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Transistors in logic gates. Basic question.

Discussion in 'General Electronics Discussion' started by JooJar, Sep 27, 2011.

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  1. JooJar


    Sep 27, 2011
    Sorry for the basic question.

    Basically we're learning about logic gates at university and I have a lot of electronics equipment, prototype boards, and components around the house that I collected some years back when I became interested (but not actively interested) in electronics.

    Now, my idea was to build these logic gates so I can get hands on understanding.

    I have some NPN transistors, but the problem I am having is understanding the collector, base and emitter.

    I tried connecting the positive end of the battery to the collector, and an L.E.D on the emitter. One wire on the L.E.D to emitter, other wire to the negative on the battery. Does this sound right?

    What do I put on the base? I just want to use the transistor as a switch. 0 or 1. I assume with an NPN a charge to the base is going to switch the transistor and activate the LED. This would be an AND logic gate?

    Thanks for any help.
  2. jackorocko


    Apr 4, 2010
    I don't think so, AND gate compares two inputs, when both are the same the output is the same as the input. Turning a transistor on or off doesn't make a gate, Think it is a little more complicated then that.
  3. davenn

    davenn Moderator

    Sep 5, 2009
    ok you are getting there :)

    preferably put the LED and an appropriate resistor in series with it and in then in the collector cct to the battery. take the emitter to 0V (negative of the battery)

    With a hi value resistor say 10k Ohms ( 10,000 Ohms) take it from the battery positive and touch the base leg of the transistor and the LED should light :)

    see diag below



    Attached Files:

    Arpan Sarkar likes this.
  4. QwertyTSecond


    Jul 31, 2011
    This would be an AND gate:

    When both inputs are off, both transistors are off. If a voltage is applied to only one of the transistors, only one transistor turns on. The other transistor blocks the current flow.
    When voltage is applied to both transistors they both allow current flow, setting the output high.
  5. davenn

    davenn Moderator

    Sep 5, 2009
    HI Qwerty

    indeed it is :)

    that diag looks very familiar and one I was thinking of using if the OP pushed for more info on transistor gates
  6. JooJar


    Sep 27, 2011
    Looking at my lecture notes I got the gates confused. never the less the hands on experience is fun. I'll stick around here.

    Thanks for the responses.
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