Traffic Controller Using Micro Controller

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A Major Project Report On Traffic Light using Microcontroller Submitted in partial fulfillment of the requirement for the award of the Diploma of Electronics & Communication Engg. Submitted To: Submitted by: Ms. Simranjeet Kaur Vinod Kumar (110746185056)

Transcript of Traffic Controller Using Micro Controller

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A Major Project Report

On

Traffic Light using Microcontroller

Submitted in partial fulfillment of the requirement for

the award of the Diploma

of

Electronics & Communication Engg.

Submitted To: Submitted by:Ms. Simranjeet Kaur Vinod Kumar (110746185056)(HOD) ECE Deepak (110746121712)

Gautam Kumar (110746185039)

Department of Electronics & Communication Engg.

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Universal Group of Polytechnic CollegesLalru Mandi, Dsitt. Mohali (Pb.)

ACKNOWLEDGEMENT

We are highly grateful to Mrs. Simran Kaur (HOD) ECE department, Universal Group of

College, Lalru Mandi (Mohali), for providing this opportunity to carry out to develop this

project at Universal Group of College, Lalru Mandi (Mohali). We would like to expresses my

gratitude to other faculty members of Electronics & Communication Engineering department

of Universal Group of College, for providing academic inputs, guidance & encouragement

throughout the project development period.

The author would like to express a deep sense of gratitude and thank Er Mrs. Simran Kaur

(HOD) ECE department without whose permission, wise counsel and able guidance, it would

have not been possible to pursue my project in this manner.

Finally, We express my indebtedness to all who have directly or indirectly contributed to the

successful completion of my major project report.

Vinod Kumar Roll No. 110746185056

Deepak Roll No. 110746121712

Gautam Kumar Roll No. 110746185039

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COMPONENTS REQUIRED

1. PIC 16F72 MICROCONTROLLER

2. MICROSWITCHES 1

3. IN 4007 DIODES 4

4. 1000 UF ELECTROLYRIC CAP 1

5. 28 PIN BASE 1

6. LA 7805 REGULATOR IC 1

7. RESISTOR 10 K OHM 2

8. RESISTOR 330 OHM 1

9. TRANSFORMER 9-0-9 1

10. CRYSTAL 4 MHZ 1

11. 33 PF CAP 2

12. GENERAL PURPOSE PCBS

13. MAIN LEAD

14. CONNECTING WIRES

15. TWO PIN CONNECTOR

16. BURG STRIPS

17. 10K POTENTIOMETERS 2

18. NUT BOLTS(1/8,1 “)

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Block Diagram

MicrocontrollerPower Supply

5 volts

7-SEGMENTS

Temp sensor

LM 35

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POWER SUPPLY

The detailed working of bridge wave rectifier

The full wave bridge rectifier is designed to convert an ac sine-wave to a full-wave pulsating

DC signal.

The bridge is normally connected to the secondary of a transformer. Here circuit will be

performed using conventional current flow.

Current will flow from higher potential to lower potential

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MICROCONTROLLERS

A microcontroller is a small computer on a single integrated circuit containing a processor

core, memory, and programmable input/output peripherals. Program memory in the form of

NOR flashor OTP ROM is alsooften included on chip, as well as a typically small amount of

RAM. Microcontrollers are designed for embedded applications, in contrast to the

microprocessors used in personal computersor other general purpose applications.

USES OF MICROCONTROLLERS

Microcontrollers are used in automatically controlled products and devices, such as

automobile engine control systems, implantable medical devices, remote controls, office

machines, appliances, power tools, and toys. By reducing the size and cost compared to a

design that uses a separate microprocessor, memory, and input/output devices,

microcontrollers make it economical to digitally control even more devices and processes.

Mixed signal microcontrollers are common, integrating analog components needed to control

non-digital electronic systems.

WORKING AT LOW POWERS

Some microcontrollers may use four-bit words and operate at clock rate frequencies as low as

4 kHz, for low power consumption (milliwattsor microwatts). They will generally have the

ability to retain functionality while waiting for an event such as a button press or other

interrupt; power consumption while sleeping (CPU clock and most peripherals off) may be

just nanowatts, making many of them well suited for long lasting battery applications. other

microcontrollers may serve performance-critical roles, where they may need to act more like

a digital signal processor (DSP), with higher clock speeds and power consumption.

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Embedded design

A microcontroller can be considered a self-contained system with a processor, memory and

peripherals and can be used as an embedded system. [1] The majority of microcontrollers in

use today are embedded in other machinery, such as automobiles, telephones, appliances, and

peripherals for computer systems. These are called embedded systems. While some

embedded systems are very sophisticated, many have minimal requirements for memory and

program length, with nooperating system, and low software complexity.

INTERFACING

Typical input and output devices include switches, relays, solenoids, LEDs, LCD ,

GRAPHIC LCD displays, radio frequency devices, and sensors for data such as temperature,

humidity, light level etc. Embedded systems usually have no keyboard, screen, disks,

printers, or other recognizable I/o devices of a personal computer, and may lack human

interaction devices of any kind.

INTERRUPTS

Microcontrollers must provide real time (predictable, though not necessarily fast) response to

events in the embedded system they are controlling. When certain events occur, an interrupt

system can signal the processor to suspend processing the current instruction sequence and to

begin an interrupt service routine (ISR, or "interrupt handler"). The ISR will perform any

processing required based on the source of the interrupt before returning to the original

instruction sequence. Possible interrupt sources are device dependent, and often include

events such as an internal timer overflow, completing an analog to digital conversion, a logic

level change on an input such as from a button being pressed, and data received on a

communication link. Where power consumption is important as in battery operated devices,

interrupts may also wake a microcontroller from a low power sleep state where the processor

is halted until required to do something by a peripheral event.

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Programs

Microcontroller programs must fit in the available on-chip program memory, since it would

be costly to provide a system with external, expandable, memory. Compilers and assemblers

are used to turn high-level language and assembler language codes into a compact machine

code for storage in the microcontroller's memory. Depending on the device, the program

memory may be permanent, read-only memory that can only be programmed at the factory,

or program memory may be field-alterable flash or erasable read-only memory.

Reading sensors

Many embedded systems need to read sensors that produce analog signals. This is the

purpose of the analog-to-digital converter (ADC). Since processors are built to interpret and

process digital data, i.e. 1s and os, they won't be able to doanything with the analog signals

that may be sent to it by a device. So the analog to digital converter is used to convert the

incoming data into a form that the processor can recognize. A less common feature on some

microcontrollers is a digital-to-analog converter (DAC) that allows the processor tooutput

analog signals or voltage levels.

TIMERS

In addition to the converters, many embedded microprocessors include a variety of timers as

well. oneof the most common types of timers is the Programmable Interval Timer (PIT). A

PIT just counts down from some value to zero. once it reaches zero, it sends an interrupt to

the processor indicating that it has finished counting. This is useful for devices such as

thermostats, which periodically test the temperature around them to see if they need to turn

the air conditioner on, the heater on, etc.

Time Processing Unit (TPU) is a sophisticated timer. In addition to counting down, the TPU

can detect input events, generate output events, and perform other useful operations.

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PWM

A dedicated Pulse Width Modulation (PWM) block makes it possible for the CPU to control

power converters, resistive loads, motors, etc., without using lots of CPU resources in tight

timer loops.

Universal Asynchronous Receiver/Transmitter (UART) block makes it possible to receive

and transmit data over a serial line with very little load on the CPU. Dedicated on-chip

hardware alsooften includes capabilities to communicate with other devices (chips) in digital

formats such as I2C and Serial Peripheral Interface (SPI).

