The NOT Gate: Truth Table and Symbol

The NOT gate is the simplest logic gate. It takes a single input and produces the opposite (inverted) output. The NOT gate is also called an inverter. This is covered in OCR J277 Section 2.5.

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How the NOT Gate Works

The NOT gate performs negation. If the input is 1, the output is 0. If the input is 0, the output is 1. It simply flips the value.

In Boolean algebra, the NOT operation on input A is written as:

• NOT A
• A (with a bar over it, written as Ā)
• !A (in some programming languages)

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NOT Gate Truth Table

A	NOT A
0	1
1	0

This is the simplest truth table — just two rows because there is only one input. With one input there are 2^1 = 2 possible input values.

OCR Exam Tip: The NOT gate is the only standard logic gate with a single input. All other gates (AND, OR, XOR, NAND, NOR) take two inputs. If an exam question asks for a gate with one input, it is always NOT.

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NOT Gate Circuit Symbol

The standard NOT gate symbol used in OCR exams consists of:

1. A triangle pointing to the right
2. A small circle (called a "bubble") at the output

The triangle represents the gate body, and the bubble indicates inversion. You should be able to both recognise and draw this symbol in the exam.

        +---\
  A ----|    >o---- NOT A
        +---/

The small circle at the tip of the triangle is the key feature that distinguishes the NOT gate from a simple buffer.

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NOT in Programming

In Python:

# Using the not keyword
logged_in = False
if not logged_in:
    print("Please log in")

In OCR pseudocode:

loggedIn = false
if NOT loggedIn then
    print("Please log in")
endif

The not operator inverts the Boolean value. Since logged_in is False, not logged_in evaluates to True, so the message is printed.

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Real-World Applications of NOT

The NOT gate has many practical uses in computing: