series 74LS output voltages

Discussion in 'Homework Help' started by cruizerdog92, May 9, 2013.

  1. cruizerdog92

    Thread Starter New Member

    May 7, 2013
    10
    0
    Hi all,
    I've been designing an electronic circuit using 74LS logic gates and my calculates have been using 3.4V as my logic gate high output voltage since the datasheet says thats the typical voltage. However when i perform my simulations through the NI multisim program it outputs 5V.

    I'm pretty sure i show use 3.4v but I dont have any components to test the actual output voltages so do you think that this is just an inaccurate program and i should stick with 3.4v when calculating resistor values or use 5v as my value.
     
  2. t06afre

    AAC Fanatic!

    May 11, 2009
    5,939
    1,222
    It will somehow depend on circuit. Are you driving LEDs with the LS outputs?
     
  3. MrChips

    Moderator

    Oct 2, 2009
    12,440
    3,360
    4V is a more realistic output.
     
  4. cruizerdog92

    Thread Starter New Member

    May 7, 2013
    10
    0
    sorry about that. i was driving leds but forgot i was sinking leds so it doesn't affect them. I'm only driving transistors with the high outputs.

    while im here is there such a thing as a open collector or gate. i can get away with an executive or gate but can only find a quad chip for the xor open collectors and only really need a single.
     
    Last edited: May 9, 2013
  5. MrChips

    Moderator

    Oct 2, 2009
    12,440
    3,360
    XOR stands for exclusive OR gate.

    Why do you need open collector? You can create one with a transistor.

    What are you trying to do? Show us your circuit diagram.
     
  6. cruizerdog92

    Thread Starter New Member

    May 7, 2013
    10
    0
    this is the circuit was trying to power an led with about 20mA but reduced it to about 12mA because wasn't sure what circuit could handle 20mA

    it works with a XOR or a OR gate but thought an open collector would give it more current. and trying to reduce chip space so didnt really want to have a quad chip when i didnt need the other three.
     
  7. MrChips

    Moderator

    Oct 2, 2009
    12,440
    3,360
    The bottom line is, is the LED bright enough? You can get LEDs that are very bright with 5mA.

    Can you draw a truth table showing on what conditions the LED will be on?
    If you really want to reduce space you can do it without logic gates, i.e. with a transistor, resistors and diodes.
     
  8. cruizerdog92

    Thread Starter New Member

    May 7, 2013
    10
    0
    so if the top switch was A and bottom was B

    A B C
    0 0 0
    0 1 1
    1 0 doesnt matter
    1 1 0
     
  9. MrChips

    Moderator

    Oct 2, 2009
    12,440
    3,360
    Here is a simple circuit using a transistor. All resistor values are guesses.

    (I have interchanged the A and B switches.)

    You could eliminate R2.


    [​IMG]
     
  10. cruizerdog92

    Thread Starter New Member

    May 7, 2013
    10
    0
    ok i see what you've done. At first i didn't think it was going to work but the A and B are swapped around. so the r2 acts as resistance to make a path to the transistor and then s2 works by shorting it to earth
     
  11. cruizerdog92

    Thread Starter New Member

    May 7, 2013
    10
    0
    sorry i just saw what was written in the brackets :D. that the switche were interchanged
     
  12. cruizerdog92

    Thread Starter New Member

    May 7, 2013
    10
    0
    sorry im new to this forum is there a way to rate threads or people. because you have been really helpful the last two days and hope im not being rude by not thanking you
     
  13. MrChips

    Moderator

    Oct 2, 2009
    12,440
    3,360
    Actually, if switches are just mechanical switches with no limitations on how they are connected, there are even more simple solutions.

    Solution #1
    Put resistor, LED and switches in series. Make switch B a normally closed switch.

    Solution #2
    Put resistor, LED and switch A in series.
    Put switch B (normally open) across the LED.
     
Loading...