Higher integration

In contrast to general-purpose CPUs, micro-controllers may not implement an external

address or data bus as they integrate RAM and non-volatile memory on the same chip as the

CPU. Using fewer pins, the chip can be placed in a much smaller, cheaper package.

Integrating the memory and other peripherals on a single chip and testing them as a unit

increases the cost of that chip, but often results in decreased net cost of the embedded system

as a whole. Even if the cost of a CPU that has integrated peripherals is slightly more than the

cost of a CPU and external peripherals, having fewer chips typically allows a smaller and

cheaper circuit board, and reduces the labor required to assemble and test the circuit board.

A micro-controller is a single integrated circuit, commonly with the following features:

central processing unit - ranging from small and simple 4-bit processors to complex

32- or 64-bit processors

discrete input and output bits, allowing control or detection of the logic state of an

individual package pin

serial input/output such as serial ports (UARTs)

other serial communications interfaces like I²C, Serial Peripheral Interface and

Controller Area Network for system interconnect

peripherals such as timers, event counters, PWM generators, and watchdog

volatile memory (RAM) for data storage

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A typical home in a developed country is likely to have only four general-purpose

microprocessors but around three dozen microcontrollers. A typical mid-range automobile

has as many as 3oor more microcontrollers. They can also be found in many electrical device

such as washing machines, microwave ovens, and telephones.

A PIC 18F872omicrocontroller in an 8o-pin TQFP package.

Manufacturers have often produced special versions of their microcontrollers in order to help

the hardware and software developmentof the target system. originally these included

EPRoM versions that have a "window" on the top of the device through which program

memory can be erased by ultraviolet light, ready for reprogramming after a programming

("burn") and test cycle. Since 1998, EPRoM versions are rare and have been replaced by

EEPRoM and flash, which are easier to use (can be erased electronically) and cheaper to

manufacture.

other versions may be available where the RoM is accessed as an external device rather than

as internal memory, however these are becoming increasingly rare due to the widespread

availability of cheap microcontroller programmers.

The use of field-programmable devices on a microcontroller may allow field update of the

firmwareor permit late factory revisions to products that have been assembled but not yet

shipped. Programmable memory also reduces the lead time required for deployment of a new

product.

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Where hundreds of thousands of identical devices are required, using parts programmed at

the time of manufacture can be an economical option. These 'mask programmed' parts have

the program laid down in the same way as the logic of the chip, at the same time.

[.] Programming environments

Microcontrollers were originally programmed only in assembly language, but various high-

level programming languages are now also in common use to target microcontrollers. These

languages are either designed specially for the purpose, or versions of general purpose

languages such as the C programming language. Compilers for general purpose languages

will typically have some restrictions as well as enhancements to better support the unique

characteristics of microcontrollers. Some microcontrollers have environments to aid

developing certain types of applications. Microcontroller vendors often make tools freely

available to make it easier to adopt their hardware.

Many microcontrollers are so quirky that they effectively require their own non-standard

dialects of C, such as SDCC for the 8o51, which prevent using standard tools (such as code

libraries or static analysis tools) even for code unrelated to hardware features. Interpreters are

often used to hide such low level quirks.

Interpreter firmware is also available for some microcontrollers. For example, BASICon the

early microcontrollers Intel 8o52 [4]; BASIC and FoRTHon the Zilog Z8[5] as well as some

modern devices. Typically these interpreters support interactive programming.

Simulators are available for some microcontrollers, such as in Microchip's MPLAB

environment. These allow a developer to analyze what the behavior of the microcontroller

and their program should be if they were using the actual part. A simulator will show the

internal processor state and also that of the outputs, as well as allowing input signals to be

generated. While on the one hand most simulators will be limited from being unable to

simulate much other hardware in a system, they can exercise conditions that may otherwise

be hard to reproduce at will in the physical implementation, and can be the quickest way to

debug and analyze problems.

Recent microcontrollers are often integrated with on-chip debug circuitry that when accessed

by an in-circuit emulator via JTAG, allow debugging of the firmware with a debugger

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Types of microcontrollers

See also: List of common microcontrollers

As of 2oo8 there are several dozen microcontroller architectures and vendors including:

68HC11

8o51

ARM processors (from many vendors) using ARM7or Cortex-M3 cores are generally

microcontrollers

STMicroelectronics STM8S (8-bit), ST1o (16-bit) and STM32 (32-bit)

Atmel AVR (8-bit), AVR32 (32-bit), and AT91SAM

Freescale ColdFire (32-bit) and So8 (8-bit)

Hitachi H8, Hitachi SuperH

Hyperstone E1/E2 (32-bit, First full integration of RISC and DSPon one processor

core [1996] [1])

MIPS (32-bit PIC32)

NEC V85o

PIC (8-bit PIC16, PIC18, 16-bit dsPIC33 / PIC24)

PowerPC ISE

PSoC (Programmable System-on-Chip)

Rabbit 2ooo

Texas Instruments Microcontrollers MSP43o (16-bit), C2ooo (32-bit), and Stellaris

(32-bit)

Toshiba TLCS-87o

Zilog eZ8, eZ8o

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History

The first single-chip microprocessor was the 4-bit Intel 4oo4 released in 1971, headed by

Intels lead research scientist Hunter H. Hetfeld. With the Intel 8oo8 and more capable

microprocessors available over the next several years.

These however all required external chip(s) to implement a working system, raising total

system cost, and making it impossible to economically computerise appliances.

The first computer system on a chip optimised for control applications - microcontroller was

the Intel 8o48 released in 1975[citation needed], with both RAM and RoMon the same chip. This

chip would find its way intoover one billion PC keyboards, and other numerous applications.

At this time Intels President, Luke J. Valenter, stated that the (Microcontroller) was one of

the most successful in the companies history, and expanded the division's budget over 25%.

Most microcontrollers at this time had two variants. one had an erasable EEPRoM program

memory, which was significantly more expensive than the PRoM variant which was only

programmable once.

In 1993, the introduction of EEPRoM memory allowed microcontrollers (beginning with the

Microchip PIC16x84) to be electrically erased quickly without an expensive package as

required for EPRoM, allowing both rapid prototyping, and In System Programming.

The same year, Atmel introduced the first microcontroller using Flash memory. [6].

other companies rapidly followed suit, with both memory types.

Cost has plummeted over time, with the cheapest 8-bit microcontrollers being available for

under $o.25 in quantity (thousands) in 2oo9, and some 32-bit microcontrollers around $1 for

similar quantities.

Nowadays microcontrollers are low cost and readily available for hobbyists, with large online

communities around certain processors.

In the future, MRAM could potentially be used in microcontrollers as it has infinite

endurance and its incremental semiconductor wafer process cost is relatively low.

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COMPONENTS DETAIL

Capacitor

From Wikipedia, the free encyclopedia

This article is about the electronic component. For the physical phenomenon, see capacitance.

For an overview of various kinds of capacitors, see types of capacitor.

Capacitor

Modern capacitors, by a cm rule

Type Passive

Invented Ewald Georg von Kleist (october 1745)

Electronic symbol

A typical electrolytic capacitor

A capacitor (formerly known as condenser) is a device for storing electric charge. The

forms of practical capacitors vary widely, but all contain at least two conductors separated by

a non-conductor. Capacitors used as parts of electrical systems, for example, consist of metal

foils separated by a layer of insulating film.

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A capacitor is a passive electronic component consisting of a pair of conductors separated by

a dielectric (insulator). When there is a potential difference (voltage) across the conductors, a

static electric field develops across the dielectric, causing positive charge to collect on one

plate and negative charge on the other plate. Energy is stored in the electrostatic field. An

ideal capacitor is characterized by a single constant value, capacitance, measured in farads.

