I'm designing an arithmetic circuit that takes in two one-bit binary numbers as input and outputs the result with LEDs representing 1's and 0's for each digit. OUT1 represents the state of the first digit, whereas OUT2 represents the state of the second digit.

The circuit for the first digit is working properly. If I switch IN2 (the first number) to 1, OUT1 lights up, as it should. I have not yet completed the second piece of that circuit that will set light up OUT1 should IN1 be 1 and IN2 be 0, that will come after this problem is solved.

The problem lies in OUT2. When I simulate in Multisim, if both IN1 and IN2 are switched on, OUT1 should turn off (which it does) and OUT2 should turn on (which it does not). I don't understand why it wouldn't be turning on, as I have what I believe to be a direct AND gate leading to OUT2.

A picture of my circuit is shown below, any help with this would be greatly appreciated.

enter image description here

  • 3
    \$\begingroup\$ Credit where it's due, +1 for a schematic, and for reference designators. However, your schematic would be easier to understand if the +ve rail ran across the top, ground at the bottom, with current flowing generally downwards. It may seem only style, but it's the style engineers use, as it reduces the amount of work needed to parse a circuit. \$\endgroup\$
    – Neil_UK
    Commented Mar 11, 2018 at 11:03

1 Answer 1


I applaud your willingness to experiment however you need to learn a little more about how transistors work,

  1. Give each LED its own resistor to limit the current through them remove R1.

  2. Use your NPN transistors on the ground side of the circuit to make their behaviour easier to predict.

  3. Use resistors in series with each transistor base something around 1k should be suitable.

Hopefully from this you can rethink your circuit design. You may want to check the truth table for out1 as well if this is intended to be a one bit adder.


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