Boolean Algebra · Truth Tables · Logic in Hardware
CSZoneCambridge IGCSE Computer Science 0478
What are Logic Gates?
Building Blocks of Digital Circuits
Logic gates are electronic components that take binary inputs (0 or 1) and produce a binary output based on Boolean logic rules. All computer processing is built from combinations of logic gates.
0 = False / off / low voltage; 1 = True / on / high voltage
Cambridge 0478: you must be able to draw and interpret truth tables for all gates
AND Gate & OR Gate
Truth Tables
AND Gate (A AND B)
A
B
Output
0
0
0
0
1
0
1
0
0
1
1
1
Output = 1 only when both inputs are 1
OR Gate (A OR B)
A
B
Output
0
0
0
0
1
1
1
0
1
1
1
1
Output = 1 when at least one input is 1
NOT Gate
The Inverter
NOT gate: one input, one output. Inverts the input — 0 becomes 1, 1 becomes 0. Also called an inverter.
A
NOT A
0
1
1
0
Boolean notation: NOT A = Ā (with overline) or ¬A or A'
AND: A · B or A AND B; OR: A + B or A OR B
Exam Practice
Have a go at this question
Cambridge IGCSE 0478 style
Complete the truth table for the expression: X = A AND (NOT B)
3 marks
A=0,B=0 → NOT B=1 → X=0 [1 row]. A=0,B=1 → NOT B=0 → X=0. A=1,B=0 → NOT B=1 → X=1. A=1,B=1 → NOT B=0 → X=0 [all 4 rows correct = 3 marks].
Key Takeaways
What to Remember
AND: output 1 only when ALL inputs are 1
OR: output 1 when AT LEAST ONE input is 1
NOT: inverts the input — 0→1, 1→0
Build truth tables systematically: list all input combinations, then calculate output