intelligent automatic street light control system using high sensitivity ldr

7/30/2019 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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http://en.wikipedia.org/wiki/Image:JFET_N-Channel_Labelled.svg

http://en.wikipedia.org/wiki/Image:BJT_NPN_symbol.svg

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http://en.wikipedia.org/wiki/Image:BJT_PNP_symbol.svg







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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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http://en.wikipedia.org/wiki/Image:Common_emitter_amplifier.svg

http://en.wikipedia.org/wiki/Image:Transistor_as_switch.svg







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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.






intelligent automatic street light control system using high sensitivity ldr

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