June 18, 2026 · 5 min read
How to Build a Half Adder and Full Adder Circuit
We’re recording the walkthrough for this tutorial — the written steps below already cover everything you need.
A half adder is one of the first combinational circuits covered in any digital logic course, and a full adder — built from two half adders plus an OR gate — usually follows right after. LogicBench's sub-circuit feature makes this exact progression easy: build the half adder once, save it, then reuse it to assemble the full adder without rewiring from scratch.
Wire the half adder from XOR and AND gates
Place a 2-input XOR gate for the Sum output and a 2-input AND gate for the Carry output. Feed both inputs A and B into both gates — that's the complete half adder logic.
Test the truth table
Toggle A and B through all four input combinations and confirm Sum and Carry match the half adder truth table: only A=1,B=1 should produce a Carry of 1.
Save it as a sub-circuit
Once it behaves correctly, save the half adder as a sub-circuit from the menu. It now appears as a single reusable block, with its two inputs and two outputs exposed.
Combine two half adders into a full adder
Drop two half-adder sub-circuits onto a fresh breadboard. Feed A and B into the first, feed its Sum output and your Carry-in into the second, then OR the two Carry outputs together for the final Carry-out.
Verify the full adder truth table
Run through all eight input combinations of A, B, and Carry-in. The Sum and Carry-out should match the standard full adder truth table exactly.