NOT gate

Outputs the opposite of its single input.

Truth table

a	Q
0	1
1	0

At a glance

Boolean expression: ¬a
Engineering notation: a'
Inputs: One
Output is high when: its single input is low

How the NOT gate behaves

A NOT gate, also called an inverter, has one input and one output. It turns 1 into 0 and 0 into 1. Inverting twice gets you back where you started, which is the double negation law.

It is the only gate that does not combine anything. Everything else on this page merges two or more signals; the inverter just flips one. That makes it the piece that turns AND into NAND, OR into NOR, and a positive condition into a negative one.

Building it from other gates

Each of these is equivalent to the NOT gate. Paste any of them into the simulator with ctrl+E to see the circuit.

Construction	Expression	Equals
From a NAND gate	!(a & a)	!a
From a NOR gate	!(a \| a)	!a
From XOR with a 1	a ^ 1	!a

Reference card

The symbol in both standards and the truth table on one image, for notes or a slide.

NOT gate reference: ANSI and IEC symbols, the boolean expression ¬a, and the full truth table

Click to download the NOT reference card

Where the NOT gate is used

Turning an active-low signal into an active-high one, which is most of what inverters do in real boards.
Completing a set: AND and OR alone cannot express everything, but add NOT and you can express any boolean function at all.
Building a ring oscillator, where an odd number of inverters in a loop never settles and oscillates instead.
Producing the complement of a variable for a sum of products expression.

In the simulator

NOT is in the Logic menu. It is the one gate with a fixed single input, so there is no input count to set.

Open the simulator

Questions about the NOT gate

Why is a NOT gate called an inverter?

Because it inverts its input: the output is always the opposite level. The two names mean the same thing, and in circuit diagrams it is drawn as a triangle with a small bubble on the output, the bubble being the part that means inversion.

What happens if I chain two NOT gates?

You get the original signal back, since inverting twice cancels out. It is not useless though: a pair of inverters is often used as a buffer, to strengthen a weak signal or to add a deliberate delay.

The other gates

AND High only when every input is high. OR High when at least one input is high. XOR High when its two inputs disagree. NAND The inverse of AND, and enough to build everything else. NOR High only when every input is low. XNOR High when its two inputs agree.