This is the ratioof the electric chargeon each conductor to the potential difference between

them.

Capacitors are widely used in electronic circuits for blocking direct current while allowing

alternating current to pass, in filter networks, for smoothing the output of power supplies, in

the resonant circuits that tune radios to particular frequencies and for many other purposes.

The capacitance is greatest when there is a narrow separation between large areas of

conductor, hence capacitor conductors are often called "plates," referring to an early means of

construction. In practice the dielectric between the plates passes a small amount of leakage

current and also has an electric field strength limit, resulting in a breakdown voltage, while

the conductors and leads introduce an undesired inductance and resistance.

Diode(Rectifier)

A rectifier is an electrical device that converts alternating current (AC) todirect current (DC),

a process known as rectification. Rectifiers have many uses including as components of

power supplies and as detectorsof radio signals. Rectifiers may be made of solid state diodes ,

vacuum tube diodes, mercury arc valves, and other components.

A device which performs the opposite function (converting DC to AC) is known as an

inverter.

When only one diode is used to rectify AC (by blocking the negative or positive portion of the

waveform), the difference between the term diode and the term rectifier is merely one of

usage, i.e., the term rectifier describes a diode that is being used to convert AC to DC. Almost

all rectifiers comprise a number of diodes in a specific arrangement for more efficiently

converting AC to DC than is possible with only one diode. Before the development of silicon

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semiconductor rectifiers, vacuum tube diodes and copper(I) oxideor selenium rectifier stacks

were used.

Early radio receivers, called crystal radios, used a "cat's whisker" of fine wire pressing on a

crystal of galena (lead sulfide) to serve as a point-contact rectifier or "crystal detector".

Rectification may occasionally serve in roles other than to generate D.C. current per se. For

example, in gas heating systems flame rectification is used to detect presence of flame. Two

metal electrodes in the outer layer of the flame provide a current path, and rectification of an

applied alternating voltage will happen in the plasma, but only while the flame is present to

generate it.

Half-wave rectification

In half wave rectification, either the positive or negative half of the AC wave is passed, while

the other half is blocked. Because only one half of the input waveform reaches the output, it is

very inefficient if used for power transfer. Half-wave rectification can be achieved with a

single diode in a one-phase supply, or with three diodes in a three-phase supply.

The output DC voltage of a half wave rectifier can be calculated with the following two ideal

equations:

Full-wave rectification

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A full-wave rectifier converts the whole of the input waveform toone of constant polarity

(positive or negative) at its output. Full-wave rectification converts both polarities of the input

waveform to DC (direct current), and is more efficient. However, in a circuit with a non-

center tapped transformer, four diodes are required instead of the one needed for half-wave

rectification. (See semiconductors, diode). Four diodes arranged this way are called a diode

bridgeor bridge rectifier:

Graetz bridge rectifier: a full-wave rectifier using 4 diodes.

For single-phase AC, if the transformer is center-tapped, then two diodes back-to-back (i.e.

anodes-to-anode or cathode-to-cathode) can form a full-wave rectifier. Twice as many

windings are required on the transformer secondary toobtain the same output voltage

compared to the bridge rectifier above.

Full-wave rectifier using a transformer and 2 diodes.

Resistor

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A typical axial-lead resistor

Partially exposed Tesla TR-212 1 kΩ carbon film resistor

Axial-lead resistors on tape. The tape is removed during assembly before the leads are

formed and the part is inserted into the board.

Three carbon composition resistors in a 196os valve (vacuum tube) radio

A linear resistor is a two-terminal, linear, passive electronic component that implements

electrical resistance as a circuit element. The current flowing through a resistor is in a direct

proportion to the voltage across the resistor's terminals. Thus, the ratioof the voltage applied

across resistor's terminals to the intensity of current flowing through the resistor is called

resistance. This relation is represented with a well-known ohm's law:

Units

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The ohm (symbol: Ω) is the SI unit of electrical resistance, named after Georg Simon ohm.

An ohm is equivalent to a volt per ampere. Since resistors are specified and manufactured

over a very large range of values, the derived units of milliohm (1 mΩ = 1o−3 Ω), kilohm (1

kΩ = 1o3 Ω), and megohm (1 MΩ = 1o6 Ω) are also in common usage.

The reciprocal of resistance R is called conductance G = 1/R and is measured in Siemens (SI

unit), sometimes referred to as a mho. Hence, Siemens is the reciprocal of an ohm: S = Ω − 1.

Although the concept of conductance is often used in circuit analysis, practical resistors are

always specified in terms of their resistance (ohms) rather than conductance.

[.] Theory of operation

[.] ohm's law

Main article: ohm's law

The behavior of an ideal resistor is dictated by the relationship specified by ohm's law:

ohm's law states that the voltage (V) across a resistor is proportional to the current (I), where

the constant of proportionality is the resistance (R).

Equivalently, ohm's law can be stated:

This formulation states that the current (I) is proportional to the voltage (V) and inversely

proportional to the resistance (R). This is directly used in practical computations. For

example, if a 3ooohm resistor is attached across the terminals of a 12 volt battery, then a

current of 12 / 3oo = o.o4 amperes (or 4omilliamperes) occurs across that resistor.

[.] Series and parallel resistors

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Main article: Series and parallel circuits

In a series configuration, the current through all of the resistors is the same, but the voltage

across each resistor will be in proportion to its resistance. The potential difference (voltage)

seen across the network is the sum of those voltages, thus the total resistance can be found as

the sum of those resistances:

As a special case, the resistance of N resistors connected in series, each of the same resistance

R, is given by NR.

Resistors in a parallel configuration are each subject to the same potential difference

(voltage), however the currents through them add. The conductancesof the resistors then add

to determine the conductance of the network. Thus the equivalent resistance (Req) of the

network can be computed:

The parallel equivalent resistance can be represented in equations by two vertical lines "||" (as

in geometry) as a simplified notation. For the case of two resistors in parallel, this can be

calculated using:

TRANSFORMER

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A transformer is a static device that transfers electrical energy from one circuit to another

through inductively coupled conductors—the transformer's coils. A varying current in the

first or primary winding creates a varying magnetic flux in the transformer's core and thus a

varying magnetic field through the secondary winding. This varying magnetic field induces a

varying electromotive force (EMF)or "voltage" in the secondary winding. This effect is called

mutual induction.

If a load is connected to the secondary, an electric current will flow in the secondary winding

and electrical energy will be transferred from the primary circuit through the transformer to

the load. In an ideal transformer, the induced voltage in the secondary winding (Vs) is in

proportion to the primary voltage (Vp), and is given by the ratioof the number of turns in the

secondary (Ns) to the number of turns in the primary (Np) as follows:

By appropriate selection of the ratioof turns, a transformer thus allows an alternating current

(AC) voltage to be "stepped up" by making Ns greater than Np, or "stepped down" by making

Ns less than Np.

In the vast majority of transformers, the windings are coils wound around a ferromagnetic

core, air-core transformers being a notable exception.

Transformers range in size from a thumbnail-sized coupling transformer hidden inside a stage

microphone to huge units weighing hundreds of tons used to interconnect portions of power

grids. All operate with the same basic principles, although the range of designs is wide. While

new technologies have eliminated the need for transformers in some electronic circuits,

transformers are still found in nearly all electronic devices designed for household ("mains")

voltage. Transformers are essential for high-voltage electric power transmission, which

makes long-distance transmission economically practical.

