← IGCSE Computer Science Paper 2

The NOT gate

A logic gate with one input that outputs the opposite value: 0 becomes 1 and 1 becomes 0. It is drawn as a triangle with a small circle and written X = NOT A.

Part of: IGCSE Computer Science 0478 — 10 Boolean Logic — Module 6

Understanding The NOT gate in IGCSE Computer Science Paper 2

What it does: Because there is one input, there are only two rows. In each row X is the opposite of A. The triangle is the gate and the small circle at the output shows the value is inverted. One wire goes in (A) and one wire comes out (X). A NOT gate has exactly one input and one output. AND and OR gates normally have two inputs. NOT outputs the opposite value, so 1 becomes 0. NOT reverses the input, so 0 becomes 1. In the expression, NOT is written before the input: X = NOT A. A NOT gate inverts its input, so it is also called an inverter.

Common mistakes: A NOT gate outputs the opposite of its input. When A is 1, X must be 0, so the row should be 1 | 0. In a logic expression NOT is written before the input it inverts. The expression should be X = NOT A.

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This module's key terms are also in a printable crossword (Modules 539–551) and word search (Modules 539–580).

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Learning
Duo
A NOT gate has how many inputs?
One inputTwo inputs
Matches
A | X 0 | 0 1 | 1
↓
The output X is copied from the input A. A NOT gate should output the opposite, so 0 gives 1 and 1 gives 0.
Applying
Meanings
Term
NOT
Meaning
A logic gate with one input that outputs the opposite value: 0 becomes 1 and 1 becomes 0. It is drawn as a triangle with a small circle and written X = NOT A.
Practice
X = NOT A
Testing
Gaps
X = A
Check Answer
Spots
30 | 1
41 | 1

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Quick Questions

1. A NOT gate has how many inputs?
A. Two inputs
B. One input ✓
A NOT gate has exactly one input and one output. AND and OR gates normally have two inputs.
2. A NOT gate receives the input 1. What is the output?
A. 0 ✓
B. 1
NOT outputs the opposite value, so 1 becomes 0.
3. A NOT gate receives the input 0. What is the output?
A. 1 ✓
B. 0
NOT reverses the input, so 0 becomes 1.
4. Which is the correct logic expression for a NOT gate with input A and output X?
A. X = A NOT
B. X = NOT A ✓
In the expression, NOT is written before the input: X = NOT A.
5. Which symbol is used for a NOT gate?
A. A triangle with a small circle ✓
B. A D-shape with a small circle
The NOT gate is a triangle with a small circle at the output. A D-shape with a small circle is the NAND gate symbol.

Match the Pairs

X = NOT A → The output X is the opposite of input A: when A is 0, X is 1, and when A is 1, X is 0.
A = 1, X = NOT A, so X = 0 → A NOT gate turns an input of 1 into an output of 0.
A NOT gate has only one input, so its two-row truth table lists just A = 0 and A = 1 with the output X in the next column. → A | X 0 | 1 1 | 0
The NOT gate is drawn as a triangle with a small circle on its output. It is also called an inverter because it inverts the input. → A --|>o-- X
A | X 0 | 0 1 | 1 → The output X is copied from the input A. A NOT gate should output the opposite, so 0 gives 1 and 1 gives 0.
X = NOT (A, B) → NOT has only one input, so it cannot be applied to two inputs A and B at once. It should be X = NOT A.

Key Term

NOT
A logic gate with one input that outputs the opposite value: 0 becomes 1 and 1 becomes 0. It is drawn as a triangle with a small circle and written X = NOT A.

Fill in the Blanks

1. X = NOT A
Hint: Type the gate keyword that flips a single input.
2. X = NOT A
Hint: Type the label of the input that the NOT gate inverts.
3. A X 0 1 1 0
Hint: Type the output of a NOT gate when the input A is 0.
4. A X 0 1 1 0
Hint: Type the output of a NOT gate when the input A is 1.
5. NOT gate: 1 input, 1 output
Hint: Type how many inputs a NOT gate has.

Spot the Error

Question 1
A Truth table for NOT gate
B A | X
C 0 | 1
D 1 | 1
Line D — A NOT gate outputs the opposite of its input. When A is 1, X must be 0, so the row should be 1 | 0.
Question 2
A Gate: NOT
B Inputs: 1
C Expression: X = A NOT
D Output: opposite of A
Line C — In a logic expression NOT is written before the input it inverts. The expression should be X = NOT A.
Question 3
A A = 1
B X = NOT A, B
C PRINT X
Line B — A NOT gate has only one input, so it cannot take A and B. The line should be X = NOT A.
Question 4
A A X
B 0 0
C 1 0
Line B — NOT turns 0 into 1. When A is 0 the output X must be 1, so the row should be 0 1.
Question 5
A NOT gate symbol:
B Triangle shape
C Small circle at the output
D Two inputs, one output
Line D — A NOT gate (inverter) has just one input and one output. The line should say One input, one output.

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