SHADOW ALARMA mini project report submitted

in partial fulfillment of the requirements forthe award of the degree of

BACHELOR OF TECHNOLOGYIN

ELECTRONICS AND COMPUTER ENGINEERING

Submittedby

A.NAFEES AHMED R.SWETHA Sk.IBRAHIM Y7EM249 Y7EM310 Y7EM291 3/4 B.Tech 3/4 B.Tech 3/4 B.Tech

Under the esteemed guidance of Sri N.V.KrishnaRamesh

lecturerSri G.Subrahmanya Sharma

Assistant Professor

DEPARTMENT OF ELECTRONICS AND COMPUTER ENGINEERINGKONERU LAKSHMAIAH COLLEGE OF ENGINEERING

(AUTONOMOUS)Green Fields, Vaddeswaram - 522 502, Guntur District, A.P.

Approved by A.I.C.T.E. Affiliated to AcharyaNagarjuna UniversityAccredited by N.B.A. Accredited by N.A.A.C ISO 9001-2000 Certified

2008-09

DEPARTMENT OF ELECTRONICS AND COMPUTER ENGINEERINGKONERU LAKSHMAIAH COLLEGE OF ENGINEERING

(AUTONOMOUS)

CERTIFICATE

This is to certify that the mini project entitled “SHADOW

ALARM”, being submitted by

A.NAFEESAHMED(Y7EM249),R.SWETHA(Y7EM310),

Sk.IBRAHIM(Y7EM291)in partial fulfillment for the award of

degree of Bachelor of Technology (B.Tech) in Electronics

and Computer Engineering of AcharyaNagarjuna University, is a

record of confide work carried out by them under our guidance

and supervision during the academic year 2008-2009 and it

has been found worthy of acceptance according to the

requirements of the university.

MrN.V.K.Ramesh, M.Phil.

Lecturer and Project Guide.

MrSri G.Subrahmanya Sharma, B.Tech., Prof.Dr.S.Balaji(Ph.D)Lecturer and Project Guide. Head of the Department

ACKNOWLEDGEMENT

We take this opportunity to remember and acknowledge the co-operation, good will and

support both moral and technical extended by several individuals out of which this project

has evolved. We always cherish our association with them.

We are greatly elated and thankful to our Head of the Department Dr.S.BALAJI for his

enthusiastic assistance and inspiring us all the way and for arranging all the facilities and

resources needed for our project.

It is with immense pleasure that we would like to express our indebted gratitude to our guides

MrN.V.K.Ramesh and MrG.Subrahmanya Sharmawho has also guided us a lot and

encouraged us in every step of the project work. Her invaluable moral support and guidance

throughout the project helped us to a greater extent.

We are also thankful to Dr.L.S.S.REDDY, PRINCIPAL of our college for his interest in our

academic performance.

We are very grateful to God for having us with his wisdom and knowledge and guidance

through out this work. We are grateful to our family members for their moral and financial

support.

Finally, our verbal abilities limit the expression of heartfelt feelings towards our non-teaching

staff and friends who had directly or indirectly helped and supported us in completing our

project in time.

With profound gratitude,

A.Nafees Ahmed(Y7EM249)

R.Swetha(Y7EM310)

Sk.Ibrahim(Y7EM291)

DECLARATION

I here by declare that this Mini Project entitled “LASER TORCH-BASED VOICE

TRANSMITTER AND RECEIVER” submitted by us is our original project work. It does

not form part of any previous project work , thesis or report to the college or any other

college.

A.Nafees Ahmed (Y7EM249)

R.Swetha (Y7EM310)

Sk.Ibrahim(Y7EM291)

CONTENTS

TOPIC PAGE NO

CERTIFICATE

ACKNOWLEDGEMENT

DECLARATION

ABSTRACT

1.CIRCUIT DESCRIPTION

1.1 INTRODUCTION

1.2 CIRCUIT DIAGRAM

1.3 COMPONENTS LIST

2.IC PIN DESCRIPTION

2.1 IC 555

2.2 IC 741

2.3 IC UM3561

3. CIRCUIT ELEMENTS

3.1 RESISTORS

3.2 CAPACITORS

3.3 TRANSISTORS

3.4 LIGHT EMITTING DIODES

3.4 ZENER DIODE

4.PHOTOS

5.CONCLUSION AND FUTURE SCOPE

6.REFERENCES

ABSTRACT

Our mini-project is to design and check the functionality of ‘SHADOW

ALARM’ circuit . Shadow Alarm is opto-sensitive circuit that sounds an alarm

whenever a shadow falls on it .Now a days it is widely used in aspects of

security systems ,where security is our main concern .So it can be used at night

by shopkeepers to protect the valuables in their showrooms. It can also be used

to provide security at warehouses(go-downs) where storage and protection of

various types of goods is main concern, and works good for home-security too.

