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Boolean logic — Lesson 1: Logic gates

Logic gates are the building blocks of every digital circuit. Each gate takes one or more binary inputs (0 or 1) and produces a single binary output by following a fixed rule. This lesson covers the gates you need for IGCSE and the truth table for each.

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What a logic gate is

inputs (0 or 1) A B LOGIC GATE output a single 0 or 1
  • A logic gate takes one or more binary inputs (0 or 1) and gives a single binary output.
  • A 1 usually means TRUE / on / high, and a 0 means FALSE / off / low.
  • Each gate follows a fixed rule shown by its truth table, which lists the output for every possible combination of inputs.
  • Gates are wired together to build the logic circuits that let a computer make decisions.

Check you've got it

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1. A 2-input AND gate has inputs A = 1 and B = 0. What is its output?

2. Which gate outputs 1 only when all of its inputs are 0?

3. A NOT gate is given an input of 0. What is its output?

4. For a 2-input XOR gate, which pair of inputs gives an output of 1?

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