Logic gates

Logic gates are at the heart of digital electronics.

A single digital signal can be either ON or OFF, but if there is more than one signal, there are more than two possible states. For example, if there are two signals present there are four possible combinations - ON/ON, ON/OFF, OFF/ON and OFF/OFF - and each combination will produce a different outcome. This is a logic gate.

The graphic shows an AND gate with 2 inputs, both of which can be either ON (represented by 1) or OFF (represented by 0). Because this is an AND gate, both inputs must be ON for there to be an output. The table on the right shows all the possible combinations of inputs and outputs (with the output ON state represented by a 1).

Diagram showing an AND gate with possible outcomes

Now look at an OR gate with the same inputs. An OR gate allows the output to be ON as long as either A or B are ON. The table on the right - called a truth table - shows all the possible inputs and outputs

An OR gate

There are six types of primitive (basic) logic gates that are used in the design of logic systems. The output of the gates is as follows:

• An AND gate is ON if both input signals are ON.
• An OR gate is ON if either or both inputs are ON.
• A NAND (NOT + AND) gate is ON unless both inputs are ON.
• A NOR gate is ON if both inputs are OFF.
• A NOT has just one input. The gate is ON if the input is OFF, and OFF if the input is ON. This gate is sometimes called an inverter.
• An Exclusive OR (XOR) gate is ON if one input is ON and the other is OFF, but will not work if both are ON.

The activity allows you to select input states for different logic gates. The truth table shows the possible outcomes for each gate.