intelligent automatic street light control system using high sensitivity ldr
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INTELLIGENT AUTOMATIC STREET LIGHT CONTROL
SYSTEM USING HIGH SENSITIVITY LDR
Automatic Street Light Control System is a simple yet powerful concept,
which uses transistor as a switch. By using this system, manual work is eliminated
upto the maximum extent. It automatically switches ON the street lights when the
sunlight goes below the visible region of our eyes. This is done by a sensor called
Light Dependent Resistor (LDR) which senses the light actually like our eyes. It
automatically switches OFF lights whenever the intensity of sunlight is highly
sufficient for the person to see.
By using this system, energy consumption is also reduced because nowadays
the manually operated street lights are switched off late in the morning and are
switched on early before sunset. This project clearly demonstrates the working of
transistor in saturation region and cutoff region and also the working of all the
components is clearly explained in this project.
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CIRCUIT DIAGRAM:
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R1
Power
Relay
N/O
Common
N/CRs
LDR
GND
+Vcc
Step
down
T/F
Bridge
Rectifier
Filter
Circuit Regulator
Power supply to all sections
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BLOCK DESCRIPTION:
POWER SUPPLY:
The input to the circuit is applied from the regulated power supply. The a.c. input i.e.,
230V from the mains supply is step down by the transformer to 12V and is fed to a
rectifier. The output obtained from the rectifier is a pulsating d.c voltage. So in order
to get a pure d.c voltage, the output voltage from the rectifier is fed to a filter to
remove any a.c components present even after rectification. Now, this voltage is
given to a voltage regulator to obtain a pure constant dc voltage.
Fig: Power supply
Transformer:
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RegulatorFilter
Bridge
Rectifier
Step down
transformer
230V AC
50Hz D.C
Output
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Usually, DC voltages are required to operate various electronic equipment
and these voltages are 5V, 9V or 12V. But these voltages cannot be obtained directly.
Thus the a.c input available at the mains supply i.e., 230V is to be brought down to
the required voltage level. This is done by a transformer. Thus, a step down
transformer is employed to decrease the voltage to a required level.
Rectifier:
The output from the transformer is fed to the rectifier. It converts A.C. into
pulsating D.C. The rectifier may be a half wave or a full wave rectifier. In this
project, a bridge rectifier is used because of its merits like good stability and full
wave rectification.
Filter:
Capacitive filter is used in this project. It removes the ripples from the output
of rectifier and smoothens the D.C. Output received from this filter is constant until
the mains voltage and load is maintained constant. However, if either of the two is
varied, D.C. voltage received at this point changes. Therefore a regulator is applied at
the output stage.
Voltage regulator:
As the name itself implies, it regulates the input applied to it. A voltage
regulator is an electrical regulator designed to automatically maintain a constant
voltage level. In this project, power supply of 5V and 12V are required. In order to
obtain these voltage levels, 7805 and 7812 voltage regulators are to be used. The first
number 78 represents positive supply and the numbers 05, 12 represent the required
output voltage levels.
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KIND OF RESISTORS
1. CARBON FILM
The carbon film type is the most popular resistor type. This resistor is made by
depositing a carbon film onto a small ceramic cylinder. A small spiral groove cut into
the film controls the amount of carbon between the leads, hence setting the
resistance. Such resistors show excellent reliability, excellent solderability, noise
stability, moisture stability, and heat stability. Typical power ratings range from 1/4
to 2 W. Resistances range from about 10 Ohm to 1 Mega ohm, with tolerances
around 5 percent.
2. CARBON COMPOSITION
This type is also popular. It is made from a mixture of carbon powder and glue like
binder. To increase the resistance, less carbon is added. These resistors show
predictable performance, low inductance, and low capacitance. Power ratings range
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from about 1/4 to 2 W. Resistances range from 1 Ohm to about 100 Mega ohms, with
tolerances around +/- 5 percent.
3. METAL OXIDE FILM
This type is general purpose resistor. It uses a ceramic core coated with a metal oxide
film. These resistors are mechanically and electrically stable and readable during
high temperature operation. They contain a special paint on their outer surfaces
making them resistant to flames, solvents, heat, and humidity. Typical resistances
range from 1 Ohm to 200 kilo ohm, with typical tolerances of +/- 5 percent.
