need the relay to pull-in at the start of the input pulse?

    Ken

    Ken
     
  10. Adjuster

    Well-Known Member

    Dec 26, 2010
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    Note ErnieM's point about the violation of causality: it is a good one.

    Unless the pulses are repetitive AND we don't mind missing the first one, the 500ms has to come off the leading edge.
     
  11. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    I got that, but does the OP understand "violation of causality"?

    Ken
     
  12. Brownout

    Well-Known Member

    Jan 10, 2012
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    It doesn't matter if he understands it or not. The point is, there is really only one option.

    If the two pulses need to fire at the same time, the original one can be delayed for .5S. If we can't delay it, we're screwed.
     
  13. Brownout

    Well-Known Member

    Jan 10, 2012
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    Here is an adaptation of erineM's idea. Sorry I dont have the complete schematic, but this shows one way to delay the start of the pulse by .5s. Basically, the output of A1 ( a scmitt trigger device ) is connected to an AND gate, along with the original pulse. The output of the AND nees to feed a driver for the relay, typically a small transistor.
     
  14. XtinctElise

    Thread Starter New Member

    Jan 21, 2012
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    Yes at the start of the input pulse. The pulses will be at least 500ms appart.
     
  15. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    As was mentioned, what you want to achieve is impossible, unless these are repetitive pulses.

    If you trigger the relay when the input pulse starts you can not turn it off with exactly 500ms before the input pulses ends.

    The end of the input pulse is unknown. You would want to foresee the future.
     
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