The 'NOT' gate

The NOT gate gives an output state that is the opposite of the input state.

The truth table below shows that this is the case:

The NOT gate is often called an inverter or an inverting buffer. You cannot have multiple inputs to a NOT gate.

Boolean Expression for the NOT gate:

The overscore - a line over the top of the letter - indicates an inverse state (a 0 becomes 1 and vice versa).

Switch circuit equivalent

To help you understand what a NOT gate does we can consider a simple electrical circuit. When the switch is open the state of the gate input can be thought of as being low (0), when it is closed the gate input can be thought of as being high (1).

When the lamp lights up the output state is high (1) and when it is unlit the state is low(0).

An open switch allows the lamp to light - a closed switch shorts it out.