RELAY

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A relay is an electrically operated switch. Many relays use an electromagnet to operate a

switching mechanism mechanically, but other operating principles are also used. Relays are

used where it is necessary to control a circuit by a low-power signal (with complete electrical

isolation between control and controlled circuits), or where several circuits must be

controlled by one signal. The first relays were used in long distance telegraph circuits,

repeating the signal coming in from one circuit and re-transmitting it to another. Relays were

used extensively in telephone exchanges and early computers to perform logical operations.

A type of relay that can handle the high power required to directly control an electric motor is

called a contactor. Solid-state relays control power circuits with no moving parts, instead

using a semiconductor device to perform switching. Relays with calibrated operating

characteristics and sometimes multiple operating coils are used to protect electrical circuits

from overload or faults; in modern electric power systems these functions are performed by

digital instruments still called "protective relays".

Basic design and operation

Simple electromechanical relay

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Small relay as used in electronics

A simple electromagnetic relay consists of a coil of wire surrounding a soft iron core, an iron

yoke which provides a low reluctance path for magnetic flux, a movable iron armature, and

one or more sets of contacts (there are two in the relay pictured). The armature is hinged to

the yoke and mechanically linked to one or more sets of moving contacts. It is held in place

by a spring so that when the relay is de-energized there is an air gap in the magnetic circuit.

In this condition, one of the two sets of contacts in the relay pictured is closed, and the other

set is open. other relays may have more or fewer sets of contacts depending on their function.

The relay in the picture also has a wire connecting the armature to the yoke. This ensures

continuity of the circuit between the moving contacts on the armature, and the circuit track on

the printed circuit board (PCB) via the yoke, which is soldered to the PCB.

When an electric current is passed through the coil it generates a magnetic field that attracts

the armature, and the consequent movement of the movable contact(s) either makes or breaks

(depending upon construction) a connection with a fixed contact. If the set of contacts was

closed when the relay was de-energized, then the movement opens the contacts and breaks

the connection, and vice versa if the contacts were open. When the current to the coil is

switched off, the armature is returned by a force, approximately half as strong as the

magnetic force, to its relaxed position. Usually this force is provided by a spring, but gravity

is also used commonly in industrial motor starters. Most relays are manufactured to operate

quickly. In a low-voltage application this reduces noise; in a high voltage or current

application it reduces arcing.

When the coil is energized with direct current, a diode is often placed across the coil to

dissipate the energy from the collapsing magnetic field at deactivation, which would

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otherwise generate a voltage spike dangerous to semiconductor circuit components. Some

automotive relays include a diode inside the relay case. Alternatively, a contact protection

network consisting of a capacitor and resistor in series (snubber circuit) may absorb the

surge. If the coil is designed to be energized with alternating current (AC), a small copper

"shading ring" can be crimped to the end of the solenoid, creating a small out-of-phase

current which increases the minimum pull on the armature during the AC cycle.[1]

A solid-state relay uses a thyristor or other solid-state switching device, activated by the

control signal, to switch the controlled load, instead of a solenoid. An optocoupler (a light-

emitting diode (LED) coupled with a photo transistor) can be used to isolate control and

controlled circuits.

Types

Latching relay

Latching relay with permanent magnet

A latching relay has two relaxed states (bistable). These are also called "impulse", "keep", or

"stay" relays. When the current is switched off, the relay remains in its last state. This is

achieved with a solenoid operating a ratchet and cam mechanism, or by having two opposing

coils with an over-center spring or permanent magnet to hold the armature and contacts in

position while the coil is relaxed, or with a remanent core. In the ratchet and cam example,

the first pulse to the coil turns the relay on and the second pulse turns it off. In the two coil

example, a pulse to one coil turns the relay on and a pulse to the opposite coil turns the relay

off. This type of relay has the advantage that one coil consumes power only for an instant,

while it is being switched, and the relay contacts retain this setting across a power outage. A

remanent core latching relay requires a current pulse of opposite polarity to make it change

state.

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Reed relay

A reed relay is a reed switch enclosed in a solenoid. The switch has a set of contacts inside

an evacuated or inert gas-filled glass tube which protects the contacts against atmospheric

corrosion; the contacts are made of magnetic material that makes them move under the

influence of the field of the enclosing solenoid. Reed relays can switch faster than larger

relays, require only little power from the control circuit, but have low switching current and

voltage ratings. In addition, the reeds can become magnetized over time, which makes them

stick 'on' even when no current is present.

Top, middle: reed switches, bottom: reed relay

Mercury-wetted relay

A mercury-wetted reed relay is a form of reed relay in which the contacts are wetted with

mercury. Such relays are used to switch low-voltage signals (one volt or less) where the

mercury reduces the contact resistance and associated voltage drop, for low-current signals

where surface contamination may make for a poor contact, or for high-speed applications

where the mercury eliminates contact bounce. Mercury wetted relays are position-sensitive

and must be mounted vertically to work properly. Because of the toxicity and expense of

liquid mercury, these relays are now rarely used. See also mercury switch.

Polarized relay

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A polarized relay placed the armature between the poles of a permanent magnet to increase

sensitivity. Polarized relays were used in middle 2oth Century telephone exchanges to detect

faint pulses and correct telegraphic distortion. The poles were on screws, so a technician

could first adjust them for maximum sensitivity and then apply a bias spring to set the critical

current that would operate the relay.

[.] Machine tool relay

A machine tool relay is a type standardized for industrial control of machine tools, transfer

machines, and other sequential control. They are characterized by a large number of contacts

(sometimes extendable in the field) which are easily converted from normally-open to

normally-closed status, easily replaceable coils, and a form factor that allows compactly

installing many relays in a control panel. Although such relays once were the backbone of

automation in such industries as automobile assembly, the programmable logic controller

(PLC) mostly displaced the machine tool relay from sequential control applications.

[.] Contactor relay

A contactor is a very heavy-duty relay used for switching electric motors and lighting loads,

although contactors are not generally called relays. Continuous current ratings for common

contactors range from 1o amps to several hundred amps. High-current contacts are made with

alloys containing silver. The unavoidable arcing causes the contacts to oxidize; however,

silver oxide is still a good conductor.[2] Such devices are often used for motor starters. A

motor starter is a contactor with overload protection devices attached. The overload sensing

devices are a form of heat operated relay where a coil heats a bi-metal strip, or where a solder

pot melts, releasing a spring to operate auxiliary contacts. These auxiliary contacts are in

series with the coil. If the overload senses excess current in the load, the coil is de-energized.

Contactor relays can be extremely loud to operate, making them unfit for use where noise is a

chief concern.

[.] Solid-state relay

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Solid state relay, which has no moving parts

25 A or 4o A solid state contactors

A solid state relay (SSR) is a solid state electronic component that provides a similar

function to an electromechanical relay but does not have any moving components, increasing

long-term reliability. With early SSR's, the tradeoff came from the fact that every transistor

has a small voltage drop across it. This voltage drop limited the amount of current a given

SSR could handle. The minimum voltage drop for such a relay is equal to the voltage drop

across one transistor (~o.6-2.o volts), and is a function of the material used to make the

transistor (typically silicon). As transistors improved, higher current SSR's, able to handle

1oo to 1,2oo Amperes, have become commercially available. Compared to electromagnetic

relays, they may be falsely triggered by transients.

Solid state contactor relay

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A solid state contactor is a heavy-duty solid state relay, including the necessary heat sink,

used for switching electric heaters, small electric motors and lighting loads; where frequent

on/off cycles are required. There are no moving parts to wear out and there is no contact

bounce due to vibration. They are activated by AC control signals or DC control signals from

Programmable logic controller (PLCs), PCs, Transistor-transistor logic (TTL) sources, or

other microprocessor and microcontroller controls.