A dim lighting in the room is necessary to detect the moving shadow. Unlike

opto-interruption alarms based on light-dependent resistors (LDRs), it does not

require an aligned light beam to illuminate the photo-sensor.

1.CIRCUIT DESCRIPTION

1.1.INTRODUCTION

This opto-sensitive circuit sounds an alarm whenever a shadow falls on it. So it can be used

at night by shopkeepers to protect the valuables in their showrooms. A dim light in the room

is necessary to detect the moving shadow. Unlike opto interruption alarms based on light-

dependent resistors (LDRs), it does not require an aligned light beam to illuminate the photo-

sensor.

The circuit is powered by a 9V PP3 battery and uses the most sensitive photo-sensor L14F1

to detect shadows. It is portable and can be used at any place that is to be monitored. Op-amp

μA741 (IC1) is used as a voltage comparator. Its inverting input is biased by the voltage

obtained from the junction of 100k resistor R1 and the collector of phototransistor T1. The

non-inverting input of IC1 gets a controlled voltage from potential divider R2 and VR1. In

the presence of ambient light, the phototransistor conducts and the inverting input (pin 2) of

IC1 gets a lower voltage than its non-inverting input (pin 3). This makes the output of IC1

high, which is indicated by the glowing of LED1. When a shadow falls on the photo sensor,

the output of IC1 goes low. This low pulse triggers the monostable (IC2) designed for a delay

of 51 seconds using R6 and C3. The output of IC2 is used to light up LED2 and activate the

alarm. Slide switch S2 is used to select either the buzzer or siren. When it is towards left the

buzzer beeps, and when it is towards right IC UM3561 (IC3) activates to give a loud alarm

simulating a police siren. Resistor R8 and zener diode ZD1 provide 3.1V DC to IC UM3561.

1.2.CIRCUIT DIAGRAM

1.3.COMPONENTS LIST:

1. IC’s

1. IC1 741 1No

2. IC2 NE555 1No

3. IC3 UM3561 1No

2.TRANSISTORS

1. T1 L14F1 1No

2. T2 BC548 1No

3.RESISTORS

1. 10 2 No’s

2. 100K 1No

3. 4.7K 1No

4. 100 Ohm 1No

5. 680 Ohm 1No

6. 47 Ohm 1No

7. 1 Megaohms 1No

8. 220 K 1No

9. 1 K 1No

4. CAPACITORS

1. 10UF 1No

2. 0.01UF 1No

3. 47UF

5.PRESET 47K 1No

6.LED 2No’S

7.ZENER DIODE 3.1V 1No

8.SPEAKER 8Ohms/0.5w 1No

2. IC PIN DESCRIPTION

2.1 IC 7555 PIN DIAGRAM

The ICM7555 is a CMOS timer providing significantly improved performance over the standard NE/SE555 timer, while at the same time being a direct replacement for those devices in most applications. The ICM7555 is a stable controller capable of producing accurate time delays or frequencies.

PIN DIAGRAM

FEATURES:• Exact equivalent in most applications for NE/SE555• Low supply current: 80mA (typ)• Extremely low trigger, threshold, and reset currents: 20pA (typ)• High-speed operation: 500kHz guaranteed• Wide operating supply voltage range guaranteed 3 to 16V over full automotive temperatures• Normal reset function; no crowbarring of supply during output transition• Can be used with higher-impedance timing elements than the bipolar 555 for longer time constants• Timing from microseconds through hours• Operates in both astable and monostable modes• Adjustable duty cycle• High output source/sink driver can drive TTL/CMOS• Typical temperature stability of 0.005%/oC at 25°C• Rail-to-rail outputs

OPERATION

ASTABLE OPERATION

The circuit can be connected to trigger itself and free run as a multivibrator, see Figure 2A. The output swings from rail to rail, and is a true 50% duty cycle square wave. (Trip points and output swings are symmetrical.) Less than a 1%frequency variation is observed over a voltage range of +5V to +15V.The duty cycle is controlled by the values of RA and RB,

MONOSTABLE OPERATION

In this mode of operation, the timer functions as a one-shot.See Figure 3. Initially the external capacitor (C) is held discharged by a transistor inside the timer. Upon application of a negative TRIGGER pulse to pin 2, the internal flip-flop is set which releases the short circuit across the external capacitor and drives the OUTPUT high. The voltage across the capacitor

now increases exponentially with a time constant t = RAC.When the voltage across the capacitor equals 2/3 V+, the comparator resets the flip-flop, which in turn discharges the capacitor rapidly and also drives the OUTPUT to its low state.TRIGGER must return to a high state before the OUTPUT canreturn to a low state.