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4. PRECISION METAL FILM
This type is very accurate, ultra low noise resistor. It uses a ceramic substrate coated
with a metal film, all encased in an epoxy shell. These resistors are used in precisiondevices, such as test instruments, digital and analog devices, and audio and video
devices. Resistances range from about 10 Ohm to 2 MOhm, with power rating from
1/4 to about 1/2 W, and tolerances of +/- 1 percent.
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5. FOIL RESISTORS
Foil resistors are similar in characteristics to metal film resistors. Their main
advantages are better stability and lower temperature coefficient of resistance
(TCR). They have excellent frequency response, low TCR, good stability, and
are very accurate. They are manufactured by rolling the same wire materials
as used in precision wire wound resistors to make thin strips of foil. This foil
is then bonded to a ceramic substrate and etched to produce the value
required. They can be trimmed further by abrasive processes, chemical
machining, or heat treating to achieve the desired tolerance. Their main
disadvantage is that the maximum value is less than metal film resistors. The
accuracy is about the same as metal film resistors, the TCR and stability
approaches precision wire wounds but are somewhat less because the rolling
and packaging processes produce stresses in the foil. The resistive materials
used in precision wire wound resistors are very sensitive to stresses, which
result in instability and higher TCS. Any stresses on these materials will result
in a change in the resistance value and TCR, the greater the stresses, the
larger the change. This type can be used as strain gauges, strain being
measured as a change in the resistance. When used as a strain gauge, the foil
is bonded to a flexible substrate that can be mounted on a part where the
stress is to be measured.
6. FILAMENT RESISTORS
Filament resistors are similar to bathtub or boat resistors except that they are not packaged in a ceramic shell (boat). The individual resistive element with the
leads already crimped is coated with an insulating material, generally a high
temperature varnish. They are used in applications where tolerance, TCR, and
stability are not important but the cost is the governing consideration. The
cost of this type is slightly higher that of carbon composition and the
electrical characteristics are better.
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7. POWER FILM
Power film resistors are similar in manufacture to metal film or carbon film resistors.
They are manufactured and rated as power resistors, with the power rating
being the most important characteristic. Power film resistors are available in
higher maximum values than the power wire wound resistors and have a very
good frequency response. They are generally used in applications requiring
good frequency response and/or higher maximum values. Generally, for
power applications the tolerance is wider. The temperature rating is changed
so that under full load, the resistor will not exceed the maximum design
temperature. The physical sizes are larger and, in some cases, the core may be
made from a more head conductive material and other means employed to
help radiate heat.
8. PRECISION WIRE WOUND
The precision wire wound resistor is a highly accurate resistor (within 0.005%) with
a very low TCR. A TCR as little as 3ppm/ oC can be achieved. However these
components are too expensive for general use and are normally used in highlyaccurate dc applications.
9. HIGH POWER WIRE WOUND
These resistors are used for high power applications. Types include vitreous enamel
coated, cement, and aluminum housed wire wound resistors. Resistive
elements are made from a resistive wire that is coiled around a ceramic
cylinder. These are the most durable of the resistors, with high heat
dissipation and high temperature stability. Resistances range from 0.1 Ohm to
about 150 kilo ohms, with power ratings from around 2 W to as high as 500
W, or more.
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10. PHOTORESISTORS AND THERMISTORS
These are special types of resistors that change resistance when heat or light is
applied. Photoresistors are made from semiconductive materials, such as
cadmium sulfide. Increasing the light level will decrease the resistance. This
type also called LDR (Light Dependent Resistor). Thermistors are
temperature sensitive resistors. Increasing the temperature will decrease theresistance (in most cases). This type also called Thermistor NTC (Negative
Temperature Coefficient). The reciprocal type is Thermistor PTC (Positive
Temperature Coefficient). Increasing the temperature will increase its
resistance.
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12. VARIABLE RESISTORS
Variable resistors provide varying degrees of resistance that can be set with the turn
of a knob. Special kinds of variable resistors include potentiometers,
rheostats, and trimmers. Potentiometers and rheostats are essentially the same
thing, but rheostats are used specially for high power AC electricity, whereas
potentiometers typically are used with lower level DC electricity. Both
potentiometers and rheostats are designed for frequent adjustment. Trimmers,
on the other hand, are miniature potentiometers that are adjusted infrequently
and usually come with pins that can be inserted into PCB. They are used for
fine tuning circuits (eg. fine tuning a circuit that goes astray as it ages), and
they are usually hidden within a circuits enclosure box. Variable resistors
come with 2 or 3 terminals. There are 2 kinds of taper, ie., linear tapered and
nonlinear tapered (logarithmic). The 'taper' describes the way in which the
resistance changes as the control knob is twisted. Linear taper usually has
coded as 'A' while nonlinear tapes has coded as 'B'.