Buchholz relay

A Buchholz relay is a safety device sensing the accumulation of gas in large oil-filled

transformers, which will alarm on slow accumulation of gas or shut down the transformer if

gas is produced rapidly in the transformer oil.

Forced-guided contacts relay

A forced-guided contacts relay has relay contacts that are mechanically linked together, so

that when the relay coil is energized or de-energized, all of the linked contacts move together.

If one set of contacts in the relay becomes immobilized, no other contact of the same relay

will be able to move. The function of forced-guided contacts is to enable the safety circuit to

check the status of the relay. Forced-guided contacts are also known as "positive-guided

contacts", "captive contacts", "locked contacts", or "safety relays".

overload protection relay

Electric motors need overcurrent protection to prevent damage from over-loading the motor,

or to protect against short circuits in connecting cables or internal faults in the motor

windings.[3]one type of electric motor overload protection relay is operated by a heating

element in series with the electric motor. The heat generated by the motor current heats a

bimetallic strip or melts solder, releasing a spring to operate contacts. Where the overload

relay is exposed to the same environment as the motor, a useful though crude compensation

for motor ambient temperature is provided.

[.] Pole and throw

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Circuit symbols of relays. (C denotes the common terminal in SPDT and DPDT types.)

Since relays are switches, the terminology applied to switches is also applied to relays. A relay will switch one or more poles, each of whose contacts can be thrown by energizing the coil in one of three ways:

Normally-open (No) contacts connect the circuit when the relay is activated; the

circuit is disconnected when the relay is inactive. It is also called a Form A contact or

"make" contact. No contacts can also be distinguished as "early-make" or NoEM,

which means that the contacts will close before the button or switch is fully engaged.

Normally-closed (NC) contacts disconnect the circuit when the relay is activated; the

circuit is connected when the relay is inactive. It is also called a Form B contact or

"break" contact. NC contacts can also be distinguished as "late-break" or NCLB,

which means that the contacts will stay closed until the button or switch is fully

disengaged.

Change-over (Co), or double-throw (DT), contacts control two circuits: one normally-

open contact and one normally-closed contact with a common terminal. It is also

called a Form C contact or "transfer" contact ("break before make"). If this type of

contact utilizes a "make before break" functionality, then it is called a Form D

contact.

The following designations are commonly encountered:

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SPST – Single Pole Single Throw. These have two terminals which can be connected

or disconnected. Including two for the coil, such a relay has four terminals in total. It

is ambiguous whether the pole is normally open or normally closed. The terminology

"SPNo" and "SPNC" is sometimes used to resolve the ambiguity.

SPDT – Single Pole Double Throw. A common terminal connects to either of two

others. Including two for the coil, such a relay has five terminals in total.

DPST – Double Pole Single Throw. These have two pairs of terminals. Equivalent to

two SPST switches or relays actuated by a single coil. Including two for the coil, such

a relay has six terminals in total. The poles may be Form A or Form B (or one of

each).

DPDT – Double Pole Double Throw. These have two rows of change-over terminals.

Equivalent to two SPDT switches or relays actuated by a single coil. Such a relay has

eight terminals, including the coil.

The "S" or "D" may be replaced with a number, indicating multiple switches connected to a

single actuator. For example 4PDT indicates a four pole double throw relay (with 14

terminals).

EN 5ooo5 are among applicable standards for relay terminal numbering; a typical EN 5ooo5-

compliant SPDT relay's terminals would be numbered 11, 12, 14, A1 and A2 for the C, NC,

No, and coil connections, respectively.

Applications

Relays are used to and for:

Control a high-voltage circuit with a low-voltage signal, as in some types of modems

or audio amplifiers,

Control a high-current circuit with a low-current signal, as in the startersolenoid of an

automobile,

Detect and isolate faults on transmission and distribution lines by opening and closing

circuit breakers (protection relays),

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A DPDT AC coil relay with "ice cube" packaging

Isolate the controlling circuit from the controlled circuit when the two are at different

potentials, for example when controlling a mains-powered device from a low-voltage

switch. The latter is often applied to control office lighting as the low voltage wires

are easily installed in partitions, which may be often moved as needs change. They

may also be controlled by room occupancy detectors in an effort to conserve energy,

Logic functions. For example, the boolean AND function is realised by connecting

normally open relay contacts in series, the oR function by connecting normally open

contacts in parallel. The change-over or Form C contacts perform the XoR (exclusive

or) function. Similar functions for NAND and NoR are accomplished using normally

closed contacts. The Ladder programming language is often used for designing relay

logic networks.

o Early computing. Before vacuum tubes and transistors, relays were used as

logical elements in digital computers. See ARRA (computer), Harvard Mark

II, Zuse Z2, and Zuse Z3.

o Safety-critical logic. Because relays are much more resistant than

semiconductors to nuclear radiation, they are widely used in safety-critical

logic, such as the control panels of radioactive waste-handling machinery.

Time delay functions. Relays can be modified to delay opening or delay closing a set

of contacts. A very short (a fraction of a second) delay would use a copper disk

between the armature and moving blade assembly. Current flowing in the disk

maintains magnetic field for a short time, lengthening release time. For a slightly

longer (up to a minute) delay, a dashpot is used. A dashpot is a piston filled with fluid

that is allowed to escape slowly. The time period can be varied by increasing or

decreasing the flow rate. For longer time periods, a mechanical clockwork timer is

installed.

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Light-emitting diode

Red, green and blue LEDs of the 5mm type

Type Passive, optoelectronic

Working principle Electroluminescence

Invented Nick Holonyak Jr. (1962)

Electronic symbol

Pin configuration Anode and Cathode

A light-emitting diode (LED) (pronounced /ˌɛl iːˈdiː/[1]) is a semiconductor light source.

LEDs are used as indicator lamps in many devices, and are increasingly used for lighting.

Introduced as a practical electronic component in 1962,[2] early LEDs emitted low-intensity

red light, but modern versions are available across the visible, ultraviolet and infrared

wavelengths, with very high brightness.

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. An LED is usually small in area (less than 1 mm2), and integrated optical

components are used to shape its radiation pattern and assist in reflection. [3] LEDs present

many advantages over incandescent light sources including lower energy consumption,

longer lifetime, improved robustness, smaller size, faster switching, and greater durability

and reliability. LEDs powerful enough for room lighting are relatively expensive and require

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more precise current and heat management than compact fluorescent lamp sources of

comparable output.

They are used in applications as diverse as replacements for aviation lighting, automotive

lighting (particularly indicators) and in traffic signals. The compact size of LEDs has allowed

new text and video displays and sensors to be developed, while their high switching rates are

useful in advanced communications technology. Infrared LEDs are also used in the remote

control units of many commercial products including televisions, DVD players, and other

domestic appliances.

Discoveries and early devices

Green electroluminescence from a point contact on a crystal of SiC recreates H. J. Round's

original experiment from 19o7.

Electroluminescence was discovered in 19o7 by the British experimenter H. J. Round of

Marconi Labs, using a crystal of silicon carbide and a cat's-whisker detector.[4][5] Russian

olegVladimirovichLosev independently reported on the creation of an LED in 1927.[6][7] His

research was distributed in Russian, German and British scientific journals, but no practical

use was made of the discovery for several decades. Rubin Braunstein of the Radio

Corporation of America reported on infrared emission from gallium arsenide (GaAs) and

other semiconductor alloys in 1955.[1o]Braunstein observed infrared emission generated by

simple diode structures using gallium antimonide (GaSb), GaAs, indium phosphide (InP), and

silicon-germanium (SiGe) alloys at room temperature and at 77 kelvin.

