logic gates

GROUP MEMBERS
 Hira Aman
 Zermeen Fatima
 Mishal Malik
 Sayeda Sajida
 Bushra Bibi
 Areeba Komal

CONTENTS
 History
 Definition
 The OR gate
 The AND gate
 The NOT gate
 The NAND gate
 The NOR gate
 Applications

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.

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”.

 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

 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.

THREE INPUT OR GATE USING
2/1 OR GATE

 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.

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.

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

THREE INPUT AND GATE USING(2/1
AND
GATE)

 To determine the total number of possible combination
of binary inputs to gate is determined by a following
formula.
 When N is the number of possible input combinations
and n is the number of possible variables.
Example,

 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

MODE OF ACTION
 In this circuit two resistors are used. The resistor
used with the base terminal is higher than with
the collector.
 To control the base current we use the high
resistance.
 When A is at “+5V”, the emitter base junction is
forward biased and the transistor is ‘ON’ and
maximum current flows through the collector
lead. We get the point A at “0V”.
 Conversely, when we connect A with ‘0V’. The
transistor stops working and we get the point A
at “+5V”.

 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 of gates
and circuits
 Boolean expressions
 logic diagrams
 truth tables

MODE OF ACTION
 The combination of OR gate and NOT gate makes
NOR gate.
 The function of Nor gate is just of the reverse of OR
gate.
 Nor gate will have an output 1 only when both input
A and B are at logically 0 v.
 If any of input is 1v the output is 0v.
 If both inputs are 1v then output is 0v.

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

LOGIC NAND GATE DEFINITION
 A combination of AND gate and NOT gate makes
a NAND gate.Its symbolic representation is
shown below.

COMPONENT
 Two ideal p_n junction diode(D1 &D2)
 Ideal n-P-n transistor

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 is voltage
at c w.r.t earth

CONTI....
 In boolean expresion the NAND gate is
expressed as y=A.B
 And is being read as A AND B negated

WORKING
 The folowing inerference can be easily drawn
from working of electrical circuit:
a) If switch A &B open (A=0,B=0) hence Y=1.
b) If switch Aopen B closed (A=0,B=1)hence Y=1
c) If switch A closed B OPEN(A=1,B=0)hence Y=1
d) If switch A&B are 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.

LOGIC GATES
APPLICATIONS
By
Zermeen Fatima

 Wherever the occurrence of any one or more than one
event is needed to be detected or some actions are to be
taken after their occurrence, in all those cases OR gates
can be used.
 In an industrial plant if one or more than one parameter
exceeds the safe value, some protective measure is
needed to be done.
 OR gate is used to detect exceed of temperature or
pressure and produce command signal for the system to
take required actions.
 We are going to show this with the help of a diagram.

 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.

NOT GATE
Safety Reminder in Cars

 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 that of a door
closing system of an automobile.
 A car needs to be so designed that the driver gets a
visual indication if any of the doors of the car is
open so that 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 that 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.

NAND GATE
Freezer warning buzzer

 When the thermistor ( sensor
for temperature) is COLD its
resistance is LARGE and the
input to the NAND gate is
high.
 Since the NAND gate is
connected as an INVERTER
the output is LOW.
 As the thermistor warms up its
resistance decreases, the
voltage across it falls and the
input to the NAND gate falls.
 When it becomes low enough
the output becomes HIGH and
the buzzer sounds.

 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 that 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.

logic gates

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