Topic 10 · Boolean Logic · 10.1

The Role of Logic Gates

Logic gates are tiny electronic decision-makers. They receive binary inputs, apply a rule and produce one binary output.

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Invitation

How does a computer make a decision?

A computer does not understand uncertainty in the way a person does. It works with clear binary states.

A decision is reduced to two possible answers: TRUE or FALSE, ON or OFF, and 1 or 0.

Logic gates help a computer decide what output should be produced from its inputs.
Two possible states
0FALSE · OFF · NO
1TRUE · ON · YES
Big Idea

A logic gate is a rule inside an electronic circuit

Each gate receives one or more binary inputs. The type of gate determines the rule that is applied.

INPUTOne or more binary values
→
LOGIC GATEApplies a Boolean rule
→
OUTPUTOne binary value
Input → Rule → Output.
Bridge

Think of switches controlling a light

A switch can be either off or on. A light can also be off or on.

The wiring between the switches and the light acts like a rule. Different wiring rules produce different results.

Logic gates perform the same job electronically. They decide whether the output should switch on.

The switches are inputs. The wiring rule is the gate. The light is the output.
Switches → Rule → Light
Input A = 1
💡
Input B = 0
○

The exact output depends on which logic-gate rule is used.

Why They Matter

Millions of simple decisions build complex systems

A single gate makes one very small decision. Digital devices combine huge numbers of gates to perform useful tasks.

🔢CalculationsProcessors combine gates to perform binary arithmetic.
🔐ControlSystems use gates to decide whether an action is allowed.
🚨MonitoringAlarms use inputs from sensors to decide when to activate.
Complex computer behaviour is built from many simple TRUE or FALSE decisions.
Worked Example

A simple security alarm decision

Imagine an alarm system with two inputs:

ADoor sensor
BSystem armed
XAlarm output

Possible situation

The door opens, so input A becomes 1. The system is armed, so input B is also 1. A logic gate uses these inputs to decide whether output X should become 1 and activate the alarm.

Inputs are facts. The gate applies the decision rule. The output controls the action.
Exam Explainer

Know the three parts of every logic circuit

InputsUsually labelled A, B or C.
GateApplies a Boolean operation.
OutputUsually labelled X or Z.
Exam answer: “A logic gate takes one or more binary inputs, applies a Boolean operation and produces a single binary output.”
Common Mistake

A logic gate does not store the input

✗ Incorrect idea

A logic gate stores binary data for later.

✓ Correct idea

A logic gate processes binary inputs and immediately produces an output.

Remember: Storage is performed by memory components. A logic gate performs a Boolean operation.
Summary

One small rule, one clear output

BinaryUses the states 0 and 1.
Logic GateApplies a decision rule.
OutputProduces 0 or 1.
Final rule: Logic gates are the electronic building blocks that allow digital systems to make decisions.