Critique my circuit for chips with RESET pins

Discussion in 'General Electronics Chat' started by JerryH, Jan 13, 2011.

  1. JerryH

    Thread Starter New Member

    Jan 10, 2011
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    I'm using a 74LS192 DECADE counter which has a RESET pin on it. I've come up with a simple circuit that seems to work just fine for now. Should I be concerned that my switch effectively shorts out the capacitor? We're only talking 5 volts at most here, with pretty low capacitance, but still....

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    Last edited: Jan 13, 2011
  2. eblc1388

    Senior Member

    Nov 28, 2008
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    You have a valid concern. The shorting of the capacitor will create a spark and cause premature failure of the reset switch contacts.

    The solution is simple. Just place in series a resistor with the reset switch of any value between 10Ω and 100Ω as this will then reduce the short-circuit current to a safe level for the switch contacts.
     
  3. JerryH

    Thread Starter New Member

    Jan 10, 2011
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    Thank you...now I just need to throw in a de-bouncer of some kind and call it done. :)
     
  4. t06afre

    AAC Fanatic!

    May 11, 2009
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    If you follow the the tip from eblc1388 the circuit will act as de-bouncer. Due to the charge and discharge of the capacitor
     
  5. JerryH

    Thread Starter New Member

    Jan 10, 2011
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    True, but wouldn't the concern then be that as the capacitor charges/discharges, it will have to pass through TTL's undefined input voltage zone of .8 to 2.0? Theoretically there could be problems...but probably not.
     
  6. t06afre

    AAC Fanatic!

    May 11, 2009
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    Take a look here. Debounching is due to the mechanical properties of a switch.
    http://www.ikalogic.com/debouncing.php
     
  7. eblc1388

    Senior Member

    Nov 28, 2008
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    If the pin is a gate input, then undefined input voltage zone will cause problem as the gate output might change state unpredictably.

    For undefined voltage on a reset pin, it could also cause multiple resets but since what reset operation does is to force the chip into a well defined state so multiple resets will effectively changes nothing after the first reset.
     
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