NOT Gates

The NOT gate has a single input and one output.

The little bubble on the output indicates that the output goes LOW when the input goes HIGH.

We can say that the output goes LOW when the input is ACTIVATED.

The opposite happens when the input is LOW. The output goes HIGH.

The TRUTH TABLE shows that the output is the opposite of the input.

The NOT gate is also called an INVERTER. It inverts the input.
_
The Boolean expression is A = Z

Which is read as, NOT A EQUALS Z

or IF A IS LOW THEN Z IS HIGH

or BAR A = Z

Exclusive OR (EXOR)

Before reading this page read the one on LOGIC GATES.

If you look at the truth table for the OR gate not only does the output go high when A OR B are high but it goes high when A AND B together are high.
This can cause problems in some applications.

Therefore the EXCLUSIVE OR or EXOR is used.

The output of the EXOR only goes high when A or B on its own goes high.
That is when A or B exclusively goes high.

When A and B are both high the output stays low.

This is shown in the truth table.

OR Gates

Before reading this page read the one on LOGIC GATES.

The OR gate has two or more inputs and one output.

The output voltage goes high only when one or more input voltages are high.

In the switch diagram the lamp lights up when A OR B (or both) are operated.

In the truth table Z = 1 when A or B = 1.

The Boolean expression is A+B = Z which translated says, A or B high makes Z high.
The plus sign + translates as OR.

NOR Gates

Before reading this page read the one on LOGIC GATES.

The NOR gate has two or more inputs and one output.

The output voltage goes low only when one or more input voltages are high.

In the switch diagram the lamp goes out when A OR B (or both) are operated. (A short circuit is placed across the lamp)

In the truth table Z = 0 when A or B = 1
_
The Boolean expression is A+B = Z which translated says, A or B high makes Z low.
_
The plus sign, + translates as OR. Z (called Z bar) means Z is low.