Build a Half Adder

CSC-116 · Semester I · Digital Logic

beginner20 minAvailable

Live simulation6 components · 6 wires

OFF

XOR

AND

Sum0

Carry0

Click a switch to toggle it · drag output → input to wire · drag to move · Delete removes the selection · Ctrl+Z undoes.

Truth table

live from the current circuit

A	B	Sum	Carry
0	0	0	0
0	1	1	0
1	0	1	0
1	1	0	1

Lab brief

Objective

Construct a half adder from logic gates and confirm that it correctly adds two binary digits, producing a sum and a carry.

Theory & concepts

A half adder adds two single bits A and B. Its two outputs are Sum and Carry. The sum bit is 1 when exactly one input is 1, which is exactly what an XOR gate computes. The carry is 1 only when both inputs are 1 - an AND gate.

Rendering diagram…

Half adder built from XOR and AND

Sum   = A XOR B
Carry = A AND B

A B | Sum Carry
0 0 |  0   0
0 1 |  1   0
1 0 |  1   0
1 1 |  0   1

Prerequisites

Lesson: Logic gates and truth tables
Comfort with truth tables

Instructions

1Drag two switches (inputs A and B) onto the canvas.
2Add an XOR gate and an AND gate.
3Wire the inputs to both gates.
4Add LEDs labelled Sum and Carry, wired to the gate outputs.
5Toggle all four input combinations and record the outputs.

Tasks

Wire the circuit
Connect switches → gates → LEDs so the layout is readable.
Verify the truth table
For each input pair, confirm the LED pattern matches Sum/Carry.
Explain it
In your own words, explain why XOR produces the sum bit.

Self-check

The circuit produces correct outputs for all four input combinations.
The XOR/AND gate choice is justified.
The truth table observations match the simulated behaviour.

Environment

drag deviceswire connectionslive simulationtruth table check