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Light Emitting Diode

Pulpit rock

Light emitting diodes (LEDs) are one of the most efficient lighting technology of today, and nearly used in all electronic lighting applications. A light-emitting diode (LED) is a semiconductor based lighting system, which emits light when electrical energy is applied to it. These tiny bulbs are illuminated solely by the movement of electrons in a semiconductor material. The lifespan of an LED is much greater than that of an incandescent bulb.


Working

Early in 1960s, early LEDs emitted low-intensity red light, and today we have LEDs available across the visible, ultraviolet, and infrared wavelengths, with very high brightness. When a light-emitting 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 electron-luminescence and the color of the light is determined by the energy gap of the semiconductor. This is how light is emitted from an LED.


How is light displayed?


The LED consists of a chip of semiconducting material doped with impurities to create a p-n junction. In typical diodes, current flows easily from the p-side(anode), to the n-side(cathode), but not in the reverse direction. Charge-carriers — electrons and holes — flow into the junction due to difference in 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 thus its color depends on the band gap energy of the materials forming the p-n junction.


LED Colors
 
  Wavelength
(nm)
Color Name Fwd Voltage
(Vf @ 20ma)
Intensity
5mm LEDs
Viewing
Angle
LED Dye Material
  940 Infrared 1.5 16mW
@50mA
15° GaAIAs/GaAs -- Gallium Aluminum Arsenide/Gallium Arsenide
  880 Infrared 1.7 18mW
@50mA
15° GaAIAs/GaAs -- Gallium Aluminum Arsenide/Gallium Arsenide
  850 Infrared  1.7 26mW
@50mA
15° GaAIAs/GaAs -- Gallium Aluminum Arsenide/Gallium Aluminum Arsenide
  660 Ultra Red 1.8 2000mcd
@50mA
15° GaAIAs/GaAs -- Gallium Aluminum Arsenide/Gallium Aluminum Arsenide
  635 High Eff. Red 2.0 200mcd @20mA 15° GaAsP/GaP - Gallium Arsenic Phosphide / Gallium Phosphide
  633 Super Red 2.2 3500mcd
@20mA
15° InGaAIP - Indium Gallium Aluminum Phosphide
  620 Super Orange 2.2 4500mcd
@20mA
15° InGaAIP - Indium Gallium Aluminum Phosphide
  612 Super
Orange
2.2 6500mcd
@20mA
15° InGaAIP - Indium Gallium Aluminum Phosphide
  605 Orange 2.1 160mcd @20mA 15° GaAsP/GaP - Gallium Arsenic Phosphide / Gallium Phosphide
  595 Super Yellow 2.2 5500mcd
@20mA
15° InGaAIP - Indium Gallium Aluminum Phosphide
  592 Super Pure
Yellow
2.1 7000mcd
@20mA
15° InGaAIP - Indium Gallium Aluminum Phosphide
  585  Yellow 2.1 100mcd @20mA 15° GaAsP/GaP - Gallium Arsenic Phosphide / Gallium Phosphide
  4500K "Incan-
descent"
White
3.6 2000mcd
@20mA
20° SiC/GaN -- Silicon Carbide/Gallium Nitride
  6500K Pale
White
3.6 4000mcd
@20mA
20° SiC/GaN -- Silicon Carbide/Gallium Nitride
  8000K Cool White 3.6 6000mcd
@20mA
20° SiC/GaN - Silicon Carbide / Gallium Nitride
  574 Super
Lime Yellow
2.4 1000mcd
@20mA
15° InGaAIP - Indium Gallium Aluminum Phosphide
  570 Super
Lime Green
2.0 1000mcd
@20mA
15° InGaAIP - Indium Gallium Aluminum Phosphide
  565 High 
Efficiency 
Green
2.1 200mcd
@20mA
15° GaP/GaP - Gallium Phosphide/Gallium Phosphide
  560 Super
Pure Green
2.1 350mcd
@20mA
15° InGaAIP - Indium Gallium Aluminum Phosphide
  555 Pure Green 2.1 80mcd
@20mA
15° GaP/GaP - Gallium Phosphide/ Gallium Phosphide
  525 Aqua Green 3.5 10,000mcd
@20mA
15° SiC/GaN - Silicon Carbide / Gallium Nitride
  505 Blue Green 3.5 2000mcd
@20mA
45° SiC/GaN - Silicon Carbide / Gallium Nitride
  470 Super Blue 3.6 3000mcd
@20mA
15° SiC/GaN - Silicon Carbide / Gallium Nitride
  430 Ultra Blue 3.8 100mcd
@20mA
15° SiC/GaN - Silicon Carbide / Gallium Nitride



Power supply for LEDs

The signal, in its natural form is always oscillating. If this signal is fed to LED, it fluctuates and eventually burn out. It needs a regulated (constant) voltage. If it has to be connected to 230/420V AC supply, it has to be regulated using voltage regulators. For power LEDs which require high current, LED drivers are used for current boosting.


Advantages:

  • Size: Size of LED is usually very small (smaller than 2 mm sq.) and hence they are easy to mount on PCBs.
  • Switching: Switching refers to the ON/OFF cycles. LEDs have very large switching cycles-it can be turned on and off millions of times before burning.
  • Efficiency: LEDs produce more light per watt than incandescent bulbs.
  • Focus: The LED can focus its light in a particular direction.
  • Shock resistance: LEDs, being solid state components, are difficult to damage with external shock.
  • Toxicity: LEDs do not contain mercury, lead or any other toxic elements.
  • Color: LEDs can emit light of any required color with the change of the semiconductor material.
  • Brightness: Brightness of an LED can be easily controlled by varying the duty cycle of the pulse by the method called Pulse Width Modulation (PWM).
  • Switching time: Switching time of an LED is extremely low. RF LEDs have switching time in pico seconds.



Disadvantages:

  • Temperature dependence: LED performance largely depends on the ambient temperature of the operating environment.
  • Safety: It is now evident that some colors of LEDs emit light which exceeds the Eye standards and hence it may pose a threat to human eye in long term.
  • Voltage sensitivity: LEDs must not be supplied voltage abouve its upper threshold limits. On doing so, it immediately burns out.
  • Initial cost: If cost efficiency is considered, price per lumen, of an LED is high.


Applications for LEDs

  • Display lighting in banners and street advertisments
  • Back lighting in televisions and mobile phones
  • Sign lighting used as street indicators
  • Traffic lights and signals
  • Light source for machine vision systems
  • Automobile lights used in cars and trucks
  • Toys and and entertainment systems
  • Flashlights, glow-lights, and lasers.
  • Elevator Push Button Lighting
  • Red or yellow LEDs are used in indicator and alphanumeric displays in environments
  • Red, yellow, green, and blue LEDs can be used for model railroading applications
  • In optical fiber and Free Space Optics communications.
  • In dot matrix arrangements for displaying messages.


Interfacing LED with Arduino Microcontroller










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