In 1961, American experimenters Robert Biard and Gary Pittman working at Texas

Instruments,[11] found that GaAs emitted infrared radiation when electric current was applied

and received the patent for the infrared LED.

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The first practical visible-spectrum (red) LED was developed in 1962 by Nick Holonyak Jr.,

while working at General Electric Company.[2]Holonyak is seen as the "father of the light-

emitting diode".[12] M. George Craford, a former graduate student of Holonyak, invented the

first yellow LED and improved the brightness of red and red-orange LEDs by a factor of ten

in 1972.[14] In 1976, T.P. Pearsall created the first high-brightness, high efficiency LEDs for

optical fiber telecommunications by inventing new semiconductor materials specifically

adapted to optical fiber transmission wavelengths.[15]

Up to 1968 visible and infrared LEDs were extremely costly, on the order of US $2oo per

unit, and so had little practical application.[16] The Monsanto Company was the first

organization to mass-produce visible LEDs, using gallium arsenide phosphide in 1968 to

produce red LEDs suitable for indicators.[16]Hewlett Packard (HP) introduced LEDs in 1968,

initially using GaAsP supplied by Monsanto. The technology proved to have major

applications for alphanumeric displays and was integrated into HP's early handheld

calculators. In the 197os commercially successful LED devices at under five cents each were

produced by Fairchild optoelectronics. These devices employed compound semiconductor

chips fabricated with the planar process invented by Dr. Jean Hoerni at Fairchild

Semiconductor.[17] The combination of planar processing for chip fabrication and innovative

packaging techniques enabled the team at Fairchild led by optoelectronics pioneer Thomas

Brandt to achieve the necessary cost reductions. These techniques continue to be used by

LED producers.[18]

[]Practical use

Red, yellow and green (unlit) LEDs used in a traffic signal in Sweden.

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The first commercial LEDs were commonly used as replacements for incandescent and neon

indicator lamps, and in seven-segment displays,[19] first in expensive equipment such as

laboratory and electronics test equipment, then later in such appliances as TVs, radios,

telephones, calculators, and even watches (see list of signal applications). These red LEDs

were bright enough only for use as indicators, as the light output was not enough to

illuminate an area. Readouts in calculators were so small that plastic lenses were built over

each digit to make them legible. Later, other colors became widely available and also

appeared in appliances and equipment. As the LED materials technology became more

advanced, the light output was increased, while maintaining the efficiency and the reliability

to an acceptable level. The invention and development of the high power white light LED led

to use for illumination[2o][21] (see list of illumination applications). Most LEDs were made in

the very common 5 mm T1¾ and 3 mm T1 packages, but with increasing power output, it has

become increasingly necessary to shed excess heat in order to maintain reliability,[22] so more

complex packages have been adapted for efficient heat dissipation. Packages for state-of-the-

art high power LEDs bear little resemblance to early LEDs.

Illustration of Haitz's Law. Light output per LED as a function of production year, note the

logarithmic scale on the vertical axis.

Continuing development

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The first high-brightness blue LED was demonstrated by Shuji Nakamura of Nichia

Corporation and was based on InGaN borrowing on critical developments in GaN nucleation

on sapphire substrates and the demonstration of p-type doping of GaN which were developed

by Isamu Akasaki and H. Amano in Nagoya. In 1995, Alberto Barbieri at the Cardiff

University Laboratory (GB) investigated the efficiency and reliability of high-brightness

LEDs and demonstrated a very impressive result by using a transparent contact made of

indium tin oxide (ITo) on (AlGaInP/GaAs) LED. The existence of blue LEDs and high

efficiency LEDs quickly led to the development of the first white LED, which employed a

Y3Al5o12:Ce, or "YAG", phosphor coating to mix yellow (down-converted) light with blue to

produce light that appears white. Nakamura was awarded the 2oo6 Millennium Technology

Prize for his invention.[23]

The development of LED technology has caused their efficiency and light output to increase

exponentially, with a doubling occurring about every 36 months since the 196os, in a way

similar to Moore's law. The advances are generally attributed to the parallel development of

other semiconductor technologies and advances in optics and material science. This trend is

normally called Haitz's Law after Dr. Roland Haitz. [24]

In February 2oo8, Bilkent university in Turkey reported 3oo lumens of visible light per watt

luminous efficacy (not per electrical watt) and warm light by using nanocrystals.[25]

In January 2oo9, researchers from Cambridge University reported a process for growing

gallium nitride (GaN) LEDs on silicon. Production costs could be reduced by 9o% using six-

inch silicon wafers instead of two-inch sapphire wafers. The team was led by Colin

Humphreys.

Technology

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I-V diagram for a diode an LED will begin to emit light when the on-voltage is exceeded.

Typical on voltages are 2-3 Volt

Physics

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-radiative transition 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.

LED development began with infrared and red devices made with gallium arsenide.

Advances in materials science have made possible the production of devices with ever-

shorter wavelengths, producing light in a variety of colors.

LEDs are usually built on an n-type substrate, with an electrode attached to the p-type layer

deposited on its surface. P-type substrates, while less common, occur as well. Many

commercial LEDs, especially GaN/InGaN, also use sapphire substrate.

Most materials used for LED production have very high refractive indices. This means that

much light will be reflected back into the material at the material/air surface interface.

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Therefore Light extraction in LEDs is an important aspect of LED production, subject to

much research and development.

Efficiency and operational parameters

Typical indicator LEDs are designed to operate with no more than 3o–6o milliwatts [mW] of

electrical power. Around 1999, Philips Lumileds introduced power LEDs capable of

continuous use at one watt [W]. These LEDs used much larger semiconductor die sizes to

handle the large power inputs. Also, the semiconductor dies were mounted onto metal slugs

to allow for heat removal from the LED die.

one of the key advantages of LED-based lighting is its high efficiency, as measured by its

light output per unit power input. White LEDs quickly matched and overtook the efficiency

of standard incandescent lighting systems. In 2oo2, Lumileds made five-watt LEDs available

with a luminous efficacy of 18–22 lumens per watt [lm/W]. For comparison, a conventional

6o–1oo W incandescent lightbulb produces around 15 lm/W, and standard fluorescent lights

produce up to 1oo lm/W. A recurring problem is that efficiency will fall dramatically for

increased current. This effect is known as droop and effectively limits the light output of a

given LED, increasing heating more than light output for increased current.[27][28][29]

In September 2oo3, a new type of blue LED was demonstrated by the company Cree, Inc. to

provide 24 mW at 2o milliamperes [mA]. This produced a commercially packaged white

light giving 65 lm/W at 2o mA, becoming the brightest white LED commercially available at

the time, and more than four times as efficient as standard incandescents. In 2oo6 they

demonstrated a prototype with a record white LED luminous efficacy of 131 lm/W at 2o mA.

Also, Seoul Semiconductor has plans for 135 lm/W by 2oo7 and 145 lm/W by 2oo8, which

would be approaching an order of magnitude improvement over standard incandescents and

better even than standard fluorescents.[3o]Nichia Corporation has developed a white LED with

luminous efficacy of 15o lm/W at a forward current of 2o mA.[31]

High-power (≥ 1 W) LEDs are necessary for practical general lighting applications. Typical

operating currents for these devices begin at 35o mA.

