Boolean Logic Diagrams (OCR GCSE Computer Science): Revision Note

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 1 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 

AND Gates, OR Gates & NOT Gates

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

AND (Conjunction)

Symbol	Circuit symbol	Explanation
A ∧ B		Returns TRUE only if both inputs are TRUE  TRUE AND TRUE = TRUE Otherwise = FALSE

OR (Disjunction)

Symbol	Circuit symbol	Explanation
A ∨ B		Returns TRUE if either input is TRUE  TRUE OR FALSE = TRUE  FALSE OR FALSE = FALSE

NOT (Negation)

Symbol	Circuit symbol	Explanation
¬ A		Reverses the input value NOT TRUE = FALSE NOT FALSE = TRUE

What is a truth table?

• A truth table is a tool used in logic and computer science to visualise the results of Boolean expressions

• They represent all possible inputs and the associated outputs for a given Boolean expression

AND (Conjunction) 

Circuit symbol	Truth Table
	A B A ^ B 0 0 0 0 1 0 1 0 0 1 1 1

OR (Disjunction) 

Circuit symbol	Truth Table
	A B A V B 0 0 0 0 1 1 1 0 1 1 1 1

NOT (Negation) 

Circuit symbol	Truth Table
	A ¬A 0 1 1 0