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This circuit is a simple and introductory approach to understanding the NAND Gate. What makes the NAND Gate so special is that it is a Universal Gate. This means that ALL logic gates (AND, OR, XOR, NOR, XNOR, NOT) can be made from NAND Gates.
But what is NAND anyways? Not-AND. Or to put it in better terms, Negated AND. You can view this in the circuit as well. The lower right circuit takes an AND gate and puts an inverter, or a NOT gate at the output, making it a "Negated AND", or a NAND gate.
To see how this works at the transistor level, look at the circuit in the upper right. This is the NAND gate at its most basic level.
NAND gates have a special characteristic. Note the result, C, in the Truth Table.
A - B - C
0 - 0 - 1
0 - 1 - 1
1 - 0 - 1
1 - 1 - 0
So long as at least one input, A or B, is LOW or "0" then the result is a HIGH or a "1". The circuit illustrates how you can use these properties to construct a NOT gate. By tying inputs A and B together of the NAND Gate, if we look at the truth table again we can see that when A and B are the same, their result is NOT A and B.
Play around with the NAND gates to create an OR gate. Next try a NOR gate. And go from there. Understanding the fundamental logic behind all other logic will help tremendously in your understanding of digital circuitry and electronics.
For more information also Google DeMorgan's Theory. This is one of the fundamental theorems behind logic gates, boolean algebra and heaps more. If this has helped you, please be sure to bookmark. =]
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I have put together two other circuits that show how to make all logic gates via just the NAND and the NOR gates. Search for those as well if interested. They can be found just searching for my username on EveryCircuit. :-D
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