Logic gates are the basic building blocks of any digital system. It is an electronic circuit having one or more than one input and only one output. The relationship between the input and the output is based on a certain logic. Based on this, logic gates are named as AND gate, OR gate, NOT gate etc.

1. LOGIC GATES
ELECTRONICS 2

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3. CONTENTS
o History
o Definition
o The OR gate
o The AND gate
o The NOT gate
o The NAND gate
o The NOR gate
o The XOR gate
o The XNOR gate
o Applications

4. HISTORY
 The binary number system was
refined by Gottfried Wilhelm
Leibniz (published in 1705).
 In an 1886 letter, Charles
Sanders Peirce described
how logical operations
could be carried out by
electrical switching
circuits.

5. INTRODUCTION
 Logic gates perform basic logical functions and are
the fundamental building blocks of digital
integrated circuits. Most logic gates take an input of
two binary values, and output a single value of a 1
or 0.
LOGIC
•Formal logic is a branch of the mathematics that
deals with true and false values instead of
numbers.
•In the mid-19thcentury, George Bool developed
many Logic ideas.
•Boolean logic deals with equations where the
operators are “AND” or “OR” instead of “add” and
“multiply”.

6. TYPES OF GATES
 AND - True if A and B are both True
 OR - True if either A or B is True
 NOT - Inverts value: True if input is False; False if input is
True
 NAND - AND followed by NOT: False only if A and B both are
True
 NOR - OR followed by NOT: True only if A and B both are False
 XOR -The XOR logic gate can be used as a one-bit adder that
adds any two bits together to output one bit.
 XNOR - An XNOR Gate is a type of digital logic gate that
receives two inputs and produces one output. Both inputs are
treated with the same logic,

8. • OR gate has two or more inputs and
performs what is known as logical addition.
• The output of OR gate is low when all inputs
are low otherwise all inputs are high.
THE OR GATE
OPERATION OF OR GATE

9. CIRCUIT DIAGRAM

10. • An AND gate has two or more inputs and
perform what is known as multiplication.
• The output of AND gate is high when all inputs
are high otherwise all inputs are low.
THE AND GATE

11. OPERATION OF AND GATE
• An AND gate produces a HIGH output only when all
inputs are HIGH. When any of the input is LOW the
output is LOW. Therefore the basic purpose of AND gate
is to determine when certain conditions are
simultaneously true, as indicated by HIGH levels on all
of its input and produces a HIGH on its output.

12. CIRCUIT DIAGRAM

13. LOGICAL EXPRESSION
• If the logical expression is, X=A.B

14. • This gate consists of three elements, a transistor
and two resistors. It is called the NOT Gate.
• Its output is opposite to its input therefore it is
also called an Inverter.
• It has one input and one output.
• Boolean expression of NOT Gate is:
A = A
NOT GATE
TRUTH TABLE

15. SYMBOLIC REPRESENTATION

16. CIRCUIT DIAGRAM

17. • A gate is a device that performs a basic operation on
electrical signals
• Gates are combined into circuits to perform more
complicated tasks
• There are three different, but equally powerful,
notational methods
for describing the behavior ofgates and
circuits
• Boolean expressions
• logic diagrams
• truth tables
N O R G A T E

18. CIRCUIT DIAGRAM

19. • Logic diagram: a graphical representation of a
circuit
• Each type of gate is represented by a specific graphical
symbol.
Boolean expressions:

21. LOGIC NAND GATE DEFINITION
• Acombination of AND gate and NOT gate makes a NAND
gate.Its symbolic representation is shown below.
NAND GATE
COMPONENT
• Two ideal p_n junction diode(D1&D2)
• Ideal n-P-n transistor

22. THEORY AND CONSTRUCTION
• If we connect the output y of AND gate to the input
of a NOT gate the gate obtained is called NAND
gate.
• The output y isvoltage at c w.r.t earth
• In boolean expresion the NAND gate is expressed as
y=A.B
• And is being read as A AND B negated

23. WORKING
• The folowing inerference can beeasilydrawn from working of
electricalcircuit:
a) If switch A &B open (A=0,B=0)hence Y=1.
b) If switch Aopen B closed (A=0,B=1)henceY=1
c) If switch A closed B OPEN(A=1,B=0)hence Y=1
d) If switch A&Bare closed (A=1,B=1)hence Y=0
LOGIC SYMBOL AND TRUTH TABLE
The graphic symbol for the NAND
gate consists of an AND symbol
with a bubble on the output,
denoting that a complement
operation is performed on the
output of the AND gate.