APPLICATIONS

• Precision timing• Pulse generation• Sequential timing• Time delay generation• Pulse width modulation• Pulse position modulation• Missing pulse detector

2.2 IC 741 DESCRIPTION :

The Operational Amplifier is probably the most versatile Integrated Circuit available. It is very cheap especially keeping in mind the fact that it contains several hundred components. The most common Op-Amp is the 741 and it is used in many circuits.The OP AMP is a ‘Linear Amplifier’ with an amazing variety of uses. Its main purpose is to amplify (increase) a weak signal - a little like a Darlington Pair.The OP-AMP has two inputs, INVERTING ( - ) and NON-INVERTING (+), and one output at pin 6.

The 741 is used in two ways.1. An inverting amplifier. Leg two is the input and the output is always reversed.In an inverting amplifier the voltage enters the 741 chip through leg two and comes out of the 741 chip at leg six. If the polarity is positive going into the chip, it negative by the time it comes out through leg six. The polarity has been ‘inverted’.2. A non-inverting amplifier. Leg three is the input and the output is not reversed.In a non-inverting amplifier the voltage enters the 741 chip through leg three and leaves the 741 chip through leg six. This time if it is positive going into the 741 then it is still positive coming out. Polarity remains the same.

FEATURES:

Short Circuit ProtectionExcellent Temperature StabilityInternal Frequency CompensationHigh Input Voltage RangeNull of Offset

PIN DIAGRAM:

CONNECTION DIAGRAM:

2.3 IC UM3561A DESCRIPTION:

UM3561 is a low-cost, low-power CMOS LSI designed for use in alarm and toy applications. Since theintegrated circuit includes oscillator and selector circuits, a compact sound module can be constructed with only a few additional components. The M3561 contains a programmed mask ROM to simulate siren sound

FEATURES: Four sounds can be selected Power on reset.

Typical 3V operating voltage A magnetic speaker can be driven by connecting 8-pin DIP package form an NPN transistor

PIN DIAGRAM:

PLAYING MODES:

3. PASSIVE ELEMENTS

3.1 RESISTORS

Resistors (R), are the most commonly used of all electronic components, to the point where they are almost taken for granted. They are "Passive Devices", that is they contain no source of power or amplification but only attenuate or reduce the voltage signal passing through them. When used in DC circuits the voltage drop produced is measured across their terminals as the circuit current flows through them while in AC circuits the voltage and current are both in-phase producing 0o phase shift.

In all Electrical and Electronic circuit diagrams and schematics, the most commonly used resistor symbol is that of a "zig-zag" type line with the value of its resistance given in Ohms, Ω.

RESISTOR SYMBOLThe symbol used in schematic and electrical drawings for a Resistor can either be a "zig-zag" type line or a rectangular box.

RESISTOR TYPES

All modern resistors can be classified into four broad groups;

Carbon Composition Resistor - Made of carbon dust or graphite paste, low wattage values

Film or Cermet Resistor - Made from conductive metal oxide paste, very low wattage values

Wire-Wound Resistors. - Metallic bodies for heat sink mounting, very high wattage ratings

Semiconductor Resistors - High frequency/precision surface mount thin film technology

RESISTOR COLOUR CODE

The resistance value, tolerance, and watt rating of the resistor are generally printed onto the body of the resistor as numbers or letters when the resistor is big enough to read the print, such as large power resistors. When resistors are small such as 1/4W Carbon and Film types, these specifications must be shown in some other manner as the print would be too small to read. So to overcome this, small resistors use coloured painted bands to indicate both their resistive value and their tolerance with the physical size of the resistor indicating its wattage rating. These coloured painted bands are generally known as a Resistors Colour Code.

The Standard Resistor Colour Code Chart.

 

The Resistor Colour Code Table.