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NPN GENERAL PURPOSE TRANSISTORS BC546; BC547
FEATURES
· Low current (max. 100 mA)
· Low voltage (max. 65 V).
APPLICATIONS
· General purpose switching and amplification.
DESCRIPTION
NPN transistor in a TO-92; SOT54 plastic package.
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LIMITING VALUES
In accordance with the Absolute Maximum Rating System (IEC 134).
1. Transistor mounted on an FR4 printed-circuit board.
SYMBOL PARAMETER CONDITIONS MIN. MAX. UNIT
VCBO collector-base voltage open emitter
BC546 - 80 V
BC547 - 50 V
VCEO collector-emitter voltage open base
BC546 - 65 V
BC547 - 45 V
VEBO emitter-base voltage open collector
BC546 - 6 V
BC547 - 6 V
IC collector current (DC) - 100 mA
ICM peak collector current - 200 mA
IBM peak base current - 200 mA
Ptot total power dissipation Tamb £ 25 °C; note 1 - 500 mW
Tstg storage temperature -65 +150 °C
Tj junction temperature - 150 °C
Tamb operating ambient temperature -65 +150 °C
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BRIEF DESCRIPTION OF TRANSISTOR:
In electronics, a transistor is a semiconductor device commonly used to amplify or switch electronic signals. The transistor is the fundamental building block of
computers, and all other modern electronic devices. Some transistors are packaged
individually but most are found in integrated circuits.
INTRODUCTION:
An electrical signal can be amplified by using a device that allows a small current or
voltage to control the flow of a much larger current. Transistors are the basic devices
providing control of this kind. Modern transistors are divided into two main
categories: bipolar junction transistors (BJTs) and field effect transistors (FETs).
Applying current in BJTs and voltage in FETs between the input and common
terminals increases the conductivity between the common and output terminals,
thereby controlling current flow between them. The characteristics of a transistor
depend on its type.
The term "transistor" originally referred to the point contact type, which saw very
limited commercial application, being replaced by the much more practical bipolar
junction types in the early 1950s. Today's most widely used schematic symbol, like
the term "transistor", originally referred to these long-obsolete devices.
In analog circuits, transistors are used in amplifiers, (direct current amplifiers, audio
amplifiers, radio frequency amplifiers), and linear regulated power supplies.
Transistors are also used in digital circuits where they function as electronic switches,
but rarely as discrete devices, almost always being incorporated in monolithic
integrated circuits. Digital circuits include logic gates, random access memory
(RAM), microprocessors and digital signal processors (DSPs).
IMPORTANCE:
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The transistor is considered by many to be the greatest invention of the twentieth
century. It is the key active component in practically all modern electronics. Its
importance in today's society rests on its ability to be mass produced using a highly
automated process (fabrication) that achieves astonishingly low per-transistor costs.
Although several companies each produce over a billion individually-packaged
(known as discrete) transistors every year, the vast majority of transistors produced
are in integrated circuits (often abbreviated as IC and also called microchips or
simply chips) along with diodes, resistors, capacitors and other electronic
components to produce complete electronic circuits. A logic gate consists of about
twenty transistors whereas an advanced microprocessor, as of 2006, can use as many
as 1.7 billion transistors (MOSFETs). "About 60 million transistors were built this
year [2002] ... for [each] man, woman, and child on Earth."
The transistor's low cost, flexibility and reliability have made it a universal device for
non-mechanical tasks, such as digital computing. Transistorized mechatronics
circuits have replaced electromechanical devices for the control of appliances and
machinery as well. It is often easier and cheaper to use a standard microcontroller and
write a computer program to carry out a control function than to design an equivalent
mechanical control function.
Because of the low cost of transistors and hence digital computers, there is a trend to
digitize information, such as the Internet Archive. With digital computers offering the
ability to quickly find, sort and process digital information, more and more effort has
been put into making information digital. As a result, today, much media data is
delivered in digital form, finally being converted and presented in analog form to the
user. Areas influenced by the Digital Revolution include television, radio and
newspapers.
Advantages
The key advantages that have allowed transistors to replace their vacuum tube
predecessors in most applications are:
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• Small size and minimal weight, allowing the development of miniaturized
electronic devices.
• Highly automated manufacturing processes, resulting in low per-unit cost.
