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PROJECT REPORT "LOGIC GATES"

A gate is defined as a digital circuit which follows some logical relationship between the input and output voltages. It is a

digital circuit which either allows a signal to pass through as stop, it is called a gate.

The Logic Gates are building blocks at digital electronics. They are used in digital electronics to change on voltage

level (input voltage) into another (output voltage) according to some logical statement relating them.

A logic gate may have one or more inputs, but it has only one output. The relationship between the possible values of

input and output voltage is expressed in the form of a table called truth table or table of combinations.

Truth table of a Logic Gates is a table that shows all the input and output possibilities for the logic gate.

George Boole in 1980 invented a different kind of algebra based on binary nature at the logic, this algebra of logic called

BOOLEAN ALGEBRA. A logical statement can have only two values, such as HIGH/LOW, ON/OFF,

CLOSED/OPEN, YES/NO, TRUE/FALSE, CONDUCTING/NON-CONDUCTING etc. The two values of logic

statements one denoted by the binary number 1 and 0. The binary number 1 is used to denote the HIGH value. The

logical statements that logic gates follow are called Boolean expressions.

TYPES OF GATES

There are three types of basic logic gates which follows Boolean expression.

i) OR gate

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ii) AND gate

iii) NOT gate

THE “OR GATE”

The OR gate is a two inputs and one output logic gate. It combing the input A and B with the output Y following the

Boolean expression.

Y = A + B

The Boolean algebra, the addition symbol (+) is called OR (i.e. OR operation OR operator).

The various possible combinations of the input and output of the OR gate can be easily understand with the help of

the electrical circuit. In this electric circuit, a parallel combination of two switches A and B is connected to a battery and

a lump L.

The following interference can be easily drawn from the working of electrical circuit is :

a) If switch A & B are open lamp do not glow (A=0, B=0)

b) If Switch A open B closed then (A=0, B=1) Lamp glow.

c) If switch A closed B open then (A=1, B=0) Lamp glow.

d) If switch A & B are closed then (A=1, B=1) Lamp glow.

As we see truth table we found same as it is observation.

THE “AND GATE”

The AND gate is also a two inputs and one output logic gate. It combines the input A and B with the output Y following

the Boolean expression.

Y = A . B

The Boolean algebra, the multiplication symbol (. dot or x Gross) is taken to mean AND.

Y = A . B have Y is equal to A AND B.

The various possible combination of the input and outputs of the AND gate can be easily found with the help of the

electrical circuit. Here a series combination of the switch A and B is connected to a battery and a lump L.

The following conclusions can be easily drawn from the working of electrical circuit :

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a) If both switches A&B are open (A=0, B=0) then lamp will not glow. (y=0)

b) If Switch A closed & B open (A=1, B=0) then Lamp will not glow. (y=0)

c) If switch A open & B closed (A=0, B=1) then Lamp will not glow. (y=0)

d) If switch A & B both closed (A=1, B=1) then Lamp will glow. (y=1)

As we see truth table we found same as it is observed experimentally.

THE “NOT GATE”

The NOT gate is a one inputs and one output logic gate. It combines the input A with the output following the

Boolean expression.

Y = A

i.e. Y not equal A. The way, the NOT gate gives the output it is also called inverter. It is represented by the symbol.

The Boolean algebra, the negative sign (-) is called NOT. The equation Y= A called Boolean expression.

The possible input and output combination of a NOT gate can be easily discussed with the help of electrical circuit.

Here, the switch is connected in parallel to the lump of the battery. The following conclusion can be easily drawn from

the working of the electrical circuit.

a) If switch A is open (i.e. A=0), the lump will glow (i.e. Y=1)

b) If Switch A is closed (i.e. A=1), the lump will not glow (Y=0).

It follows that in the given electrical circuit, the lump glows (or output is obtained), when the switch A is not closed. Far

this reason, the electrical circuit is called not gate. The two possible input-output combinations can be written in the form

of the table. It is called truth table of NOT gate.

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