Short for light-emitting diode, LED is a semiconductor that illuminates when an electrical charge passes through it. LEDs are commonly green, amber or red, but they can be an assortment of other colors as they've become popular with case lighting. Below are examples of how an LED could be used with a computer.
An example of an LED is the led status indicators on your keyboard for Num Lock, Caps Lock, and Scroll Lock, as shown in the picture. In this image, the blue LED on the keyboard indicates the Num Lock is enabled, but since the Caps Lock and Scroll Lock LEDs are not, that indicates they are not enabled. The location of these indicators depend on the keyboard manufacturer, but are usually located in the top center or top-right side of the keyboard.
Not all keyboard num lock, caps lock, or scroll lock indicators have identifiers like those shown in the picture. If you are not sure what LED goes to what key, press the Num Lock, Caps Lock, or Scroll Lock key while watching the LEDs and see which one illuminates.
Apple keyboards and some others have the Caps Lock LED indicator on the Caps Lock key and may not have other indicators because there are no Scroll Lock or Num Lock keys.
Most computers today use an optical mouse, which is a mouse that uses an LED to help track the cursor on the screen.
Many motherboards also have an onboard LED that lets anyone looking at the motherboard know there is power getting to the motherboard and, in some cases, if there are errors. Pictured at right is an LED/PLED located between the PCI slots on a motherboard.
Do not work inside a computer or disconnect any internal plugs while this LED is lit.
When the power cable is disconnected from the computer, the LED light stays on for several seconds and then slowly fades off, which is normal.
Floppy, hard drive, CD-ROM, and other drives LED
Printer, speakers, monitor, and other devices
Red, amber, green, white, and blue LED meanings
There are only de facto standards when classifying the meanings of an LED's color. Below are explanations of what an LED and its color may mean.
Solid green, blue, or white LED
A solid green, blue, or white LED on the front of a computer, monitor, or another device is often an indication of power. For example, when you press the power button on your computer, if the computer is getting power and is working, the light should illuminate and stay solid.
Blinking green, blue, or white LED
A blinking green, blue, or white LED is an indication that data is being transmitted or the device is working.
Most blinking LEDs, regardless of the color, are an indication that something is working, being processed, or that data is being transferred.
Solid amber or yellow LED
A solid amber or yellow LED often means the computer, monitor, or other device has power, but is not getting a signal. For example, a monitor may have a yellow LED with no display or a black screen when the data cable is disconnected and it cannot display a picture.
Blinking amber or yellow LED
When dealing with an amber or yellow LED on the back of a computer, on a network card, modem, switch, or router this LED is an indication that data is being transferred over the device. In the case of a router, this light may also blink so fast that sometimes it may appear to be solid.
Solid red LED
Blinking red LED
A blinking red LED is often usually only used as an alert to notify the user that there is a problem.