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Learning about logic gates

  1. Aug 4, 2013 #1
    Hi I am learning about logic gates and am having some fun with them but somtimes in my designs I only have use for maybe 1 or 2 gates, this is not convenient sence the chips I use have 4 or 6 gates in one chip, does anyone know where I can find NAND, AND, and inverter gates in 8pin dip or less chips? so I could have 2 and gates or 3 invertes in just an 8pin chip?
     
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  3. Aug 5, 2013 #2

    jim hardy

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    8 pin restriction will be difficult, standard was 14 or 16.

    The venerable 555 can do logic and is cheap in 8 pin. Tie TRIG and THRESH together and it's an inverter with hysteresis - great for logic signal conditioning. And it's good practice for you learning logic, add diodes to build your desired gates.

    does this help?
    http://en.wikipedia.org/wiki/List_of_7400_series_integrated_circuits
     
  4. Aug 5, 2013 #3

    davenn

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    Im not aware of any chips with only 1 or 2 gates in them, it doesnt matter if the chips you are using
    have more gates than you need you just dont use the other ones. But one thing you may not know
    is that you should tie the unused inputs to the GND or +5V rail via a 1k resistor. This is for stability
    reasons. It stops erratic behaviour of the gates you are using

    Your other choice, if you are only wanting a couple of gates is to experiment with diodes and transistors
    and build gates out of discrete components. It will REALLY teach you about how a gate operates

    here's an AND gate....

    attachment.php?attachmentid=60694&stc=1&d=1375696885.gif

    do some googling for other examples :)

    cheers
    Dave
     

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  5. Aug 5, 2013 #4

    berkeman

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    74AHC1Gxx logic gates are singles: http://ics.nxp.com/products/ahc/1g/

    :smile:
     
  6. Aug 5, 2013 #5
    Ya I thought of a similar npn AND gate circuit like that one but what I really need is a simple inverter, any ideas?
     
  7. Aug 5, 2013 #6

    berkeman

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    Keep in mind that if you build your logic gates with discrete transistors, they will be *very slow* gates. Quiz Question -- Why?
     
  8. Aug 6, 2013 #7

    Averagesupernova

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    Ah yes. Leave it to good ol' berkeman to turn it into a quiz. LOL You should have been a teacher. I suspect you have a talent for it. Keep it up.
     
  9. Aug 6, 2013 #8
    Sure. Take the AND gate from above, remove the top NPN, add a resistor in the collector terminal of the remaining NPN and move the output from the emitter of the bottom NPN into its collector. How would you size the resistor between the collector of the NPN and the power supply?
     
  10. Aug 6, 2013 #9

    berkeman

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    :smile:
     
  11. Aug 6, 2013 #10

    davenn

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    I think I know why ( not sure) been waiting for tesladude to respond before I spoke

    Dave
     
  12. Aug 6, 2013 #11

    berkeman

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    Yeah, let's let the young'uns take a shot at it. I was surprised when I first encountered it, but the explanation makes sense.
     
  13. Aug 8, 2013 #12
    Perhaps it is the physical size of the transistors. Consequentially there will exist a larger depletion region between the junctions in the transistors (assuming TTL logic, but DTL and RTL will have a similar issue). Is it that the PN junctions may take longer to overcome and begin conducting as compared to their physically smaller counterparts?
     
    Last edited: Aug 8, 2013
  14. Aug 8, 2013 #13

    rbj

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    "slow" is relative. slow compared to 74xx TTL? slow compared to what?

    because i think that "properly biased" (like the old ECL logic was), a discrete transistor will turn on and off as fast as a counterpart that lives on a chip.

    the distances between devices eventually come up regarding speed, but i don't think we're there regarding this application. would an inverter made out of a single discrete transistor be all that much slower than an inverter that is 1/6th of a 74LS06?
     
  15. Aug 8, 2013 #14

    berkeman

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    No, it's *really* slow. :smile:
     
  16. Aug 8, 2013 #15

    berkeman

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    It does have to do with the size of the transistors, specifically the capacitances...
     
  17. Aug 8, 2013 #16

    rbj

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    you have interelectrode and junction capacitance with chip transistors. and there is some capacitance with the substrate. in fact, i've always thought that monolithic analog crapped out at lower frequencies than discrete analog because of these parasitic capacitance. how is it that MOS transistors on chips are so much faster than a sufficiently biased discrete transistor? is it purely the dimensions of the device?
     
  18. Aug 8, 2013 #17

    berkeman

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    Yeah, the typical logic gate has a number of transistors, but their input capacitances are small compared to each single packaged transistor's input capacitance. In many cases it's the Miller capacitance, which makes speed problems even worse.
     
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