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Three transistor AND Gate (FAIL)

February 12, 2012

Today i tried and i am still trying to figure out how to make AND gate with 3 transistors (NPN). I tried switching T3 transistor to simple mosfet since it needs higher current on base to go to high state but i ended up with same result. LED 1 and LED 2 will work according to wich button is pressed but LED 3 is dead non stop. Any ideas what should be the reason for my failure. I belive it’s some simple mistake that i cannot see it clearly atm so please if you have any advice or explanation how and why please feel free to post comment.


Chart bellow is my idea how this circuit should operate

Button 1  | Button 2  | LED 3
ON          |      OFF      |   OFF
OFF         |       ON       |   OFF
OFF         |      OFF      |   OFF
ON          |       ON       |    ON

  1. Keep it simple permalink

    Simple: place the wires from T3 direct behind the buttons 🙂

  2. Thank you for very helpfull link, i really apprieciate the kindness. I will definetly look into it since on first look it has very nicely defined and explained how things works.

  3. slateraptor permalink

    Hope this helps:

    You’ll need to implement pull-up or pull-down resistors at the base of each input to prevent them from floating if you want to use SPST buttons in your test circuit.

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