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Logic Gates Explained – AND, OR & NOT (O Level Computer Science 2210 / IGCSE 0478)

What Are Logic Gates?

Logic gates are electronic circuits that perform logical operations on binary inputs (0 and 1).

Computers operate using binary signals, where:

  • 0 represents OFF / False

  • 1 represents ON / True

Logic gates process these signals and produce a binary output.

They are the basic building blocks of digital circuits used in computers and electronic devices.

The AND Gate

The AND gate produces an output of 1 only when both inputs are 1.

If either input is 0, the output will be 0.

Truth Table

Input AInput BOutput000010100111

Example:

A security system may activate only when two conditions are true, such as entering the correct password AND fingerprint.

The OR Gate

The OR gate produces an output of 1 if at least one input is 1.

The output becomes 0 only when both inputs are 0.

Truth Table

Input AInput BOutput000011101111

Example:

A device may turn on if button A OR button B is pressed.

The NOT Gate

The NOT gate is different from other gates because it has only one input.

It produces the opposite value of the input.

Truth Table

InputOutput0110

Example:

If a sensor detects no light (0), the system may activate a light (1).

Summary of Basic Logic Gates

GateFunctionANDOutput is 1 only if both inputs are 1OROutput is 1 if at least one input is 1NOTOutput is the opposite of the input

These gates form the foundation of digital circuits used in computers.

Why Logic Gates Are Important

Logic gates are used in many computing systems such as:

  • Processors

  • Memory circuits

  • Digital devices

  • Control systems

They allow computers to perform logical decisions using binary data.

Exam Tip (2210 / 0478)

Students are often asked to:

  • Draw logic gate symbols

  • Complete truth tables

  • Explain how logic gates work

A common exam question is:

Complete the truth table for an AND gate.

Practice Question

Explain one difference between an AND gate and an OR gate.

Answer

An AND gate outputs 1 only when both inputs are 1, while an OR gate outputs 1 if at least one input is 1.

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