Note that these efficiencies are for the LED chip only, held at low temperature in a lab. In a

lighting application, operating at higher temperature and with drive circuit losses, efficiencies

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are much lower. United States Department of Energy (DoE) testing of commercial LED

lamps designed to replace incandescent lamps or CFLs showed that average efficacy was still

about 46 lm/W in 2oo9 (tested performance ranged from 17 lm/W to 79 lm/W).[32]

Cree issued a press release on February 3, 2o1o about a laboratory prototype LED achieving

2o8 lumens per watt at room temperature. The correlated color temperature was reported to

be 4579 K.[33]

Lifetime and failure

Main article: List of LED failure modes

Solid state devices such as LEDs are subject to very limited wear and tear if operated at low

currents and at low temperatures. Many of the LEDs produced in the 197os and 198os are

still in service today. Typical lifetimes quoted are 25,ooo to 1oo,ooo hours but heat and

current settings can extend or shorten this time significantly. [34]

The most common symptom of LED (and diode laser) failure is the gradual lowering of light

output and loss of efficiency. Sudden failures, although rare, can occur as well. Early red

LEDs were notable for their short lifetime. With the development of high-power LEDs the

devices are subjected to higher junction temperatures and higher current densities than

traditional devices. This causes stress on the material and may cause early light output

degradation. To quantitatively classify lifetime in a standardized manner it has been

suggested to use the terms L75 and L5o which is the time it will take a given LED to reach

75% and 5o% light output respectively.[35]

Like other lighting devices, LED performance is temperature dependent. Most

manufacturers’ published ratings of LEDs are for an operating temperature of 25°C. LEDs

used outdoors, such as traffic signals or in-pavement signal lights, and that are utilized in

climates where the temperature within the luminaire gets very hot, could result in low signal

intensities or even failure.[36]

LEDs maintain consistent light output even in cold temperatures, unlike traditional lighting

methods. Consequently, LED technology may be a good replacement in areas such as

supermarket freezer lighting[37][38][39] and will last longer than other technologies. Because

LEDs do not generate as much heat as incandescent bulbs, they are an energy-efficient

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technology to use in such applications such as freezers. on the other hand, because they do

not generate much heat, ice and snow may build up on the LED luminaire in colder climates.[4o] This has been a problem plaguing airport runway lighting, although some research has

been done to try to develop heat sink technologies in order to transfer heat to alternative areas

of the luminaire.[41]

Colors and materials

Conventional LEDs are made from a variety of inorganic semiconductor materials, the

following table shows the available colors with wavelength range, voltage drop and material:

ColorWavelength

(nm)Voltage (V) Semiconductor Material

Infrared λ> 76o ΔV< 1.9Gallium arsenide (GaAs)

Aluminium gallium arsenide (AlGaAs)

Red 61o <λ< 76o1.63 < ΔV<

2.o3

Aluminium gallium arsenide (AlGaAs)

Gallium arsenide phosphide (GaAsP)

Aluminium gallium indium phosphide (AlGaInP)

Gallium(III) phosphide (GaP)

orange 59o <λ< 61o2.o3 < ΔV<

2.1o

Gallium arsenide phosphide (GaAsP)

Aluminium gallium indium phosphide (AlGaInP)

Gallium(III) phosphide (GaP)

Yellow 57o <λ< 59o2.1o < ΔV<

2.18

Gallium arsenide phosphide (GaAsP)

Aluminium gallium indium phosphide (AlGaInP)

Gallium(III) phosphide (GaP)

Green 5oo <λ< 57o1.9[42]< ΔV<

4.o

Indium gallium nitride (InGaN) / Gallium(III)

nitride (GaN)

Gallium(III) phosphide (GaP)

Aluminium gallium indium phosphide (AlGaInP)

Aluminium gallium phosphide (AlGaP)

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Blue 45o <λ< 5oo2.48 < ΔV<

3.7

Zinc selenide (ZnSe)

Indium gallium nitride (InGaN)

Silicon carbide (SiC) as substrate

Silicon (Si) as substrate — (under development)

Violet 4oo <λ< 45o2.76 < ΔV<

4.oIndium gallium nitride (InGaN)

Purple multiple types2.48 < ΔV<

3.7

Dual blue/red LEDs,

blue with red phosphor,

or white with purple plastic

Ultraviolet λ< 4oo3.1 < ΔV<

4.4

Diamond (235 nm)[43]

Boron nitride (215 nm)[44][45]

Aluminium nitride (AlN) (21o nm)[46]

Aluminium gallium nitride (AlGaN)

Aluminium gallium indium nitride (AlGaInN) —

(down to 21o nm)[47]

White Broad spectrum ΔV = 3.5 Blue/UV diode with yellow phosphor  

Ultraviolet and blue LEDs

Blue LEDs.

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Blue LEDs are based on the wide band gap semiconductors GaN (gallium nitride) and InGaN

(indium gallium nitride). They can be added to existing red and green LEDs to produce the

impression of white light, though white LEDs today rarely use this principle.

The first blue LEDs were made in 1971 by Jacques Pankove (inventor of the gallium nitride

LED) at RCA Laboratories.[48] These devices had too little light output to be of much

practical use. However, early blue LEDs found use in some low-light applications, such as

the high-beam indicators for cars.[49] In the late 198os, key breakthroughs in GaNepitaxial

growth and p-typedoping[5o] ushered in the modern era of GaN-based optoelectronic devices.

Building upon this foundation, in 1993 high brightness blue LEDs were demonstrated.[51]

By the late 199os, blue LEDs had become widely available. They have an active region

consisting of one or more InGaNquantum wells sandwiched between thicker layers of GaN,

called cladding layers. By varying the relative InN-GaN fraction in the InGaN quantum

wells, the light emission can be varied from violet to amber. AlGaNaluminium gallium

nitride of varying AlN fraction can be used to manufacture the cladding and quantum well

layers for ultraviolet LEDs, but these devices have not yet reached the level of efficiency and

technological maturity of the InGaN-GaN blue/green devices. If the active quantum well

layers are GaN, as opposed to alloyed InGaN or AlGaN, the device will emit near-ultraviolet

light with wavelengths around 35o–37o nm. Green LEDs manufactured from the InGaN-GaN

system are far more efficient and brighter than green LEDs produced with non-nitride

material systems.

With nitrides containing aluminium, most often AlGaN and AlGaInN, even shorter

wavelengths are achievable. Ultraviolet LEDs in a range of wavelengths are becoming

available on the market. Near-UV emitters at wavelengths around 375–395 nm are already

cheap and often encountered, for example, as black light lamp replacements for inspection of

anti-counterfeiting UV watermarks in some documents and paper currencies. Shorter

wavelength diodes, while substantially more expensive, are commercially available for

wavelengths down to 247 nm.[52] As the photosensitivity of microorganisms approximately

matches the absorption spectrum of DNA, with a peak at about 26o nm, UV LED emitting at

25o–27o nm are to be expected in prospective disinfection and sterilization devices. Recent

research has shown that commercially available UVA LEDs (365 nm) are already effective

disinfection and sterilization devices.[53]

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Deep-UV wavelengths were obtained in laboratories using aluminium nitride (21o nm),[46]boron nitride (215 nm)[44][45] and diamond (235 nm).[43]

White light

There are two primary ways of producing high intensity white-light using LEDs. one is to use

individual LEDs that emit three primary colors[54]—red, green, and blue—and then mix all the

colors to produce white light. The other is to use a phosphor material to convert

monochromatic light from a blue or UV LED to broad-spectrum white light, much in the

same way a fluorescent light bulb works.

Due to metamerism, it is possible to have quite different spectra that appear white.

