GCSE · Computer Science · AQA · Spec 8525

NOT logic gate

Say "It is NOT the weekend" on a Saturday and it is False. A NOT gate does that to a 1 or 0: whatever goes in, the opposite comes out.

Computer Science · Logic gates

Meet the gate that always disagrees

Say "It is NOT the weekend" on a Saturday and the statement flips to False. A NOT gate does the same to a 1 or a 0. Step through both possible inputs and watch what it does.

Spot the pattern first. Step 1 shows an input of 0. Before you press Next, predict the output for an input of 1.

NOTgate

Input	Output
0	1
1	0

Input=0 → Output = 1

Output

Step 1: The input is 0, which means False. The gate reverses it, so the output is 1 (True).

1 / 2

A truth table lists every possible input and the output it gives. One input that can be 0 or 1 means two rows, and the highlight shows which row you are on.

Step 1 of 2: The input is 0, which means False. The gate reverses it, so the output is 1 (True)..

One input or two?

NOT gatevsAND and OR gates

AND, OR and NOT are the three fundamental logic gates. NOT is the odd one out.

Focus

Number of inputs

NOT gate

One

AND and OR gates

Two

The insight

This is the big difference. Not every logic gate has two inputs, and NOT is the one with a single input.

What it does to what it receives

NOT gate

Reverses its input

AND and OR gates

Takes in two inputs (each gate has its own lesson)

Where the output comes from

NOT gate

The one input

AND and OR gates

The two inputs

What they share

NOT gate

Input and output are 1 (True) or 0 (False)

AND and OR gates

Inputs and output are 1 (True) or 0 (False)

Work it backwards

This time you can see the output, but not what went in.

A NOT gate's output is 1. What was its input?

Computer Science · Logic gates

Which idea is closest to yours?

Four students each describe a NOT gate and the 1s and 0s that flow through it.

Which description is closest to what you think right now?

NOT means "switch off", so a NOT gate always outputs 0.A 1 is a low voltage and a 0 is a high voltage.A NOT gate reverses the one input it receives: 0 becomes 1 and 1 becomes 0.A NOT gate has two inputs, just like AND and OR.

How sure are you?

I'm sureI think soI'm guessing

NOT means "switch off", so a NOT gate always outputs 0.

This is a common misconception

Easy to believe, because "not" sounds like "off". But check the table: an input of 0 gives an output of 1. A NOT gate reverses its input, so the output is 0 only when the input is 1.

A 1 is a low voltage and a 0 is a high voltage.

This is a common misconception

It is the other way round. In the circuit, 1 (True) is a high voltage and 0 (False) is a low voltage. A 0 is still a real signal: it is just the low one.

A NOT gate reverses the one input it receives: 0 becomes 1 and 1 becomes 0.

This matches the evidence

Yes. One input in, one output out, and the output is always the opposite of the input.

A NOT gate has two inputs, just like AND and OR.

This is a common misconception

NOT is the odd one out. It has only one input (and one output), which is why its truth table needs just two rows.

NOT reverses whatever comes in through its single input: 1 (True, high voltage) becomes 0 (False, low voltage), and 0 becomes 1.

Computer Science · Boolean statements

NOT in everyday sentences

Decide whether each statement evaluates to True or False. Trick: first ask whether the plain condition, without the NOT, is True. Then reverse it.

Still to sort

Evaluates to True (0)

The condition inside the NOT is False, so NOT reverses it to True.

Evaluates to False (0)

The condition inside the NOT is True, so NOT reverses it to False.

5 of 5 still to sort.

A Boolean statement is either True or False, and NOT reverses a condition. Right now: the lights are on, the door is shut, and it is raining.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

The gate that always disagrees: one input in, the opposite comes out.

What you need to know

A logic gate carries out a logical operation. AND, OR and NOT are the three fundamental gates.
A NOT gate has one input and one output. AND and OR have two inputs, so not every gate has two.
A NOT gate reverses its input: an input of 0 gives an output of 1, and an input of 1 gives an output of 0.
A truth table shows the output for every possible input. NOT has one input that can be 0 or 1, so its table has two rows.
Inputs and outputs are True or False, written 1 or 0. In a circuit, 1 is a high voltage and 0 is a low voltage.

The big picture

A NOT gate has one input and one output, and it reverses whatever it receives: 0 becomes 1 and 1 becomes 0. Because it has just one input, its truth table is only two rows. That lets you work out a missing input or output, and use NOT to reverse a condition in a Boolean statement.

Key points

1A NOT gate has a single input and a single output.

2Its output is always the opposite of its input.

3Its truth table has one input column and two rows.

4Given either the input or the output of a NOT gate, the other is the opposite value.

5NOT reverses a condition in a Boolean statement, turning True into False and False into True.

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Worked example

Problem

A NOT gate's output is 0. Work out what its input was, then check your answer.

⚠ Watch out

Giving a NOT gate a second input because AND and OR have two. NOT has one input, so its truth table has a single input column and just two rows.

🧠

Memory hook

NOT is the gate that always disagrees. Say "it is NOT the weekend" and the truth flips. Feed a NOT gate a 1 and the output flips to 0.

✓

Check yourself

Without looking back: write out the truth table for a NOT gate, then say whether "It is NOT the weekend" is True or False on a Wednesday.

Flashcards

(13)

What is a logic gate?

An electronic component that carries out a logical operation.

Name the three fundamental logic gates.

AND, OR and NOT. Each has its own symbol in circuit diagrams.

What values can a logic gate's inputs and outputs take?

True or False, written as 1 (True) or 0 (False).

In a circuit, what do 1 and 0 stand for?

1 is a high voltage and 0 is a low voltage.

How many inputs does a NOT gate have?

One input (and one output). Not all logic gates have two inputs.

What does a NOT gate do?

It reverses its input. True becomes False and False becomes True.

What is a truth table?

A table that shows the output for every possible combination of inputs.

Why does a NOT gate's truth table have only two rows?

It has one input, and that input can only be 0 or 1.

What input gives a NOT gate an output of 0?

An input of 1.

A NOT gate's output is 1. What was the input?

0. The input is always the opposite of the output.

When is the statement "It is NOT the weekend" True?

Exactly when "it is the weekend" is False.

True or false: a NOT gate always outputs 0.

False. It reverses its input, so it outputs 1 when the input is 0.

What is the key difference between NOT and the AND and OR gates?

NOT has one input. AND and OR have two.

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Meet the gate that always disagrees

Which idea is closest to yours?

NOT in everyday sentences

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