Colour Digit Multiplier Tolerance

Black 0 1  

Brown 1 10 ± 1%

Red 2 100 ± 2%

Orange 3 1K  

Yellow 4 10K  

Green 5 100K ± 0.5%

Blue 6 1M ± 0.25%

Violet 7 10M ± 0.1%

Grey 8    

White 9    

Gold   0.1 ± 5%

Silver   0.01 ± 10%

None     ± 20%

VARIABLE RESISTORS

CONSTRUCTION

Variable resistors consist of a resistance track with connections at both ends and a wiper which moves along the track as you turn the spindle. The track may be made from carbon, cermet (ceramic and metal mixture) or a coil of wire (for low resistances). The track is usually rotary but straight track versions, usually called sliders, are also available.

Variable resistors may be used as a rheostat with two connections (the wiper and just one end of the track) or as a potentiometer with all three connections in use. Miniature versions called presets are made for setting up circuits which will not require further adjustment.

Variable resistors are often called potentiometers in books and catalogues. They are specified by their maximum resistance, linear or logarithmic track, and their physical size. The standard spindle diameter is 6mm.

RHEOSTAT

This is the simplest way of using a variable resistor. Two terminals are used: one connected to an end of the track, the other to the moveable wiper. Turning the spindle changes the resistance between the two terminals from zero up to the maximum resistance. 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. 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

Rheostat Symbol 

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.

PRESET(OPEN STYLE) MULTITURN PRESET PRESET(CLOSED STYLE)

3.2 CAPACITORS

INTRODUCTION

Just like the Resistor, the Capacitor or sometimes referred to as a Condenser is a passive device, and one which stores energy in the form of an electrostatic field which produces a potential (Static Voltage) across its plates. When a voltage is applied to these plates, a current flows charging up the plates with electrons giving one plate a positive charge and the other plate an equal and opposite negative charge. This flow of electrons to the plates is known as the Charging Current and continues to flow until the voltage across the plates (and hence the capacitor) is equal to the applied voltage Vc. At this point the capacitor is said to be fully charged and this is illustrated below.

Capacitor Construction

Q = C x V

UNITS OF CAPACITANCE Microfarad  (μF)   1μF = 1/1,000,000 = 0.000001 = 10-6 F Nanofarad  (nF)   1nF = 1/1,000,000,000 = 0.000000001 = 10-9 F  Picofarad  (pF)   1pF = 1/1,000,000,000,000 = 0.000000000001 = 10-12 F

TYPES OF CAPACITORS

There are a very large variety of different types of Capacitors available in the market place and each one has its own set of characteristics and applications from small delicate trimming capacitors up to large power metal can type capacitors used in high voltage power correction and smoothing circuits.

1.DIELECTRIC Dielectric Capacitors are usually of the variable type such as used for tuning transmitters, receivers and transistor radios. They have a set of fixed plates and a set of moving plates that mesh with the fixed plates and the position of the moving plates with

respect to the fixed plates determines the overall capacitance. The capacitance is generally at maximum when the plates are fully meshed.

Variable Capacitor Symbols

As well as the continuously variable types, preset types are also available called Trimmers. These are generally small devices that can be adjusted or "pre-set" to a particular capacitance with the aid of a screwdriver and are available in very small capacitances of 100pF or less and are non-polarized.

2. FILM CAPACITORS Film Capacitors are the most commonly available of all types of capacitors, consisting of a relatively large family of capacitors with the difference being in their dielectric properties. These include polyester (Mylar), polystyrene, polypropylene, polycarbonate, metallized paper, teflon etc. Film type capacitors are available in capacitance ranges from 5pF to 100uF depending upon the actual type of capacitor and its voltage rating. Film capacitors also come in an assortment of shapes and case styles which include:

Wrap & Fill (Oval & Round)  Epoxy Case (Rectangular & Round)  Metal Hermetically Sealed (Rectangular & Round) 

3. CERAMIC CAPACITORS

Ceramic Capacitors or Disc Capacitors as they are generally called, are made by coating two sides of a small porcelain or ceramic disc with silver and are then stacked together to make a capacitor. For very low capacitance values a single ceramic disc of about 3-6mm is used. Ceramic capacitors have a high dielectric constant (High-K) and are available so that relatively high capacitances can be obtained in a small physical size. They exhibit large non-linear changes in capacitance against temperature and as a result are used as de-coupling or by-pass capacitors as they are also non-polarized devices. Ceramic capacitors have values ranging from a few picofarads to one or two microfarads but their voltage ratings are generally quite low.

