The Logic NOT Function

The Logic NOT Function

The Logic NOT Function is simply a single input inverter that changes the input of a logic level "1" to an output of logic level "0" and vice versa. The logic NOT function is so called because its output state is NOT the same as its input state. It is generally denoted by a bar or overline ( ¯ ) over its input symbol which denotes the inversion operation. As NOT gates perform the logic INVERT or COMPLEMENTATION function they are more commonly known as Inverters because they invert the signal. In logic circuits this negation can be represented by a normally closed switch.

Switch Representation of the NOT Function

If A means that the switch is closed, then NOT A or simply A says that the switch is NOT closed or in other words, it is open. The logic NOT function has a single input and a single output as shown.

Truth Table

Switch Output 1 0 0 1 Boolean Expression A or A

The inversion indicator for a logic NOT function is a "bubble", ( O ) symbol on the output (or input) of the logic elements symbol. In Boolean algebra the Logic NOT Function follows the Complementation Law producing inversion.

NOT gates or Inverters can be used with standard AND and OR gates to produce NAND and NOR gates. Inverters can also be used to produce "Complementary" signals in more complex decoder/logic circuits for example, the complement of logic A is A and a double inversion will give the original value of A.

When designing logic circuits and you need only one or two inverters, but do not have the space or the money for a dedicated Inverter chip such as the 74LS04, you can easily make inverter functions using any spare NAND or NOR gates by simply connecting their inputs together as shown below.