GCSE Computer Science: Boolean Logic and Logic Gates Guide

What is Boolean logic?

Boolean logic deals with values that are only ever true or false (1 or 0), forming the basis of how computer circuits make decisions.

The three basic logic gates

An AND gate outputs true only if both inputs are true. An OR gate outputs true if at least one input is true. A NOT gate simply reverses its single input, turning true into false and false into true.

Truth tables

A truth table lists every possible combination of inputs and shows the resulting output for a logic gate or circuit, letting you check a circuit's behaviour systematically rather than guessing.

Combining logic gates

Logic gates can be combined into more complex circuits, such as XOR (true only if inputs are different) built from combinations of AND, OR and NOT gates, to carry out more sophisticated logical operations.

Common mistakes

• Mixing up AND (needs both true) and OR (needs at least one true).
• Forgetting a truth table needs every possible input combination, not just a few examples.
• Losing track of which gate's output feeds into which gate next in a combined circuit.