74LS08

Discussion in 'General Electronics Chat' started by Linden, Feb 28, 2015.

  1. Linden

    Thread Starter New Member

    Feb 28, 2015
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    I am trying to use mini float switches to drive my inputs but when I hook up my Vcc and Gnd, they start off in a false logic "1" state.(measuring 1.4V on I and 3.9V on the O). What is going on? They are brand new chips.
     
  2. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Can you post a schematic of your current set-up?

    Bertus
     
  3. Linden

    Thread Starter New Member

    Feb 28, 2015
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    JK is used as a piezo hush
     
  4. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    I do not see any pull-up or pull-down resistors on the inputs of the gates.
    When you leave then floating the results may be unpredictable.

    [​IMG]

    Bertus
     
  5. Linden

    Thread Starter New Member

    Feb 28, 2015
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    I figured that would solve it just wanted to make sure. What size resistors would you suggest?
     
  6. bertus

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  7. Linden

    Thread Starter New Member

    Feb 28, 2015
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    Thanks for all your help. I am sure this will work.
     
  8. MrChips

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    Oct 2, 2009
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    Unconnected inputs of TTL gates usually start out at logic 1. But it is better to be certain by using 1kΩ pullups.
     
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  9. cmartinez

    AAC Fanatic!

    Jan 17, 2007
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    I normally use 4.7K, and up to 10K resistors as TTL pullups... is that bad? should I switch to 1K?
     
  10. MrChips

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    To answer your question, we have to look at the datasheets of TTL gates.

    For 74xx and 74LSxx, VIH is quoted as 2V. This is the minimum Vin for logic HIGH.
    Fro 74xx, IIH is 40μA and 20μA for 74LSxx. This is the input leakage current when Vin is HIGH.

    Suppose you want Vin to be 4V. Then the pullup resistor has to drop 1V while conducting IIH.

    Rpullup = (Vcc - Vin)/IIH

    For 74xx, Rpullup = 1V/40μA = 25kΩ

    For 74LSxx, Rpullup = 1V/20μA = 50kΩ

    Hence, Rpullup of 10kΩ is fine.
     
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  11. ScottWang

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    Aug 23, 2012
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    Your values of resistors are quite normal, if you want to reducing the current then you can increasing some more, normally resistor of pull high for TTL ic around 1K~33K, but if the pull low should be used 330Ω, because the internal diode of input, although using less 330Ω is ok, but it will draw more current.
     
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  12. cmartinez

    AAC Fanatic!

    Jan 17, 2007
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    Thanks! ... Ah, the magic of datasheets... if only more of us bothered to actually read them... ;)
     
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