Ceramic types of capacitors generally have a 3-digit code printed onto their body to identify their capacitance value. For example, 103 would indicate 10 x 103 PF

4. ELECTROLYTIC CAPACITORS

Electrolytic Capacitors are generally used when very large capacitance values are required. Here instead of using a very thin metallic film layer for one of the electrodes, a semi-liquid electrolyte solution in the form of a jelly or paste is used which serves as the second electrode (usually the cathode). The dielectric is a very thin layer of oxide which is grown electro-chemically in production with the thickness of the film being less than ten microns. This insulating layer is so thin that it is possible to make large value capacitors of a small size. The

majority of electrolytic types of capacitors are Polarized, that is the voltage applied to the capacitor terminals must be of the correct polarity as an incorrect polarization will break down the insulating oxide layer and permanent damage may result.

Electrolytic Capacitors are generally used in DC power supply circuits to help reduce the ripple voltage or for coupling and decoupling applications. Electrolytic's generally come in two basic forms; Aluminum Electrolytic and Tantalum Electrolytic capacitors.

3.3.TRANSISTOR

A transistor is a semiconductor device used to amplify and switch electronic signals. It is made of a solid piece of semiconductor material, with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current flowing through another pair of terminals. Because the controlled (output) power can be much more than the controlling (input) power, the transistor provides amplification of a signal. Some transistors are packaged individually but many more are found embedded in integrated circuits.

PIN CONFIGURATION:-

L14F1

TRANSISTOR CHARACTERSTICS

ADVANTAGES

The key advantages that have allowed transistors to replace their vacuum tube predecessors in most applications are

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. Lower power dissipation and generally greater energy efficiency. Higher reliability and greater physical ruggedness. Extremely long life. Some transistorized devices have been in service for more than

30 years.

LIMITATIONS Silicon transistors do not operate at voltages higher than about 1,000 volts SiC

devices can be operated as high as 3,000 volts). In contrast, electron tubes have been developed that can be operated at tens of thousands of volts.

High power, high frequency operation, such as that used in over-the-air television broadcasting, is better achieved in electron tubes due to improved electron mobility in a vacuum.

Silicon transistors are much more sensitive than electron tubes to an electromagnetic pulse, such as generated by an atmospheric nuclear explosion.

3.4.LIGHT EMITTING DIODES:

A light-emitting diode (LED) is a semiconductor light source. The LED is based on the semiconductor diode. When a diode is forward biased (switched on), electrons are able to recombine with holes within the device, releasing energy in the form of photons. This effect is called electroluminescence and the color of the light (corresponding to the energy of the photon) is determined by the energy gap of the semiconductor.

In a diode formed from a direct band-gap semiconductor, such as gallium arsenide, carriers that cross the junction emit photons when they recombine with the majority carrier on the other side. Depending on the material, wavelengths (or colors) from the infrared to the near ultraviolet may be produced. The forward potential of these diodes depends on the wavelength of the emitted photons: 1.2 V corresponds to red, 2.4 V to violet. The first LEDs were red and yellow, and higher-frequency diodes have been developed over time. All LEDs produce incoherent, narrow-spectrum light; “white” LEDs are actually combinations of three LEDs of a different color, or a blue LED with a yellow scintillator coating. LEDs can also be used as low-efficiency photodiodes in signal applications. An LED may be paired with a photodiode or phototransistor in the same package, to form an opto-isolator.

Like a normal diode, the LED consists of a chip of semiconducting material doped with impurities to create a p-n junction. As in other diodes, current flows easily from the p-side, or anode, to the n-side, or cathode, but not in the reverse direction. Charge-carriers—electrons and holes—flow into the junction from electrodes with different voltages. When an electron meets a hole, it falls into a lower energy level, and releases energy in the form of a photon.The wavelength of the light emitted, and therefore its color, depends on the band gap energy of the materials forming the p-n junction. In silicon or germanium diodes, the electrons and holes recombine by a non-radiativetransition which produces no optical emission, because these are indirect band gap materials. The materials used for the LED have a direct band gap with energies corresponding to near-infrared, visible or near-ultraviolet light.

