[<< wikibooks] GCSE Computer Science/Logic gates
Logic gates are the basic building blocks of any digital system. It is a circuit having one or more than one input and only one output.  There are four main types of gate; NOT, AND, OR and XOR.  There are also three gates which are the inverse of one of the main gates; NAND, NOR and XNOR.  These inverse gates are the equivalent of the main gate type followed by a NOT.


== Truth Tables ==
A truth table shows all of the possible inputs and outputs for a logic circuit.  The example below is for a NOT gate.  It shows that if the input is a 1 then the output is a 0 and vice-versa.


== Gate Types ==


=== NOT ===
A NOT gate will invert, or flip, any input.  If the input is a 1 then it will output a 0 and vice-versa.


=== AND ===
An AND gate will return a 1 if, and only if, both of the inputs were a 1.  If either, or both, inputs are a 0 then the output will also be 0.


=== OR ===
An OR gate will return a 1 if either, or both, of the inputs are a 1. It will only return a 0 if both inputs were a 0.


=== XOR ===
The XOR gate will return a 1 if either, but not both, of the inputs are a 1.  If both, or neither, input is a 1 then it will return 0.


=== NAND ===
The NAND gate is the same as an AND followed by a NOT, so is the inverse of an AND gate.


=== NOR ===
The NOR gate is the same as an OR followed by a NOT, so is the inverse of an OR gate.


=== XNOR ===
The XNOR gate is the same as an XOR followed by a NOT, so is the inverse of an XOR gate.


== Logic Circuits ==
A logic circuit is an electronic circuit which includes one or more logic gates.  Below is an example of a logic circuit together with its logic table: