The NOT gate

The gates of Weeks 8 and 11 had two inputs. The NOT gate has one input and one output, and the output is the opposite of the input: 1 gives 0 and 0 gives 1. That is why it is also called an inverter. A circuit for it uses one switch and one lamp, but the switch is placed beside the lamp as a shortcut. When the switch is open, the current can only pass through the lamp, so the lamp is lit. When the switch is closed, the current takes the shortcut around the lamp and the lamp goes out. (A real circuit also needs a small resistor to protect the battery; we ignore it.) So an open switch, 0, gives a lit lamp, 1, and a closed switch, 1, gives an unlit lamp, 0. One input can be 0 or 1, so the table has 2¹ = 2 rows. The symbol is a triangle pointing at the output, with a small circle at its tip; the small circle always means “reverse”. The formula is Y = NOT A, written with a bar over A or as Y = ¬A, and we write ¬A. Note that the output does not disappear: it is the opposite value. Two NOTs in a row cancel: NOT NOT A is A again.