Logic gates and truth tables - Computer Science IGCSE Study Notes

Imagine you have a light switch. When you flip it ON, the light turns on. When you flip it OFF, the light turns off. That's a very basic kind of 'logic'. Logic gates are like these simple switches, but instead of just one switch, they can have one or more inputs, and they follow specific rules to decide what their single output will be.

Think of a logic gate as a tiny, super-fast 'decision-making' machine inside a computer. It takes in electrical signals (which are either 'on' or 'off', like 'yes' or 'no', or 'true' or 'false', or '1' or '0') and then produces another electrical signal as its answer. These 'on' or 'off' signals are called binary (meaning 'two states').

Each type of logic gate has its own special rule for making decisions. For example, some gates only give a 'yes' output if all their inputs are 'yes'. Others give a 'yes' output if any of their inputs are 'yes'. We use truth tables to show all the possible inputs and the resulting output for each type of gate. It's like a cheat sheet for what the gate will do!

Let's think about a simple alarm system for a treasure chest. We want the alarm to go off if both the lid is opened AND the key is not in the lock.

• Input A: Is the lid open? (Yes = 1, No = 0)
• Input B: Is the key NOT in the lock? (Yes = 1, No = 0)
• Output: Does the alarm sound? (Yes = 1, No = 0)

We need a logic gate that only gives a 'yes' (alarm sounds) when both Input A is 'yes' and Input B is 'yes'. This is exactly what an AND gate does! If the lid is open (1) AND the key is not in the lock (1), then the alarm sounds (1). If either the lid is closed (0) or the key is in the lock (0), then the alarm won't sound (0).

This simple 'AND' decision is happening billions of times a second inside your computer to make all sorts of complex things work, from checking your password to controlling traffic lights.