RGB systems

Combined spectral curves for blue, yellow-green, and high brightness red solid-state

semiconductor LEDs.FWHM spectral bandwidth is approximately 24–27 nm for all three

colors.

[]Phosphor-based LEDs

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Spectrum of a “white” LED clearly showing blue light which is directly emitted by the GaN-

based LED (peak at about 465 nm) and the more broadband Stokes-shifted light emitted by

the Ce3+:YAG phosphor which emits at roughly 5oo–7oo nm.

This method involves coating an LED of one color (mostly blue LED made of InGaN) with

phosphor of different colors to produce white light, the resultant LEDs are called phosphor-

based white LEDs.[57] A fraction of the blue light undergoes the Stokes shift being

transformed from shorter wavelengths to longer. Depending on the color of the original LED,

phosphors of different colors can be employed. If several phosphor layers of distinct colors

are applied, the emitted spectrum is broadened, effectively increasing the color rendering

index (CRI) value of a given LED.[58]

Phosphor based LEDs have a lower efficiency than normal LEDs due to the heat loss from

the Stokes shift and also other phosphor-related degradation issues. However, the phosphor

method is still the most popular technique for manufacturing high intensity white LEDs. The

design and production of a light source or light fixture using a monochrome emitter with

phosphor conversion is simpler and cheaper than a complex RGB system, and the majority of

high intensity white LEDs presently on the market are manufactured using phosphor light

conversion.

The greatest barrier to high efficiency is the seemingly unavoidable Stokes energy loss.

However, much effort is being spent on optimizing these devices to higher light output and

higher operation temperatures. For instance, the efficiency can be increased by adapting

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better package design or by using a more suitable type of phosphor. Philips Lumileds'

patented conformal coating process addresses the issue of varying phosphor thickness, giving

the white LEDs a more homogeneous white light.[59] With development ongoing, the

efficiency of phosphor based LEDs is generally increased with every new product

announcement.

Technically the phosphor based white LEDs encapsulate InGaN blue LEDs inside of a

phosphor coated epoxy. A common yellow phosphor material is cerium-doped yttrium

aluminium garnet (Ce3+:YAG).

White LEDs can also be made by coating near ultraviolet (NUV) emitting LEDs with a

mixture of high efficiency europium-based red and blue emitting phosphors plus green

emitting copper and aluminium doped zinc sulfide (ZnS:Cu, Al). This is a method analogous

to the way fluorescent lamps work. This method is less efficient than the blue LED with

YAG:Ce phosphor, as the Stokes shift is larger and more energy is therefore converted to

heat, but yields light with better spectral characteristics, which render color better. Due to the

higher radiative output of the ultraviolet LEDs than of the blue ones, both approaches offer

comparable brightness. Another concern is that UV light may leak from a malfunctioning

light source and cause harm to human eyes or skin.

other white LEDs

Another method used to produce experimental white light LEDs used no phosphors at all and

was based on homoepitaxially grown zinc selenide (ZnSe) on a ZnSe substrate which

simultaneously emitted blue light from its active region and yellow light from the substrate.

Organic light-emitting diodes (oLEDs)

Main article: organic light-emitting diode

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If the emitting layer material of the LED is an organic compound, it is known as an organic

light emitting diode (oLED). To function as a semiconductor, the organic emitting material

must have conjugated pi bonds. [61] The emitting material can be a small organic molecule in a

crystalline phase , or a polymer. Polymer materials can be flexible; such LEDs are known as

PLEDs or FLEDs.

Compared with regular LEDs, oLEDs are lighter, and polymer LEDs can have the added

benefit of being flexible. Some possible future applications of oLEDs could be:

Inexpensive, flexible displays

Light sources

Wall decorations

Luminous cloth

oLEDs have been used to produce visual displays for portable electronic devices such as

cellphones, digital cameras, and MP3 players. Larger displays have been demonstrated, [62] but

their life expectancy is still far too short (<1,ooo hours) to be practical[citation needed].

Today, oLEDs operate at substantially lower efficiency than inorganic (crystalline) LEDs.[63]

Types

LEDs are produced in a variety of shapes and sizes. The 5 mm cylindrical package (red, fifth

from the left) is the most common, estimated at 8o% of world production.[citation needed] The color

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of the plastic lens is often the same as the actual color of light emitted, but not always. For

instance, purple plastic is often used for infrared LEDs, and most blue devices have clear

housings. There are also LEDs in SMT packages, such as those found on blinkies and on cell

phone keypads (not shown).

The main types of LEDs are miniature, high power devices and custom designs such as

alphanumeric or multi-color.

Miniature LEDs

Different sized LEDs. 8 mm, 5 mm and 3 mm, with a wooden match-stick for scale.

Main article: Miniature light-emitting diode

These are mostly single-die LEDs used as indicators, and they come in various-sizes from

2 mm to 8 mm, through-hole and surface mount packages. They are usually simple in design,

not requiring any separate cooling body.[67] Typical current ratings ranges from around 1 mA

to above 2o mA. The small scale sets a natural upper boundary on power consumption due to

heat caused by the high current density and need for heat sinking.

High power LEDs

See also: Solid-state lighting and LED lamp

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High-power light emiting diodes (Luxeon, Lumileds)

High power LEDs (HPLED) can be driven at currents from hundreds of mA to more than an

ampere, compared with the tens of mA for other LEDs. Some can produce over a thousand [68]

[69]lumens. Since overheating is destructive, the HPLEDs must be mounted on a heat sink to

allow for heat dissipation. If the heat from a HPLED is not removed, the device will burn out

in seconds. A single HPLED can often replace an incandescent bulb in a torch, or be set in an

array to form a powerful LED lamp.

Some well-known HPLEDs in this category are the Lumileds Rebel Led, osramopto

Semiconductors Golden Dragon and Cree X-lamp. As of September 2oo9 some HPLEDs

manufactured by Cree Inc. now exceed 1o5 lm/W [7o] (e.g. the XLamp XP-G LED chip

emitting Cool White light) and are being sold in lamps intended to replace incandescent,

halogen, and even fluorescent style lights as LEDs become more cost competitive.

LEDs have been developed by Seoul Semiconductor that can operate on AC power without

the need for a DC converter. For each half cycle part of the LED emits light and part is dark,

and this is reversed during the next half cycle. The efficacy of this type of HPLED is

typically 4o lm/W.[71] A large number of LED elements in series may be able to operate

directly from line voltage. In 2oo9 Seoul Semiconductor released a high DC voltage capable

of being driven from AC power with a simple controlling circuit. The low power dissipation

of these LEDs affords them more flexibility than the original AC LED design.

Mid-range LEDs

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Medium power LEDs are often through-hole mounted and used when a output of a few

lumen is needed. They sometimes have the diode mounted to four leads (two cathode leads,

two anode leads) for better heat conduction and carry an integrated lens. An example of this

is the Superflux package, from Philips Lumileds. These LEDs are most commonly used in

light panels, emergency lighting and automotive tail-lights. Due to the larger amount of metal

in the LED, they are able to handle higher currents (around 1oo mA). The higher current

allows for the higher light output required for tail-lights and emergency lighting.

Application-specific variations

Flashing LEDs are used as attention seeking indicators without requiring external

electronics. Flashing LEDs resemble standard LEDs but they contain an integrated

multivibrator circuit which causes the LED to flash with a typical period of one

second. In diffused lens LEDs this is visible as a small black dot. Most flashing LEDs

emit light of a single color, but more sophisticated devices can flash between multiple

colors and even fade through a color sequence using RGB color mixing.

Calculator LED display, 197os.