Initialized by

Almas Akram

LIGHT EMITTING DIODES (LED)

OPTOELECRONIC

A light emitting diode (LED) is essentially a PN junction opto-semiconductor that emits a monochromatic (single color) light when operated in a forward biased direction.

LEDs converrt electrical energy into light energy. They are frequently used as "pilot" lights in electronic appliances to indicate whether the circuit is closed or not

HOW DOES A LED WORK?

– When sufficient voltage is applied to the chip across the leads of the LED, electrons can move easily in only one direction across the junction between the p and n regions.

– In the p region there are many more positive than negative charges.

– When a voltage is applied and the current starts to flow, electrons in the n region have sufficient energy to move across the junction into the p region

How does LED work?

How Does A LED Work?

– Each time an electron recombine swith a positive charge, electric potential energy is converted into electromagnetic energy.

– For each recombination of a negative and a positive charge, a quantum of electromagnetic energy is emitted in the form of a photon of light with a frequency characteristic of the semi-conductor material(usually a combination of the chemical elements gallium, arsenic and phosphorus.

How Much Energy Does an LED Emit?

The energy (E) of the light emitted by an LED is related to the electric charge (q) of an electron and the voltage (V) required to light the LED by the expression: E = qv Joules.

– Finding the Energy from the Voltage Suppose you measured the voltage across the leads of an LED , and you wished to find the corresponding energy required to light the LED. Let us say that you have a red LED, and the voltage measured between the leads of is 1.71 Volts. So the Energy required to light the LED is E = qv or E = -1.6 x 10-9

Finding the Energy from the Voltage

– Voltage Suppose you measured the voltage across the leads of an LED, and you wished to find the corresponding energy required to light the LED.

– Let us say that you have a red LED, and the voltage measured between the leads of is 1.71 Volts.

– So the Energy required to light the LED is

– E = qV or E = -1.6 x 10-19(1.71)

– Joule, since a Coulomb-Volt is a Joule. Multiplication of these numbers then gives

– E = 2.74 x 10-19Joule.

NEXT proceeded by

MUBEEN TALHA.

LED VOLTAGE AND CURRENT

To prevent from destruction of LED we use a resistor

There are two voltages Source voltage and Diode voltage.

LED Brightness

the brightness of an LED depends on the current

When Vs is much greater than Vd the brightness of LED is approximately constant.

If Vs is only slightly more than Vd, the LED brightness will vary noticeably.

To keep the brightness of LED constant the source current must be constant .

Colours of LEDs

– LEDs are available in red, orange, amber, yellow, green, blue and white. Blue and white LEDs are much more expensive than the other colours. The colour of an LED is determined by the semiconductor material, not by the colouring of the package (the plastic body).

– LEDs are made from gallium-based crystals that contain one or more additional materials such as phosphorous to produce a distinct color. Different LED chip technologies emit light in specific regions of the visible light spectrum and produce different intensity levels.

BREAKDOWN VOLTAGE

– LED have very low breakdown voltage , typically between 3V and 5V

– LED destroyed if the too much voltage applied.

– It works only in farward biased

NEXT proceeded by

AQSA

Seven-Segment Display

It contains seven rectangular LEDs ( A through G ) Each LED is called a segment.

External series resistors are included to limit the current to safe levels.

Using seven segment we can form any digit from 1 to 9 . It can also display the capital letters(A,C,E,F) .

In seven-segment indicator all anodes are connected together.

It is also in common cathode type in which all cathodes are connected together

Seven segment Display

NEXT proceeded by

Zoya

Photodiode

–What is photodiode ?Photodiode is a reverse of led

In photodiode input is light and the output is voltage

It works in reverse biased.

When the covalent bond break electrons emit and it moves from lower to higher orbit and gain energy. When it come back to lower energy level it emits radiation in form of voltage .

photodiode

- Principle of photodiode When light energy bombarded a pn junction ,it dislodge valance

electrons

More the light strike the pn-junction more is the reverse current produce

Incoming light produces free electrons and holes

The stronger the light the larger the minority carriers and greater the reverse current

As the light become brighter , the reverse current increases.

NEXT proceeded by

Muneeba Nisar

optocoupler

Optocoupler combines led and the photodiode

-led convert voltage into light and work as a input

-photodiode convert light into a voltage and work as a output.

When the input voltage is varying ,the amount of light is fluctuating

optocoupler

– Advantage of optocouplerEasily availaible

No gas , No pollutant produce

Less area required for installation

– Disadvantage of optocouplerNot always produce constant electricity

Due to temperature variation after some time decrease the efficiency

Laser diode

In a led free electrons radiate light when falling high energy level to lower one

A laser diode is different , it produce the coherent light

A laser diode produce a narrow beam of light which is very intense ,focused and pure.

These are also known as semi-conductor laser, it can produce visible and invisible lights.

NEXT proceeded by

M. ABDULLAH HASHMI (GROUP leader )

Applications

– Sensor Application

– Mobile Applications

– Sign Applications

– Automobile Applications

– LED Signals

– Indicators

Sensor Applications

– Color Bar Code Readers

– Medical Instrumentation (blood group detection using fiber optics)

– Money sensors

– Optical switches

– Fiber optical cummunication

Mobile Applications

– General Backlighting

– Lap Tops

– Digital Cameras

– PDAs

– Mobile Phone

Sign Applications

– Transportation Passenger Information

– Traffic

– Monochrome Message Board

– Full color video

Automobile Applications

Interior Lighting - Instrument Panels& switches,courtesy lighting

Exterior lighting,rear stop/turn/tail

Truck/bus lighting ,new turn/tail/marker lights

Signal Appications

– Runway

– Tower Lights

– Aviation

– Rail

– Traffic

– Emergency/Police Vehicle Lighting

– LEDs offer enormous benefits Lights

– Energy savings (up to 85% less power than incandescent)

– Reduction in maintenance costs

– Increased visibility in daylight and adverse weather conditions

Indication

• VCR/ DVD/ Stereo and other audio and video devices

• Household appliances

• Security Equipment

• Instrumentation

• Toys/Games

• Switches

Some type of LEDs

THE END

THANKS FOR LISTENING

ANY QUESTION ??