24. XOR GATE
What is an XOR Gate?
• An XOR gate (also known as an EOR, or EXOR gate) – pronounced as Exclusive OR gate – is a
digital logic gate that gives a true (i.e. a HIGH or 1) output when the number of true inputs is
odd. An XOR gate implements an exclusive OR, i.e., a true output result occurs if one – and
only one – of the gate’s inputs is true. If both inputs are false (i.e. LOW or 0) or both inputs are
true, the output is false.
• XOR represents the inequality function, i.e. the output is true if the inputs are not alike;
otherwise, the output is false. A common way to remember the XOR is “must have one or the
other, but not both”.
• Another way to look at an XOR gate: a modulo sum of two variables in a binary system looks
like this:

25. XOR GateTruthTable
XOR Gate Circuit Diagram
Logical Symbol of XOR Gate

26. What is an XNOR Gate?
• The XNOR gate (also known as a XORN’T, ENOR, EXNOR or NXOR) – and
pronounced as Exclusive NOR – is a digital logic gate whose function is
the logical complement of the exclusive OR gate (XOR gate). Logically, an
XNOR gate is a NOT gate followed by an XOR gate.
XNOR GateTruthTable
XNOR GATE

27. XNOR Gate Circuit Diagram
The symbol of the XNOR gate:

28. LOGIC GATES
APPLICATIONS

29. OR GATE
Industrial Plant
• Wherever the occurrenceofany oneormorethan one eventis neededtobe detectedorsomeactions
are tobe takenaftertheir occurrence,in all thosecasesOR gates canbeused.
• In an industrial plant ifoneormorethan oneparameter exceedsthe safevalue,someprotective
measureis neededtobedone.
• OR gateis usedtodetectexceedoftemperature or pressure andproducecommandsignal for the
systemto take requiredactions.
• Weare goingtoshowthis with the help ofa diagram.

31. • Microwave oven will only start if the ‘’start’’ button
is pressed and the door is closed.
• If the ‘’start’’ button is pressed and door is open then
the oven will not work and vice versa.
• Illustration is given by the diagram.
AND GATE
MICROWAVE OVEN

33. The NOT gate is also known as an inverter, simply because it changes the input
to its opposite (inverts it).
• It is used in combination with other gates.
• The application discussed here is t h a t of a door closing system of a n automobile.
• A car needs to be so designed t h a t the driver gets a visual indication if any of
the doors of the car is open so t h a t it helps to avoid accident and injury to the
passengers. Assuming there are two doors (just for simplicity, it works for more
doors as well) where this system is fitted, the circuit can be designed using a
NAND gate as follows
• You can see from the figure t h a t when any of the switches is open due to the
door position, the NAND gate energies the lamp inside the car, hence warning
the driver.
NOT GATE
Safety Reminder in Cars

34. Signal

35. • When t h e thermistor (sensor for
temperatu re) is COLD its resistance is
LARGE a n d t h e in p u t to t h e NAND gate is
high.
• Since t h e NAND gate is connected a s a n
INVERTER t h e o u tp u t is LOW.
• As t h e thermistor w a r ms u p its resistance
decreases, t h e voltage across it falls a n d t h e
in p u t to t h e NAND gate falls.
• When it becomes low enough t h e o u tp u t
becomes HIGH a n d t h e buzzer sounds.
NAND GATE
Freezer warning buzzer

36. • When the switch is closed (burglar
steps on a pressure pad) one input
of the NAND gate
is LOW.
• When the LDR (Light Dependent
Resistor) is in the light (the burglar's
torch) the other input
is LOW.
• This means t h a t if either of these things
happen,
i.e. the switch is closed or the light is on
one of the inputs is LOW, the output is
HIGH and the buzzer sounds.
BURGLAR ALARM
NAND Gate

38. NOR GATE
BLENDERS