Topic 10 · Boolean Logic · 10.4

The OR Logic Gate

The OR gate produces an output of 1 when either input, or both inputs, are 1.

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Invitation

What if either choice is enough?

Imagine your teacher says:

“You can join the sports club if you play football OR basketball.”

You do not need to play both sports. Either one is enough.

That is the behaviour of an OR gate: one or both conditions can be true.
Sports club rule
Football ✓
Basketball OR
↓
Join the club ⚽
Big Idea

The OR gate checks two inputs

An OR gate has two inputs and one output.

The output is 1 when input A, input B, or both inputs are 1.

A = 1Football
OR
B = 0Basketball
→
X = 1Join club
At least one input is 1, so the output is 1.
Visual Guide

The OR gate symbol

Two input lines enter the left side of the gate. One output line leaves the right side.

The inputs are labelled A and B. The output is labelled X.

Read the gate from left to right: inputs → rule → output.
OR gate
ABX
Bridge

Only one combination keeps the output off

Football ✗ + Basketball ✗
Cannot join
Football ✓ + Basketball ✗
Join the club
Football ✗ + Basketball ✓
Join the club
Football ✓ + Basketball ✓
Join the club
The OR gate outputs 0 only when both inputs are 0.
Truth Table

Every possible input combination

A truth table shows every possible pair of inputs and the output produced.

ABX
000
011
101
111
Find the row where both inputs are 0. That is the only row where X is 0.
Logic Expression

Writing the OR rule

The inputs are labelled A and B. The output is labelled X.

X = A OR B

This means X is 1 when A, B, or both inputs are 1.

Read it aloud as: “X equals A OR B.”
Worked Example

Find the output

Question

An OR gate receives A = 0 and B = 1. What is the output X?

“X = 1 because at least one input is 1.”
Exam Explainer

Three facts worth remembering

Two InputsAn OR gate compares A and B.
Either Is EnoughOne or both inputs can be 1.
One OutputThe gate produces X.
Exam answer: “An OR gate outputs 1 when either input, or both inputs, are 1.”
Common Mistake

OR does not mean only one

✗ Incorrect

A = 1 and B = 1 gives X = 0.

✓ Correct

A = 1 and B = 1 gives X = 1.

Remember: OR includes the case where both inputs are 1.
Summary

OR means either—or both

InputsTwo binary inputs.
RuleAt least one must be 1.
OutputOne binary result.
Final rule: X = 0 only when A = 0 and B = 0.