Boolean Logic Diagrams (AQA GCSE Computer Science)

Revision Note

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What is Boolean logic?

  • Boolean logic is used in computer science and electronics to make logical decisions

  • Boolean operators are either TRUE or FALSE, often represented as or 0

  • Inputs and outputs are given letters to represent them

  • To define Boolean logic we use special symbols to make writing expressions much easier 

Boolean Logic Diagrams

What are logic gates?

  • Logic gates are a visual way of representing a Boolean expression

  • The logic gates covered in this course are:

    • AND

    • OR

    • NOT

    • XOR

AND

Expression operator

Circuit symbol

Explanation

A space. space B
(A AND B)

and-gate

Returns TRUE only if both inputs are TRUE 

TRUE AND TRUE = TRUE

Otherwise = FALSE

OR

Expression operator

Circuit symbol

Explanation

A space plus space B

(A OR B)

or-gate

Returns TRUE if either input is TRUE 

TRUE OR FALSE = TRUE 

FALSE OR FALSE = FALSE

NOT

Expression operator

Circuit symbol

Explanation

top enclose A
(NOT A)

not-gate

Reverses the input value

NOT TRUE = FALSE

NOT FALSE = TRUE

XOR (exclusive OR)

Expression operator

Circuit symbol

Explanation

A circled plus B
(A XOR B)

xor

Returns TRUE if either input is TRUE but NOT both

TRUE OR FALSE = TRUE 

FALSE OR FALSE = FALSE

TRUE OR TRUE = FALSE

Examiner Tip

You are expected to understand and use the Boolean expression operators as well as the symbols and words.

For example the expression (A AND B) OR (NOT C) could be represented as:

open parentheses A. B close parentheses plus top enclose C

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