Light Emitting Diode ( LED )
Light emitting diodes must be choosen according to how they will be used, because there are various kinds.
The diodes are available in several colors. The most common colors are red and green, but there are even blue ones.
The device on the far right in the photograph combines a red LED and green LED in one package. The component lead in the middle is common to both LEDs. As for the remaing two leads, one side is for the green, the other for the red LED. When both are turned on simultaneously, it becomes orange.
When an LED is new out of the package, the polarity of the device can be determined by looking at the leads. The longer lead is the Anode side, and the short one is the Cathode side.
The polarity of an LED can also be determined using a resistance meter, or even a 1.5 V battery.
When using a test meter to determine polarity, set the meter to a low resistance measurement range. Connect the probes of the meter to the LED. If the polarity is correct, the LED will glow. If the LED does not glow, switch the meter probes to the opposite leads on the LED. In either case, the side of the diode which is connected to the black meter probe when the LED glows, is the Anode side. Positive voltage flows out of the black probe when the meter is set to measure resistance.
It is possible to use an LED to obtain a fixed voltage.
The voltage drop (forward voltage, or VF) of an LED is comparatively stable at just about 2V.
