# Chapter 3 Logic Gates and Logic Circuits

### Introduction:

There are many electric circuits in computers and devices. New memories and thousands of things controlling the device are made up of logic gates. Logic gates take binary inputs and produce binary outputs. Several logic gates combined makes up a logic circuit, and these circuits are designed to execute a special function. You can check the output of a logic gate or circuit by looking at its truth table.

### Logic Gates:

There are six different logic gates: ### Truth Tables:

Truth tables are used to trace the output from a logic gate or circuit. When we construct truth tables, there can be only 1s and os, and all combinations of 1s and 0s can be input are considered.

NOT Gate (Description):

The output, X, is 1 if the input, A, is 0

AND Gate (Description):

The output, X, is 1 if both inputs, A and B are 1

OR Gate (Description):

The output, X, is 1 if either input, A or B is 1

NAND Gate (NOT AND):

The output, X, is 1 if input A AND input B are NOT both 1

NOR Gate (NOT OR)
The output, X, is 1 if neither input A nor input B is 1

XOR Gate:

The output, X, is 1 if (input A is 1 AND input B is 0) OR (Input A is 0 AND input B is 1)

3 Symbols:

represents the AND operation

+         represents the OR operation

a bar above the letter represents the NOT operation

Using Logisim can help with logic gates, it can tell you the outcomes and is very easy to use, link:

http://www.cburch.com/logisim/download.html

Logic Circuits

When Logic Gates are put together, they can carry out a function, and a truth table for it. In this truth table you should have the inputs, for example if there are only 2 inputs then you will only have the values ( 0 0 ) ( 1 0 ) ( 0 1 ) and ( 1 1 ) , the intermediate values (which are values which you get in the ‘middle’ or when you have a logic gate, they can be labelled any letter you want however do not repeat any letters) and output ( mainly written as X). When you have one input for example 0 1 into an AND gate, it will output 0, this 0 if there is no more gates left will be the output, if there is more, than it is an intermediate value.

Logic Circuits in real life can be found in many devices that do not require an operating system and devices which only have a certain set of instructions such as a microwave, air conditioner, TV remote, etc.

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