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LS outputs and HC inputs

Discussion in 'General Electronics Discussion' started by 0x24a537r9, Feb 18, 2010.

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  1. 0x24a537r9

    0x24a537r9

    2
    0
    Feb 18, 2010
    This should be an easy question. I'm better with my "-wares" when they're soft :)

    So I'm working on a carry-sum binary calculator just for fun, and all of my logic chips are 74HC###, except for a couple 74LS149 shift registers. Now I would have bought them in the HC flavor, but unfortunately these were the only parallel-in parallel-out shift registers Jameco had. As I've read more about digital circuit design, though, I've found out LS outputs can't drive HC inputs. Now I've read the solution is a pull-up resistor, but I'm not really sure how that works, or what it is, or how to implement it, etc...

    I guess what I'm saying is: apart from buying some 74HC149 chips, what are my options? Each output pin of the LS will need to drive one input pin of a 74HC08 quad 2-input AND gate, one input pin of a 74HC86 quad 2-input XOR gate, and one input pin of an IR2422 darlington transistor array chip (for driving the leds). Also, since the output of the logic gates is going back into the shift registers, do I need to do anything to protect the LS inputs from the HC outputs? (Everything is running at 5V except the LEDs)

    Thanks a boatload! And when I say "boat" I mean a cruise-ship, too. Not some dinky canoe.

    (Oh and also, ascii art schematics are acceptable!)

    0x24a537r9
    Cameron
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,299
    2,737
    Jan 21, 2010
    Connect a resistor from the LS output to the +5V

    See here for all the details you need. 4.7k should be fine for most cases.

    Everything you mention should be fine. The IR2422 has reasonably large base resistors, so it should be fine, but if you have problems, use a smaller pull-up resistor for the outputs connected to one of these (say, as low as 1.5k).

    Lower value pull-up resistors will cause greater power consumption.

    If you can get hold of the HC version of the shift register it would make things easier.

    BTW I'm off on a cruise ship in about a week -- I'll accept your boatload of thanks ;)
     
  3. 0x24a537r9

    0x24a537r9

    2
    0
    Feb 18, 2010
    Perfect! You have my thanks!
     
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