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8 bit Binary Decoder

  1. Apr 11, 2004 #1
    How difficult would it be to create a circuit that uses 3 mechanical switches as inputs to light up 8 LEDs for the binary combinations? This is for my electric circuits class.

    I've done one for Logic class, but I was able to use Logic gates and some other extra equipment. So any hints on how I should go about creating one without these logic gates?
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  3. Apr 11, 2004 #2


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    Do you mean you want to build your own instead of just using something like a 74138 chip?

  4. Apr 11, 2004 #3

    yeah, I'm not allowed to use a logic chip, so will this be possible?
  5. Apr 11, 2004 #4


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    Transistors, lots of them, is one way to create AND, OR, and NOT circuits without using any chips. Relays would work too, but that's an expensive option due to the sheer number needed.

    For this, you could start with the truth table for the in/out and then end up with a large breadboard. :)

    I'd think it'd be easiest to work from most significant to the least significant to minimize the logic needed quickly, but that's only an opinion.

  6. Apr 11, 2004 #5
    this sounds complicated, I'm not even sure how how to create logic from transistors and resistors. I know how the truth table will turn out.
  7. Apr 11, 2004 #6


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  8. Apr 11, 2004 #7
    Thanks, I'll see if I can make this work
  9. Apr 12, 2004 #8


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    What are you supposed to use if you can't use logic gates of any sort?

    - Warren
  10. Apr 12, 2004 #9

    This is for a class that takes place before Logic Design, so most of the students don't know about the logic gates. I'm taking both classes at the same time, but I'm not allowed to use these gates since it gives me an advantage over others.

    The project description is as follows:

    Design a circuit that uses 3 mechanical switches for its inputs. The "up" position of each switch should represent a binary 1, and the "down" as binary 0. The circuit should light one of 8 LED's for each binary combination of switch positions.
  11. Apr 12, 2004 #10


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    A simple switch is a logic gate, when 2 switches are placed in parallel they form an OR gate, when in series they from a AND gate.

    It looks to me that the goal of this project is to find an arrangement of the 3 switches which will control each of the lights. There are 8 possible positions therefore there should be a way to wire the circuit to control 8 different lights.

    Sounds like an exercise in ladder logic. If you ever encounter a PLC this is very nice stuff to know.
  12. Apr 14, 2004 #11
    could you post the answer when you get it? thanks,
  13. Apr 19, 2004 #12


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    res-diode combinations for AND and OR. Common emitter config bjt for NOT. In principle, you can build any combinatorial logic out of res, diode and bjt.
  14. Apr 27, 2004 #13
    I dont know if this is too late, but u are allowed to use the different kind of gates to create this circuit. In your lab kit you should have all the needed gates like for Exaple the chip (74LS00) is 4 NAND gates in one. Try to figure out which one is which I do not have time right now to go over them all, but all the different gates are avalable in your labkit.
  15. May 21, 2004 #14
    If you are allowed to use (1) a single pole-two position (2) a double pole-two position, and (3) a four pole-two position switch, it is very easy. Simply tie the single pole to power; then tie its two outputs to the two poles of the double pole switch. Then tie the four outputs of the double pole switch to the four inputs of the four pole switch. The eight four pole switch outputs then go to the eight LEDs. They are grounded together.
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