• Lower possible operating voltages, making transistors suitable for small,
battery-powered applications.
• No warm-up period for cathode heaters required after power application.
• Lower power dissipation and generally greater energy efficiency.
• Higher reliability and greater physical ruggedness.
• Extremely long life. Some transistorized devices produced more than 30 years
ago are still in service.
• Complementary devices available, facilitating the design of complementary-
symmetry circuits, something not possible with vacuum tubes.
• Though in most transistors the junctions have different doping levels and
geometry, some allow bidirectional current flow.
• Ability to control very large currents, as much as several hundred amperes.
• Insensitivity to mechanical shock and vibration, thus avoiding the problem of
microphonics in audio applications.• More sensitive than the hot and macroscopic tubes.
Disadvantages
• Silicon transistors do not operate at voltages higher than about 1 kV, SiC go
to 3 kV.
• The electron mobility is higher in a vacuum, so that high power, high
frequency operation is easier in tubes.
• Silicon transistors, compared to vacuum tubes, are highly sensitive to
electromagnetic pulses.
Types
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PNP P-channel
NPN N-
channel
BJT JFET
Transistors are categorized by:
• Semiconductor material: germanium, silicon, gallium arsenide, silicon
carbide, etc.
• Structure: BJT, JFET, IGFET (MOSFET), IGBT, "other types"
• Polarity: NPN, PNP (BJTs); N-channel, P-channel (FETs)
• Maximum power rating: low, medium, high
• Maximum operating frequency: low, medium, high, radio frequency (RF),
microwave (The maximum effective frequency of a transistor is denoted by
the term f T, an abbreviation for "frequency of transition". The frequency of
transition is the frequency at which the transistor yields unity gain).
• Application: switch, general purpose, audio, high voltage, super-beta,
matched pair
• Physical packaging: through hole metal, through hole plastic, surface mount,
ball grid array, power modules
• Amplification factor hfe (transistor beta).
Thus, a particular transistor may be described as: silicon, surface mount, BJT, NPN,
low power, high frequency switch.
NPN TRANSISTORS:
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An NPN transistor can be considered as two diodes with a shared anode region. In
typical operation, the emitter–base junction is forward biased and the base–collector
junction is reverse biased. In an NPN transistor, for example, when a positive voltage
is applied to the base–emitter junction, the equilibrium between thermally generated
carriers and the repelling electric field of the depletion region becomes unbalanced,
allowing thermally excited electrons to inject into the base region. These electrons
wander (or "diffuse") through the base from the region of high concentration near the
emitter towards the region of low concentration near the collector. The electrons in
the base are called minority carriers because the base is doped p-type which would
make holes the majority carrier in the base.
The base region of the transistor must be made thin, so that carriers can diffuse across
it in much less time than the semiconductor's minority carrier lifetime, to minimize
the percentage of carriers that recombine before reaching the collector–base junction.
To ensure this, the thickness of the base is much less than the diffusion length of the
electrons. The collector–base junction is reverse-biased, so little electron injection
occurs from the collector to the base, but electrons that diffuse through the base
towards the collector are swept into the collector by the electric field in the depletion
region of the collector–base junction.
NPN BJT with forward-biased E–B junction and reverse-biased B–C junction
TRANSISTORS IN CIRCUITS
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The diagram shown is a schematic representation of an npn transistor connected to
two voltage sources. To make the transistor conduct appreciable current (on the order
of 1 mA) from C to E, V BE must be above a minimum value sometimes referred to as
the cut-in voltage. The cut-in voltage is usually about 600 mV for silicon BJTs, but
can be different depending on the current level selected for the application and the
type of transistor. This applied voltage causes the lower p-n junction to 't urn-on'
allowing a flow of electrons from the emitter into the base. Because of the electric
field existing between base and collector (caused by V CE ), the majority of these
electrons cross the upper p-n junction into the collector to form the collector current,
I C . The remainder of the electrons recombine with holes, the majority carriers in the base, making a current through the base connection to form the base current, I B . As
shown in the diagram, the emitter current, I E , is the total transistor current which is
the sum of the other terminal currents. That is:
In the diagram, the arrows representing current point in the direction of the electric or
conventional current—the flow of electrons is in the opposite direction of the arrows
since electrons carry negative electric charge. The ratio of the collector current to the
base current is called the DC current gain. This gain is usually quite large and is
often 100 or more.
