Basic Understanding of Logic Gates

Here is a good information on Logic Gates with sufficient diagrams and truth tables. Have a look at it.

Logic gate is a digital circuit with one or more input signals and only one output signal. The input and output signal are either low or high voltages (0 or 1). They are called logic circuits because they can be analysed with Boolean algebra.

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The basic logic gates are:

  • AND Gate
  • OR Gate
  • NOT Gate

Let us see about these three basic gates in more detail:

AND Gate

The AND gate is a logic gate of which output is high (1) only when all the inputs are high (1). The truth table of a gate is a table which shows input and output conditions. The table below show truth table for AND gate having two inputs ‘A’ and ‘B’.

INPUT                  OUTPUT

A       B                                 Y=A.B

_____________________________                    

0       0                                    0

0       1                                    0

1       0                                    0

1       1                                    1

If A and B are inputs, AND operation is indicated by logic equation: Y=A.B

The AND operation is similar to that of multiplication operation.

IC 7408 is Quad 2-input AND gate.

OR Gate

The OR gate is a logic circuit of which output is high, when any one of the input is high. The output of OR gate is low, when all the inputs are low. The following table shows truth table for OR gate with two inputs.

INPUT                  OUTPUT

A       B                                 Y=A+B

_____________________________                    

0       0                                    0

0       1                                    1

1       0                                    1

1       1                                    1

The logic equation for OR gate is, Y=A+B

IC 7432 is Quad 2-input OR gate.

NOT Gate

The NOT gate is also known as an inverter. It has one input and one output. The NOT gate is a logic circuit of which output is opposite of the input.

The truth table of NOT gate is as follows:

INPUT                  OUTPUT

       A                                   Y=A raised to bar i.e.‾

       0                                      1

       1                                      0

The logic equation of NOT gate is: Y=A raised to bar i.e.‾

IC 7404 is a hex inverter.

Thus we have seen all the basic gates i.e. AND, OR and NOT gates. Other gates can be built by using these basic gates. Let us discuss some other gates now.

NAND Gate

The NAND gate is a logic circuit of which output is low only when all the inputs are high. The NAND operation is a AND followed by NOT operation. The following table shows truth table of NAND gate.

INPUT                  OUTPUT

A       B                                 Y=A.B the whole raised to bar i.e.‾

_____________________________                    

0       0                                    1

0       1                                    1

1       0                                    1

1       1                                    0

The logic equation for NAND gate is: Y=A.B the whole raised to bar i.e.‾

The 2-input NAND operation.

IC 7400 is Quad 2-input NAND Gate.

NOR Gate

The NOR gate is logic circuit of which output is high only when all the inputs are low. The NOR operation is a OR followed by NOT operation.

The truth table is shown below:

INPUT                  OUTPUT

A       B                                 Y=A+B the whole raised to bar i.e.‾

_____________________________                    

0       0                                    1

0       1                                    0

1       0                                    0

1       1                                    0

The Logic equation for NOR gate is: Y=A+B the whole raised to bar i.e.‾

The 2-input NOR operation.

IC 7402 is Quad 2-input NOR gate.

There is one more type of gate i.e. EX-OR gate which I will be explaining after a small time. Thanks for reading this piece of information.

10.04.11

An informative article on a very complex topic. Well done Forgiver.

10.04.11

Thanks Jimmy

10.04.11

You’ve introduce me to a new field, something I know little about but I want to know more about it.

10.04.11

I would really love to give you some more information on this field as I am into it. I am ready to give you all the information you are interested in. Thanks for reading my article Guy Hogan.
Sincere regards,
-The forgiver

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