Lesson: Introduction to Logic Gates This is the 3rd lecture of Computer and Network Technology subject of BCS HEQ certificate level examination. In this lecture we will learn about the building blocks of a computer processor called logic gates. You will learn about seven logic gates namely NOT, AND, OR, NOR, NAND, XOR, and XNOR gates.

1. Chapter: Fundamentals
Lesson: Logic Gates
Lecturer: Susantha Herath PGD in IT (MBCS), PGD in Marketing (Uni. Of Kln)
Lecture 03
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2.  Computer is a electronic device. It is a digital circuit built using
many types of electronic components. The most important
component in a computer is the processor. There are more than
one processors in a modern computer dedicated for specific
tasks.
 Processors are made of combination of millions of transistors.
Transistor is the smallest single electronic component inside a
processor. There are single transistors in a digital circuit for
other operations, but we are talking about inside a computer
processor.
 Using special combinations of different transistors (and other
components such as diodes and resistors), small operational and
individual circuit blocks are created. These blocks are called
“Logic Gates”.
 We can say that a Logic Gate is the smallest single circuit unit in
a processor.
 So, a processor is made of millions of logic gates.

3.  We now know that a computer is a digital circuit
and it uses binary number system for data
representation. We know that binary number
system uses only 1 and 0. In a digital circuit, we
can say that these are two states of current TRUE
and FALSE.
 Logic gates are all about representing these two
states of current with different inputs and
output.
 It is important to understand how computer
works in circuit level to learn other areas of
computers such as programming and
networking.

4.  There are seven types of logic gates. Each logic
gate represent with a unique symbol.
 A logic gate has one or more input and one
output.
 Logic gates are the building blocks of a digital
circuit (processor).
 Truth tables are used to show the operation of a
gate (or gates). A truth table shows output for all
possible combinations of inputs.

5.  NOT gate has one input and one output
 NOT gate also called as the inverter gate
because it convert the input state opposite.
 As shown in above truth table (A is input and
X is output), NOT gate invert the input state.
 There are only two possible combination of
inputs as shown in the above truth table.

6.  AND gate has two inputs and one output.
When there are two inputs, there are only 4
possible combinations of outputs as shown in
the following truth table. Output x TRUE only
if both inputs (A and B) are TRUE.

7.  Output x TRUE if both or either one input (A
or B, or both A and B) is TRUE. These output
of gates are calculated according to BOOLEAN
ALGEBRA which we will discuss in the next
lesson.

8.  Output x is TRUE if only one input is TRUE.

9.  In NAND gate output x is FALSE only if both
inputs (A and B) are TRUE.

10.  In NOR gate output x is TRUE only if both
inputs (A and B) are FALSE.

11.  In XNOR gate output x TRUE only if both
inputs (A and B) are TRUE or both inputs are
FALSE.

12.  Logic gates are the building blocks of a
computer processor.
 There are seven logic gates.
 Boolean Algebra is the mathematic use to
represent operations of logic gates (logic
circuits).