It should also be noted that the emitter current is related to V BE exponentially. At
room temperature, increasing V BE by about 60 mV increases the emitter current by a
factor of 10. The base current is approximately proportional to the emitter current, so
it varies the same way.
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Structure and use of npn transistor
Regions of operation
Bipolar transistors have five distinct regions of operation, defined mostly by applied
bias:
• Forward-active (or simply, active): The emitter-base junction is forward
biased and the base-collector junction is reverse biased. Most bipolar
transistors are designed to afford the greatest common-emitter current gain, β f ,
in forward-active mode. If this is the case, the collector-emitter current is
approximately proportional to the base current, but many times larger, for
small base current variations.
• Reverse-active (or inverse-active or inverted): By reversing the biasing
conditions of the forward-active region, a bipolar transistor goes into reverse-
active mode. In this mode, the emitter and collector regions switch roles.
Since most BJTs are designed to maximize current gain in forward-active
mode, the β f in inverted mode is several (2 - 3 for the ordinary germanium
transistor) times smaller. This transistor mode is seldom used, usually being
considered only for failsafe conditions and some types of bipolar logic. The
reverse bias breakdown voltage to the base may be an order of magnitude
lower in this region.
• Saturation: With both junctions forward-biased, a BJT is in saturation mode
and facilitates high current conduction from the emitter to the collector. This
mode corresponds to a logical "on", or a closed switch.
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• Cutoff : In cutoff, biasing conditions opposite of saturation (both junctions
reverse biased) are present. There is very little current flow, which
corresponds to a logical "off", or an open switch.
• Avalanche breakdown region
While these regions are well defined for sufficiently large applied voltage, they
overlap somewhat for small (less than a few hundred millivolts) biases. For example,
in the typical grounded-emitter configuration of an NPN BJT used as a pulldown
switch in digital logic, the "off" state never involves a reverse-biased junction
because the base voltage never goes below ground; nevertheless the forward bias is
close enough to zero that essentially no current flows, so this end of the forward
active region can be regarded as the cutoff region
Bipolar junction transistor
The bipolar junction transistor (BJT) was the first type of transistor to be mass-
produced. Bipolar transistors are so named because they conduct by using both
majority and minority carriers. The three terminals of the BJT are named emitter ,
base and collector . Two p-n junctions exist inside a BJT: the base/emitter junction
and base/collector junction. "The [BJT] is useful in amplifiers because the currents at
the emitter and collector are controllable by the relatively small base current." In an
NPN transistor operating in the active region, the emitter-base junction is forward
biased, and electrons are injected into the base region. Because the base is narrow,
most of these electrons will diffuse into the reverse-biased base-collector junction
and be swept into the collector; perhaps one-hundredth of the electrons will
recombine in the base, which is the dominant mechanism in the base current. By
controlling the number of electrons that can leave the base, the number of electrons
entering the collector can be controlled.
Unlike the FET, the BJT is a low–input-impedance device. Also, as the base–emitter
voltage (V be) is increased the base–emitter current and hence the collector–emitter
current ( I ce) increase exponentially according to the Shockley diode model and the
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Ebers-Moll model. Because of this exponential relationship, the BJT has a higher
transconductance than the FET.
Bipolar transistors can be made to conduct by exposure to light, since absorption of
photons in the base region generates a photocurrent that acts as a base current; the
collector current is approximately beta times the photocurrent. Devices designed for
this purpose have a transparent window in the package and are called
phototransistors.
Semiconductor material
The first BJTs were made from germanium (Ge) and some high power types still are.
Silicon (Si) types currently predominate but certain advanced microwave and high
performance versions now employ the compound semiconductor material gallium
arsenide (GaAs) and the semiconductor alloy silicon germanium (SiGe). Single
element semiconductor material (Ge and Si) is described as elemental.
Rough parameters for the most common semiconductor materials used to make
transistors are given in the table below; it must be noted that these parameters will
vary with increase in temperature, electric field, impurity level, strain and various
other factors:
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Semiconductor material characteristics
Semiconductor
material
Junction
forward
voltage
V @ 25 °C
Electron
mobility
m²/(V·s) @ 25
°C
Hole mobility
m²/(V·s) @ 25
°C
Max. junction
temp.
