Logic circuit – The chip that the gate is housed in.
Logic gate – The actual process that goes on inside the chip.
There are two chips that logic gates can be housed in,
CMOS integrated circuit• Supply rails between 0 to +5-15v
• I/O threshold needs to be halfway between the supply rails. So if the supply rails are 0 and 9v then the logic gate will read the input as off when it receives a voltage of 4.5v
• Is very prone to static damage
TTL integrated circuits• Supply rails MUST be 0-5v
• Anything below 1v is considered 0 or Off
• Much more robust than the CMOS IC less prone to static damage
The two main things you need to know about logic gates are that they can only work off of digital signals, analogue signals are too unstable to be used, and they can only have two states,
On = 1
Off = 0
There are 7 logic gates,
1.
AND 2.
OR 3.
NOT 4.
NAND 5.
NOR 6.
XOR or
EXCLUSIVE OR 7.
XNORTruth TablesTo find an input and output of a logic gate you need a Truth Table, these will display the inputs and outputs, genrelly they have 2 inputs and 1 output. i label my Inputs A and B, my output Q
Inputs > A B | Q < Output
Input A is high > 1 0 | 1 < This means the Output is high
^
Input B is low
A cookie for whoever can tell me what gates this can represent
[spoiler]OR / XOR / NAND[/spoiler]
AND
The
AND truth table
A B | Q
0 0 | 0
0 1 | 0
1 0 | 0
1 1 | 1
This gate will only become high (1) when both Inputs are high (1)
The
AND gate can be used in such places as an outside light with an IR sensor, One input will be to detect if its dark, the other to detect if there is a person coming up the path.
Also the
AND gate can be made with normal components as such ...
OR
The
OR truth table
A B | Q
0 0 | 0
0 1 | 1
1 0 | 1
1 1 | 1
The
OR gate will become high (1) when one or both of the inputs are high (1)
The
OR gate can also be made without the chip with the components as such ...
NOT
The
NOT truth table
A | Q
0 | 1
1 | 0
The
NOT gate inverts (changes) the input signal, so if the input is high (1) the output will be low (0)
NAND
The
NAND truth table
A B | Q
0 0 | 1
0 1 | 1
1 0 | 1
1 1 | 0
The
NAND gate will not give a high (1) output when both inputs are high (1)
The
NAND gate is called the universal building block as you can make all other gates using the
NAND gate wired up in different ways more on that later though. later is now
NAND LogicNOR
The
NOR truth table
A B | Q
0 0 | 1
0 1 | 0
1 0 | 0
1 1 | 0
The
NOR is the complete opposite of the
OR gate, this only allows a High (1) output when both inputs are low (0)
XOR
The
XOR truth table
A B | Q
0 0 | 0
0 1 | 1
1 0 | 1
1 1 | 0
The
XOR is a true
OR gate beacuse it only allows one Input to be high (1) to give a high (1) output.
XNOR 
The
XNOR truth table
A B | Q
0 0 | 1
0 1 | 0
1 0 | 0
1 1 | 1
The
XNOR gate is the same as the
NOR gate but allows both inputs to be high (1) to give a high (1) output as well as both inputs being low (0)
