Q:- What is L.E.D. ???

A light-emitting diode (LED) is a semiconductor device that emits visible light when an electric current passes through it. The light is not particularly bright, but in most LEDs it is monochromatic, occurring at a single wavelength. The output from an LED can range from red (at a wavelength of approximately 700 nanometers) to blue-violet (about 400 nanometers). Some LEDs emit infrared (IR) energy (830 nanometers or longer); such a device is known as an infrared-emitting diode (IRED).

An LED or IRED consists of two elements of processed material called P-type semiconductors and N-type semiconductors. These two elements are placed in direct contact, forming a region called the P-N junction. In this respect, the LED or IRED resembles most other diode types, but there are important differences. The LED or IRED has a transparent package, allowing visible or IR energy to pass through. Also, the LED or IRED has a large PN-junction area whose shape is tailored to the application.

Benefits of LEDs and IREDs, compared with incandescent and fluorescent illuminating devices, include:

• Low power requirement: Most types can be operated with battery power supplies.
• High efficiency: Most of the power supplied to an LED or IRED is converted into radiation in the desired form, with minimal heat production.
• Long life: When properly installed, an LED or IRED can function for decades.

Typical applications include:

• Indicator lights: These can be two-state (i.e., on/off), bar-graph, or alphabetic-numeric readouts.
• LCD panel back-lighting: Specialized white LED's are used in flat-panel computer displays.
• Fiber optic data transmission: Ease of modulation allows wide communications bandwidth with minimal noise, resulting in high speed and accuracy.
• Remote control: Most home-entertainment "remotes" use IREDs to transmit data to the main unit.
• Optoisolator: Stages in an electronic system can be connected together without unwanted interaction.