De-bouncing circuit

Discussion in 'General Electronics Chat' started by elleader, Jul 16, 2011.

  1. elleader

    Thread Starter New Member

    Mar 25, 2011
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    http://www.ikalogic.com/debouncing.php

    this link shows the problem of contact bounce and the two solutions of it( digital and analog ) at analog solution he used schmitt triger ic to prevent errors come from the unknown area.

    i don't understand what is the source of errors at this area ?

    please help ,very urgent
     
  2. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    Did you read the text?

    "The problem with this configuration, as described briefly before is that, due to the mechanical nature of any switch that may contains spring return action of some kind, there wont be a clean transition from a state to another, but instead there will be a series of high and low states spikes as shown in figure 1B."

    When you press or release the two contacts of a switch, they open and close intermittently for a short period of time. This is the "error" you don't want and that you need to avoid, doing so by debouncing it with the proposed solutions.
     
  3. elleader

    Thread Starter New Member

    Mar 25, 2011
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    i read it and i know this part and it's solution using RC , the o/p should be smoth after that , but he used schmitt trigger after that to solve another problem at the unkown area, i don't understand this part why there will be errors at this area , and what is the source of these errors ?
     
  4. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    Ok, got it. If you have an input signal on a logic gate which is slowly rising, the voltage level will come to a point where it detects a level change at the input, consequently changing the output level. However, any tiny noise at this input could make the gate detect a low-level again which would cause it to switch the output again.

    The schmitt-trigger will prevent this from happening, because once it detected the input as High, the voltage at the input would have to decrease to a value lower than the Low-threshold voltage, which is normally several volts away.
     
  5. Wendy

    Moderator

    Mar 24, 2008
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    Schmitt Triggers are used to clean up noisy signals, not just debouncing switching. If you have a signal with a lot of noise riding on it, the answer is usually a Schmitt Trigger.

    [​IMG]
     
    elleader likes this.
  6. hspalm

    Active Member

    Feb 17, 2010
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    I think what he refers to is the "unknown area" as described in the article.

    The unknown area is when the voltage level is either on it's rising or fallow edge, when it is between the two given HIGH or LOW max/min levels. This is called an unknown zone in the article because the voltage level is not recognized as either HIGH or LOW by digital circuits, or that it could easily switch between those two in this state.

    This unknown area depends greatly on whatever type of logic you are working on, whether it be TTL or CMOS, and even if you're on 5v. 3.3v or any other logic levels (the zone will shrink or grow).

    In the article, this problem is solved using a schmitt trigger because the unknown zone will not affect it's output state, it simply switches to either HIGH or LOW when its input is above or beyond its threshold levels.

    edit: two people beat me to it, guess I need to work on my typing skills
     
  7. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    LOL... I guess you were too occupied with your own thread :D
     
  8. hspalm

    Active Member

    Feb 17, 2010
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    Multitasking, it's called. I slow down both processes by 50% to make it work. I must be badly programmed!
     
  9. someonesdad

    Senior Member

    Jul 7, 2009
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    Contact bounce is an interesting mechanical process -- I'd imagine some theoreticians somewhere have studied it and concluded that is due to some quasi-chaotic process. When you use a scope to study the bounces of different switches, you find that the general pattern repeats, but the details are always different. I've attached a scope trace of a bounce pattern from a microswitch I have. If you're interested in some details, Jack Ganssle has studied and published some on his website.
     
  10. elleader

    Thread Starter New Member

    Mar 25, 2011
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    thanx all
     
  11. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    Interesting stuff ! I've never done this I guess, tracing the bouncing pattern (never needed to). :)
     
  12. someonesdad

    Senior Member

    Jul 7, 2009
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    I think it would be fascinating to study in detail, as what is likely to go is in no way a simple process. There's arcing, plasmas, welding, phase changes, and mechanical interactions. No doubt it has been exhaustively studied by the folks who make switches and relays, but of course they hold their data tightly to their chests.

    If you have a scope and get curious, it's interesting to capture the contact bounce waveforms from a variety of switches to see how they all vary. Or, just look at Ganssle's site for an idea of the variety.
     
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