°C
Ge 0.27 0.39 0.19 70 to 100
Si 0.71 0.14 0.05 150 to 200
GaAs 1.03 0.85 0.05 150 to 200
Al-Si junction 0.3 — — 150 to 200
The junction forward voltage is the voltage applied to the emitter-base junction of a
BJT in order to make the base conduct a specified current. The current increases
exponentially as the junction forward voltage is increased. The values given in thetable are typical for a current of 1 mA (the same values apply to semiconductor
diodes). The lower the junction forward voltage the better, as this means that less
power is required to "drive" the transistor. The junction forward voltage for a given
current decreases with increase in temperature. For a typical silicon junction the
change is approximately −2.1 mV/°C.
The density of mobile carriers in the channel of a MOSFET is a function of the
electric field forming the channel and of various other phenomena such as the
impurity level in the channel. Some impurities, called dopants, are introduced
deliberately in making a MOSFET, to control the MOSFET electrical behavior.
The electron mobility and hole mobility columns show the average speed that
electrons and holes diffuse through the semiconductor material with an electric field
of 1 volt per meter applied across the material. In general, the higher the electron
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mobility the faster the transistor. The table indicates that Ge is a better material than
Si in this respect. However, Ge has four major shortcomings compared to silicon and
gallium arsenide:
• Its maximum temperature is limited.
• It has relatively high leakage current.
• It cannot withstand high voltages.
• It is less suitable for fabricating integrated circuits.
Because the electron mobility is higher than the hole mobility for all semiconductor
materials, a given bipolar NPN transistor tends to be faster than an equivalent PNP
transistor type. GaAs has the highest electron mobility of the three semiconductors. It
is for this reason that GaAs is used in high frequency applications. A relatively recent
FET development, the high electron mobility transistor (HEMT), has a
heterostructure (junction between different semiconductor materials) of aluminium
gallium arsenide (AlGaAs)-gallium arsenide (GaAs) which has double the electron
mobility of a GaAs-metal barrier junction. Because of their high speed and low noise,
HEMTs are used in satellite receivers working at frequencies around 12 GHz.
Max. Junction temperature values represent a cross section taken from various
manufacturers' data sheets. This temperature should not be exceeded or the transistor
may be damaged.
Al-Si junction refers to the high-speed (aluminum-silicon) semiconductor-metal
barrier diode, commonly known as a Schottky diode. This is included in the table
because some silicon power IGFETs have a parasitic reverse Schottky diode formed
between the source and drain as part of the fabrication process. This diode can be a
nuisance, but sometimes it is used in the circuit.
Usage
In the early days of transistor circuit design, the bipolar junction transistor, or BJT,
was the most commonly used transistor. Even after MOSFETs became available, the
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BJT remained the transistor of choice for digital and analog circuits because of their
ease of manufacture and speed. However, desirable properties of MOSFETs, such as
their utility in low-power devices, have made them the ubiquitous choice for use in
digital circuits and a very common choice for use in analog circuits.
BJT used as an electronic switch, in grounded-emitter configuration
Amplifier circuit, standard common-emitter configuration
Switches
Transistors are commonly used as electronic switches, for both high power
applications including switched-mode power supplies and low power applications
such as logic gates.
Amplifiers
From mobile phones to televisions, vast numbers of products include amplifiers for
sound reproduction, radio transmission, and signal processing. The first discrete
transistor audio amplifiers barely supplied a few hundred milliwatts, but power and
audio fidelity gradually increased as better transistors became available and amplifier
architecture evolved.
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Transistors are commonly used in modern musical instrument amplifiers, in which
circuits up to a few hundred watts are common and relatively cheap. Transistors have
largely replaced valves (electron tubes) in instrument amplifiers. Some musical
instrument amplifier manufacturers mix transistors and vacuum tubes in the same
circuit, to utilize the inherent benefits of both devices.
Computers
The "first generation" of electronic computers used vacuum tubes, which generated
large amounts of heat, were bulky, and were unreliable. The development of the
transistor was key to computer miniaturization and reliability. The "second
generation" of computers, through the late 1950s and 1960s featured boards filled
with individual transistors and magnetic memory cores. Subsequently, transistors,
other components, and their necessary wiring were integrated into a single, mass-
manufactured component: the integrated circuit.
HOW A TRANSISTOR WORKS?
A transistor may be used to switch or to amplify. The image to the right represents a
typical transistor in a circuit. Its three components are the Base, Emitter and Collector
which correspond to regions of the mixed semiconductors from which the transistor
is made. Current may flow from the Emitter to the Collector depending on the
voltage applied to the Base, but only if this voltage exceeds a certain value this is
depicted in the graph below at A and is referred to as V be.
