Logic gate

Computational equipment, physical or theoretical, that performs a boolean logic function

A logic gate is an electronic component that can be used to conduct electricity based on a rule. The output of the gate is the result of applying this rule to one or more "inputs". These inputs may be two wires or the output of other logic gates.

Logic gates are digital components. They normally work at only two levels of voltage, a positive level and zero level. Commonly they work based on two states: On and Off. In the On state, voltage is positive. In the Off state, the voltage is at zero. The On state usually uses a voltage in the range of 3.5 to 5 volts. This range can be lower for some uses.

Logic gates compare the state at their inputs to decide what the state at their output should be. A logic gate is on or active when its rules are correctly met. At this time, electricity is flowing through the gate and the voltage at its output is at the level of its On state.

Logic gates are electronic versions of Boolean logic. Truth tables will tell you what the output will be, depending on the inputs.

AND logic gate

 
A general idea of a symbol for an AND Logic Gate

AND gates have two inputs. The output of an AND gate is on only if both inputs are on. If at least one of the inputs are off, the output will be off.

Using the image at the right, if A and B are both in an On state, the output (out) will be an On state. If either A or B is in an Off state, the output will also be in an Off state. A and B must be On for the output to be On.

Truth table
INPUTOUTPUT
ABA AND B
OnOnOn
OnOffOff
OffOnOff
OffOffOff

OR logic gate

 
A general idea of a symbol for an OR Logic Gate

OR gates have two inputs. The output of an OR gate will be on if at least one of the inputs are on. If both inputs are off, the output will be off.

Using the image at the right, if either A or B is On, the output (out) will also be On. If both A and B are Off, the output will be Off.

Truth table
INPUTOUTPUT
ABA OR B
OnOnOn
OnOffOn
OffOnOn
OffOffOff

NOT logic gate

The NOT logic gate has only one input. If the input is On then the output will be Off. In other words, the NOT logic gate changes the signal from On to Off or from Off to On. It is sometimes called an inverter.

 
A general idea of a symbol for a NOT Logic Gate
Truth table
AOutput
OffOn
OnOff

XOR logic gate

 
A general idea of a symbol for a XOR Logic Gate

XOR ("exclusive or") gates have two inputs. The output of a XOR gate will be true only if the two inputs are different from each other. If both inputs are the same, the output will be off.

Truth table
INPUTOUTPUT
ABA AND B
OnOnOff
OnOffOn
OffOnOn
OffOffOff

NAND logic gate

 
A general idea of a symbol for a NAND Logic Gate

NAND means not both. It is called NAND because it means "not and." This means that it will always output true unless both inputs are on.

Truth table
INPUTOUTPUT
ABA AND B
OnOnOff
OnOffOn
OffOnOn
OffOffOn

XNOR logic gate

 
A general idea of a symbol for a XNOR Logic Gate

XNOR means "not exclusive or." This means that it will only output true if both inputs are the same. It is the opposite of a XOR logic gate.

Truth table
INPUTOUTPUT
ABA AND B
OnOnOn
OnOffOff
OffOnOff
OffOffOn

FIND OUT WHAT IS THE USE OF LOGIC GATES AND ITS APPLICATIONS?

Applications of Logic Gates

The applications of Logic Gates are:

  • NAND Gates are used in Burglar alarms and buzzers.
  • They are basically used in circuits involving computation and processing.
  • They are also used in push button switches. E.g. Door Bell.
  • They are used in the functioning of street lights.
  • AND Gates are used to enable/inhibit the data transfer function.
  • They are also used in TTL (Transistor Transistor Logic) and CMOS circuitry.

[1]Advantages of Logic Gates

The advantages of Logic Gates are:

  • Logical Operations are performed using Boolean Algebra which makes the circuit design more economical and simple.
  • Logic ‘1’ and Logic ‘0’ can be easily distinguished.

Disadvantages of Logic Gates

The disadvantages of Logic Gates are:

  • Operating Voltage is limited.
  • Time delay occurs between input and output.
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