The NOT Gate: The Simplest Flip
Imagine a light in a room controlled by a single switch. When the switch is ON, the light is OFF — and when the switch is OFF, the light is ON. That's exactly what a NOT gate does: it inverts, or flips, whatever it receives.
A NOT gate has exactly one input and one output. The output is always the opposite of the input. If the input is 1 (high voltage, true), the output is 0 (low voltage, false). If the input is 0, the output is 1.
The NOT gate is also called an inverter because it inverts the signal. In circuit diagrams, it is drawn as a triangle pointing to the right with a small circle (called a bubble) at its tip. The bubble represents the inversion.
The Truth Table
A truth table lists every possible input and the corresponding output. For a NOT gate, there are only two possibilities:
That's the entire table. No more rows, no complexity.
The Boolean Expression
In Boolean algebra, we write the NOT operation with a bar over the variable:
You read this as "Y equals NOT A" or "Y equals A bar." The bar is the inversion symbol.
Why It Matters
The NOT gate is the foundation of all other logic gates. Every flip-flop, every memory cell, every processor — they all rely on inversion somewhere. Without the NOT gate, you cannot build a NAND gate (which is NOT + AND), a NOR gate (NOT + OR), or any sequential circuit that remembers a state. …