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Simple circuit using a transistor
Transistor as an amplifier
A varying base voltage, Vin, as long as it exceeds V be, controls current through the
transistor and thus influences the output voltage V out. The slope of the graph is such
that small swings in Vin will produce large changes in Vout. This occurs because the
base voltage controls how much of the power supply voltage Vcc causes current
through the transistor itself, and how much of it causes current through a load driven
by Vout. It is important that the operating parameters of the transistor are chosen and
the circuit designed such that as far as possible the transistor operates within a linear
portion of the graph, such as that shown between A and B, otherwise the output
signal will suffer distortion.
Transistor as a Switch
Operation graph of a transistor
It can be seen from the graph that once the Base voltage reaches a certain level,
shown at B, no more current will flow and the output will be held at a fixed voltage.
The transistor is then said to be saturated. Hence, values of input voltage can be
chosen such that the output is either completely off, or completely on. The transistor
is acting as a switch, and this type of operation is common in digital circuits where
only "on" and "off" values are relevant.
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IN 4007 DIODE
Features:
• Diffused Junction
• High Current Capability and Low Forward
• Voltage Drop
• Surge Overload Rating to 30A Peak
• Low Reverse Leakage Current
• Plastic Material: UL Flammability
• Classification Rating 94V-0
APPLICATION:
• Single phase, half wave, 50Hz, and resistive or inductive load.
• For capacitive load, derate current by 20%.
Forward Voltage Drop, Vf
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Notice that the diode conducts a small current in the forward direction up to a
threshold voltage, 0.3 for germanium and 0.7 for silicon ; after that it conducts as we
might expect. The forward voltage drop, Vf, is specified at a forward current, If.
Leakage current
In the reverse direction there is a small leakage current up until the reverse
breakdown voltage is reached. This leakage is undesirable, obviously the lower the
better, and is specified at a voltage less the than breakdown; diodes are intended to
operate below their breakdown voltage.
Current Rating
The current rating of a diode is determined primarily by the size of the diode chip,
and both the material and configuration of the package, Average Current is used, not
RMS current. A larger chip and package of high thermal conductivity are both
conducive to a higher current rating.
Switching
The switching speed of a diode depends upon its construction and fabrication. In
general the smaller the chip the faster it switches, other things being equal. The
reverse recovery time, trr, is usually the limiting parameter; trr is the time it takes a
diode to switch from on to off.
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IC 7805
7805 IC is an integrated three-terminal positive fixed linear voltageregulator. It supports an input voltage of 10 volts to 35 volts and output voltage of 5
volts. It has a current rating of 1 amp although lower current models are available. Its
output voltage is fixed at 5.0V. The 7805 also has a built-in current limiter as a safety
feature. 7805 is manufactured by many companies, including National
Semiconductors and Fairchild Semiconductors.
The 7805 will automatically reduce output current if it gets too hot.The last
two digits represent the voltage; for instance, the 7812 is a 12-volt regulator. The
78xx series of regulators is designed to work in complement with the 79xx series of
negative voltage regulators in systems that provide both positive and negative
regulated voltages, since the 78xx series can't regulate negative voltages in such a
system.
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The 7805 & 7812 is one of the most common and well-known of the 78xx
series regulators, as it's small component count and medium-power regulated 5V
make it useful for powering TTL devices.
Light-emitting diode (LED)
Light-emitting diodes are elements for light signalization in electronics. They are
manufactured in different shapes, colors and sizes. For their low price, low
consumption and simple use, they have almost completely pushed aside other light
sources- bulbs at first place. They perform similar to common diodes with the
difference that they emit light when current flows through them.
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SPECIFICATIONS IC 7805 IC 7812
Vout 5V 12V
Vin - Vout Difference 5V - 20V 5V - 20V
Operation Ambient Tmp 0 - 125°C 0 - 125°C
Output Imax 1A 1A
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It is important to know that each diode will be immediately destroyed unless its
current is limited. This means that a
conductor must be connected in
parallel to a diode. In order to correctly
determine value of this conductor, it is
necessary to know diode’s voltage
drop in forward direction, which
depends on what material a diode is
made of and what colour it is. Values
typical for the most frequently used
diodes are shown in table below: As seen, there are three main types of LEDs.
Standard ones get full brightness at current of 20mA. Low Current diodes get full
brightness at ten times lower current while Super Bright diodes produce more
intensive light than Standard ones.
