Light Emitting Diodes (LED) are very rugged, they last a very long time and they are an optical source. (A LIGHT SOURCE) LEDs produce red, green, yellow, or orange light. They are used in a range of products Infrared LEDs are also available although light from this type cannot be seen by the human eye. These are used in security devices. LEDs are part of the diode family, consequently they must be connected the right way round or current will not pass through. They are usually protected by a resistor.