Many electronic circuits have to make decisions. They look at two or more inputs and use these to determine the outputs from the circuit. The process of doing this uses electronic logic, which is based on digital switches called gates.

Logic gates allow an electronic system to make a decision based on a number of its inputs. They are digital electronic devices. Each input and output of the gates must be one of two states:

true or 1 or 'on'

false or 0 or 'off'

A single digital signal can be either on or off - for example, a light with one switch can be on or off. However, if there is more than one signal, there are more than two possible states. For example, if two signals are present there are four possible combinations: on/on, on/off, off/on and off/off.

Logical operators are used in programming. For example, you might find code written as:  

IF A=1 AND B=1 Then….. or IF A=1 OR B+1 Then….

These are known as AND, OR, NOT, XOR

A truth table for a logical operator defines the outputs for different combinations of input.

Use the table below as examples of how to tackle the Boolean algebra