Logic Gates

 

Diodes

A device that can control the direction of the flow of a current is a diode; it is made of semiconductor materials such as silicon.

Diode

As you can see, the position of the diode can turn the current on or off.

That means, a diode can be used as a one way switch: if the wider end of it faces a positive terminal of a battery, it lets current through or else it cuts the current off.

Based on this system, a couple of switches are made and they are called logic gates. The input of these gates can be combinations of 'On' or 'Off' states and they are known as '1's or '0's. These are binary digits, that has been the basis of the Binary System. The output can also be '0's or '1's. That means, we can express the output of these systems in terms of '1' and '0' - binary digits.
In other words, the output of the logic gates can be used to interpret the numbers in binary form. This is the birth of computer systems; they recognize only '0' s and '1's.

The following animations show the major logic gates, their input's and output's.

 

 

Logic Gates

OR Gate


ORGate

 

Truth Table
LeftRight
TopBottomOutput
000
011
101
111

AND Gate


ANDGate

 

Truth Table
LeftRight
TopBottomOutput
000
010
100
111

NOT Gate


NOTGate

 

Truth Table
InputOutput
01
10

NOR Gate


NORGate

 

Truth Table
LeftRight
TopBottomOutput
001
010
100
110

NAND Gate


NANDGate

 

Truth Table
LeftRight
TopBottomOutput
001
011
101
110

 

 

Practice Exercises

Construct a truth tables for each the following circuit of logic gates; then, check them with the corresponding animation that follows:

E.g.1

logicgates1

 

Since there are three inputs, the possibilities are, 23 = 8. Here is the animated answer.

 

logicgates11

 

E.g.2

logicgates2

 

Since there are four inputs, the possibilities are, 24 = 16. Here is the animated answer with some of the possibilities - not all.

 

logicgates22

 

E.g.3

logicgates3

 

Since there are three inputs, the possibilities are, 23 = 8, in this case too. Here is the animated answer with all the possibilities.

 

logicgates33

 

Uses of Logic Gates

Logic gates are used in almost every electronic device today that we take for granted; the following animation shows some of the role played by them in the things that we use everyday.

Washing Machine

washingmachine

Have you noticed when the motor is turning on?  the right temperature, right water level and of course, the closure of the door - for obvious reasons.



7-digit Decoder

washingmachine

This is how a calculator displays the digits.

Do you want to practise what you learn? Here is a book for you:

 

 

 

 

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