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.

The “logical NOT function” is generally denoted by a bar or overline ( ¯ ) over its input symbol which denotes the inversion operation, (hence its name as an inverter). 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.

NOT Function Truth Table

Switch	Output
1	0
0	1
Boolean Expression	not-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.

 

Logic NOT Gates or “Inverters” as they are more commonly called, can be connected with standard AND and OR gates to produce NAND and NOR gates respectively. Logic NOT gates can also be used to produce “Complementary” signals in more complex decoder/logic circuits for example, the complement of logic A is A and two Inverters connected together in series will give a double inversion which produces at its output the original value of A.
When designing logic circuits and you may only need one or two logic NOT gates within your design, but do not have the space or the money for a dedicated Inverter chip such as the 74LS04. Then you can easily make a logic NOT function easily by using any spare NAND or NOR gates by simply connecting their inputs together as shown below.

NOT Function Equivalents