NOT Gate

NOT gate inverts the signal, i.e., if input is 1 then output of NOT gate is 0 and for 0 input, the output is 1.

Truth Table

A Y
0 1
1 0

The circuit to implement a NOT gate is identical to that used for a transistor as a switch.

When input A is at 0 level, transistor is off and the entire VCC voltage (5V) appears at the output Y. When input A is 1 (5V), the transistor conducts and output voltage Y is 0.