Colors and Materials:

Color Wavelength Voltage (V) Semiconductor Material

(nm)

Infrared λ > 760 ΔV < 1.9Gallium arsenide (GaAs)Aluminium gallium arsenide (AlGaAs)

Red 610 < λ < 7601.63 < ΔV < 2.03

Aluminium gallium arsenide (AlGaAs)Gallium arsenide phosphide (GaAsP)Aluminium gallium indium phosphide (AlGaInP)Gallium(III) phosphide (GaP)

Orange 590 < λ < 6102.03 < ΔV < 2.10

Gallium arsenide phosphide (GaAsP)Aluminium gallium indium phosphide (AlGaInP)Gallium(III) phosphide (GaP)

Yellow 570 < λ < 5902.10 < ΔV < 2.18

Gallium arsenide phosphide (GaAsP)Aluminium gallium indium phosphide (AlGaInP)Gallium(III) phosphide (GaP)

Green 500 < λ < 5701.9[37] < ΔV < 4.0

Indium gallium nitride (InGaN) / Gallium(III) nitride (GaN)Gallium(III) phosphide (GaP)Aluminium gallium indium phosphide (AlGaInP)Aluminium gallium phosphide (AlGaP)

Blue 450 < λ < 5002.48 < ΔV < 3.7

Zinc selenide (ZnSe)Indium gallium nitride (InGaN)Silicon carbide (SiC) as substrateSilicon (Si) as substrate — (under development)

Violet 400 < λ < 4502.76 < ΔV < 4.0

Indium gallium nitride (InGaN)

Purple multiple types 2.48 < ΔV < 3.7

Dual blue/red LEDs,blue with red phosphor,

or white with purple plastic

Ultraviolet λ < 4003.1 < ΔV < 4.4

Diamond (235 nm)[38]

Boron nitride (215 nm)[39][40]

Aluminium nitride (AlN) (210 nm)[41]

Aluminium gallium nitride (AlGaN)Aluminium gallium indium nitride (AlGaInN) — (down to 210 nm)[42]

WhiteBroad spectrum

ΔV = 3.5 Blue/UV diode with yellow phosphor  

ADVANTAGES

Efficiency Size On/Off time Cycling Dimming Cool light Slow failure Lifetime

DISADVANTAGES

High initial price Temperature dependence Voltage sensitivity Light quality. Area light source Blue Blue pollution

APPLICATIONS

Application of LEDs fall into four major categories:

Visual signal application where the light goes more or less directly from the LED to the human eye, to convey a message or meaning.

Illumination where LED light is reflected from object to give visual response of these objects.

Generate light for measuring and interacting with processes that do not involve the human visual system.

Narrow band light sensors where the LED is operated in a reverse-bias mode and is responsive to incident light instead of emitting light.

3.5. ZENER DIODES :

Zener diode is a type of diode that permits current not only in the forward direction like a normal diode, but also in the reverse direction if the voltage is larger than the breakdown voltage known as "Zener knee voltage" or "Zener voltage".

A conventional solid-state diode will not allow significant current if it is reverse-biased below its reverse breakdown voltage. When the reverse bias breakdown voltage is exceeded, a conventional diode is subject to high current due to avalanche breakdown. Unless this current is limited by circuitry, the diode will be permanently damaged. In case of large forward bias (current in the direction of the arrow), the diode exhibits a voltage drop due to its junction built-in voltage and internal resistance. The amount of the voltage drop depends on the semiconductor material and the doping concentrations.

A Zener diode exhibits almost the same properties, except the device is specially designed so as to have a greatly reduced breakdown voltage, the so-called Zener voltage. By contrast with the conventional device, a reverse-biased Zener diode will exhibit a controlled breakdown and allow the current to keep the voltage across the Zener diode at the Zener voltage. The Zener diode is therefore ideal for applications such as the generation of a reference voltage (e.g. for an amplifier stage), or as a voltage stabilizer for low-current applications.

The Zener diode's operation depends on the heavy doping of its p-n junction allowing electrons to tunnel from the valence band of the p-type material to the conduction band of the n-type material. In the atomic scale, this tunneling corresponds to the transport of valence band electrons into the empty conduction band states; as a result of the reduced barrier between these bands and high electric fields that are induced due to the relatively high levels of dopings on both sides. The breakdown voltage can be controlled quite accurately in the doping process. While tolerances within 0.05% are available, the most widely used tolerances are 5% and 10%. Breakdown voltage for commonly available zener diodes can vary widely from 1.2 volts to 200 volts.

.

4.PHOTOS

5.CONCLUSION&FUTURE SCOPE

6.REFERENCES

www.electronicsforyou.com

www.howstuffworks.com

www.wikipedia.org

www.circuits-today.com

www.freedatasheets.com