Since the 8051 microcontrollers can provide only low input current and since their
pins are configured as outputs when voltage level on them is equal to 0, direct
connecting to LEDs is carried out as it is shown on figure ( Low current LED, cathode
is connected to output pin).
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Potentiometer
Fig: Variable resistor
Variable resistors used as potentiometers have all three terminals connected.
This arrangement is normally used to vary voltage, for example to set the switching
point of a circuit with a sensor, or control the volume (loudness) in an amplifier
circuit. If the terminals at the ends of the track are connected across the power
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supply, then the wiper terminal will provide a voltage which can be varied from zero
up to the maximum of the supply.
Presets
These are miniature versions of the standard variable resistor. They are designed to
be mounted directly onto the circuit board and adjusted only when the circuit is built.
For example to set the frequency of an alarm tone or the sensitivity of a light-
sensitive circuit. A small screwdriver or similar tool is required to adjust presets.
Presets are much cheaper than standard variable resistors so they are sometimes used
in projects where a standard variable resistor would normally be used.
Multiturn presets are used where very precise adjustments must be made. The screw
must be turned many times (10+) to move the slider from one end of the track to the
other, giving very fine control.
RELAYS:
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Potentiometer Symbol
Preset Symbol
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A relay is an electrically controllable switch widely used in industrial controls,
automobiles and appliances.
The relay allows the isolation of two separate sections of a system with two different
voltage sources i.e., a small amount of voltage/current on one side can handle a large
amount of voltage/current on the other side but there is no chance that these two
voltages mix up.
Inductor
Fig: Circuit symbol of a relay
Operation:
When current flows through the coil, a magnetic field is created around the
coil i.e., the coil is energized. This causes the armature to be attracted to the coil. The
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armature’s contact acts like a switch and closes or opens the circuit. When the coil is
not energized, a spring pulls the armature to its normal state of open or closed. There
are all types of relays for all kinds of applications.
Fig: Relay Operation and use of protection diodes
Transistors and ICs must be protected from the brief high voltage 'spike'
produced when the relay coil is switched off. The above diagram shows how a signal
diode (eg 1N4148) is connected across the relay coil to provide this protection. The
diode is connected 'backwards' so that it will normally not conduct. Conduction
occurs only when the relay coil is switched off, at this moment the current tries to
flow continuously through the coil and it is safely diverted through the diode.
Without the diode no current could flow and the coil would produce a damaging high
voltage 'spike' in its attempt to keep the current flowing.
In choosing a relay, the following characteristics need to be considered:
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1. The contacts can be normally open (NO) or normally closed (NC). In the NC type,
the contacts are closed when the coil is not energized. In the NO type, the contacts
are closed when the coil is energized.
2. There can be one or more contacts. i.e., different types like SPST (single pole
single throw), SPDT (single pole double throw) and DPDT (double pole double
throw) relays.
3. The voltage and current required to energize the coil. The voltage can vary from a
few volts to 50 volts, while the current can be from a few milliamps to 20milliamps.
The relay has a minimum voltage, below which the coil will not be energized. This
minimum voltage is called the “pull-in” voltage.
4. The minimum DC/AC voltage and current that can be handled by the contacts.
This is in the range of a few volts to hundreds of volts, while the current can be from
a few amps to 40A or more, depending on the relay.
LDR:
LDRs or Light Dependent Resistors are very useful especially in light/dark sensor
circuits. Normally the resistance of an LDR is very high, sometimes as high as 1000
000 ohms, but when they are illuminated with light resistance drops dramatically.
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When the light level is low the resistance of the LDR is high. This prevents current
from flowing to the base of the transistors. Consequently the LED does not light.
However, when light shines onto the LDR its resistance falls and current flows into
the base of the first transistor and then the second transistor. The LED glows.
WORKING PROCEDURE:
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LDR offers Very high Resistance in darkness. In this case the voltage drop across the
LDR is more than 0.7V. This voltage is more sufficient to drive the transistor into
saturation region. In saturation region, Ic (Collector current) is very high. Because of
this Ic, The relay gets energized, and switches on the lamp.
LDR offers Very low Resistance in brightness. In this case the voltage drop across
the LDR is less than 0.7V. This voltage is not sufficient to drive the transistor into
saturation region. Hence, the transistor will be in cut-off region. In cut-off region, Ic
(Collector current) is zero. Because of this Ic, The relay will not be energized, and
the lamp will be in ON state only. Diode is connected across the relay to neutralizethe reverse EMF generated.
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