AQA 8525 · Section 3.4 · 3.4.2

Boolean
Logic

Logic Gates · AND · OR · NOT · Truth Tables · Logic Expressions

CSZoneAQA GCSE Computer Science 8525

Logic Gates

AND · OR · NOT

AND Gate

A	B	Q
0	0	0
0	1	0
1	0	0
1	1	1

Q = A AND B

OR Gate

A	B	Q
0	0	0
0	1	1
1	0	1
1	1	1

Q = A OR B

NOT Gate

A	Q
0	1
1	0

Q = NOT A

Logic Expressions

Writing and Reading Boolean Expressions

Q = A AND B ← AND gate
Q = A OR B ← OR gate
Q = NOT A ← NOT gate

Combined expression:
Q = (A AND B) OR (NOT C)

If A=1, B=0, C=1:
A AND B = 0
NOT C = 0
Q = 0 OR 0 = 0

Tip:Evaluate innermost brackets first, then work outwards — just like maths.

Building Truth Tables

Systematically Listing All Combinations

Q = (A OR B) AND (NOT A)

A	B	A OR B	NOT A	Q
0	0	0	1	0
0	1	1	1	1
1	0	1	0	0
1	1	1	0	0

Rule:2 inputs = 4 rows. 3 inputs = 8 rows. Add intermediate columns for complex expressions.

Exam Practice

Have a go at this question

AQA-style question

Complete a truth table for the expression: Q = A AND (NOT B)
Include all four combinations of A and B.

4 marks

A	B	NOT B	Q = A AND (NOT B)
0	0	1	0
0	1	0	0
1	0	1	1
1	1	0	0

Key Takeaways

What to Remember

AND: output 1 only if BOTH inputs are 1

OR: output 1 if AT LEAST ONE input is 1

NOT: flips the input (0→1, 1→0)

Truth tables: 2 inputs = 4 rows, 3 inputs = 8 rows