Building upon the last two posts where I showed how to build NAND, NOR, AND, OR, NOT and XOR gates using NPN transistors, this post will show how these gates can be used to build a full adder. A full adder is one of the circuits used in an Arithmetic and Logic Unit (ALU) which a central component in a microprocessor. The screenshot below shows the full adder circuit using combinational logic running in Circuit Simulator.

Full Adder Circuit Using Combinational Logic

The next screen shot shows the same full adder circuit built using NPN transistors.

Full Adder Circuit Using NPN Transistors

As always, the code for the circuit can be found here. Just copy and import the code into the free Java Circuit Simulator to get a feeling for the circuit.