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Logic Gates act as switches in a circuit that performs logical operation. Let it be any form of electronics you work on, they are the most basic things to be acquainted for. This post will help you understand what are Logic Gates, their types with brief introduction, working principle, applications, advantages and disadvantages.

What are Logic Gates

Logic Gates are the building blocks of Digital Systems. They are basically used to perform logical operations by acting as a switch. Boolean Functions are implemented using Logic Gates. A Logic Gate accepts the input and operates on a required condition. If a certain condition is true, it turns “ON” and goes “OFF” when the condition is false.

Introduction to Logic Gates

Fig. 1 – Introduction to Logic Gates

Function of a Logic Gate is expressed using Truth Table. It displays all possible combination of the inputs and a column against each row shows the corresponding output value. Any number of inputs provided results in a single output with a time delay.

Fig.2 below shows an example of Logic Gate circuit which has three inputs A, B, C and single output Y.

Example of a Logic Gate Circuit

Fig. 2 – Example of Logic Gate Circuit

Types of Logic Gates

There are seven types of Logic Gates. They are:

  • NOT Gate (Inverter)
  • AND Gate (Multiplication)
  • OR Gate (Addition)
  • NAND Gate
  • NOR Gate
  • XOR Gate
  • XNOR Gate

NOT Gate (Inverter)

The output of NOT Gate is ‘High’ if the input is ‘Low’ and Vice versa.

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Fig. 3 – Symbol and Truth Table of NOT Gate

AND Gate (Multiplication)

The output of AND Gate is ‘High’ if both the inputs are High and the output is ‘Low’ if both or either of the inputs is ‘Low’.

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Fig. 4 – Symbol and Truth Table of AND Gate

OR Gate (Addition)

The output of OR Gate is ‘High’ if both the inputs are ‘High’ or one of the inputs is ‘High’. The output is ‘Low’ if both the inputs are ‘Low’.

image

Fig. 5 – Symbol and Truth Table of OR Gate

NAND Gate

The output of NAND Gate is ‘High’ if both the inputs are ‘Low’ and if either of the input is ‘Low’. The output is ‘Low’ if both the inputs are ‘High’

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Fig. 6 – Symbol and Truth Table of NAND Logic Gates

NOR Gate

The output of NOR Gate is ‘High’ if both the inputs are ‘Low’. The output is ‘Low’ if both the inputs are ‘High’ or either of the input is ‘High’

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Fig. 7 – Symbol and Truth Table of NOR Logic Gates

XOR Gate

The output of XOR Gate is ‘High’ if either of the input is ‘High’. The output is ‘Low’ if both the inputs are ‘High’ or if both the inputs are ‘Low’.

image

Fig. 8 – Symbol and Truth Table of XOR Gate

XNOR Gate

The output of XNOR Gate is ‘High’ if both the inputs are ‘High’ or if both the inputs are ‘Low’. The output is ‘Low’ if either of the input is ‘Low’

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Fig. 9 – Symbol and Truth Table of XNOR Gate

Working Principle of Logic Gates

To understand the working principle of Logic Gates, let us consider the example of Street Lighting System. Fig below shows the Circuit Diagram of Street Lighting System which mainly consists of Logic Gate (s) (NOT and OR Gate), Switch and Light Sensor. The operational condition for the street light to function is as follows:

  • Switch: On = 1, Off = 0
  • Light Sensor: Dark = 0, Bright = 1
  • Street Light: On = 1, Off = 0

Based on this, we can prepare a Truth Table as shown in Fig. 10

Working Principle of Street Lighting System and Truth Table

Fig. 10 – Circuit Diagram of Street Lighting System and Truth Table

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.

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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