Light Emitting Diode
From DT Online
Light Emitting Diodes (LED) come in different colours, shapes, sizes and brightness. The more common colours being red, green and yellow.
They are also available in bi-colour and tri-colour versions. Round ones come in 3, 5 and 8mm sizes. There are also rectangular, square and bar types.
Flashing types are available and seven segment displays are made of a number of LEDs in a particular pattern.
Features and Applications
LEDs are easily damaged by excess current therefore it is normal to connect the LED to the supply via a series current limiting Resistor. The value of this resistor is not critical but is usually within the range 100 - 330R.
Polarity must be observed when connecting LEDs (it should be noted that LEDs are damaged by reverse biased voltages larger than about 6 volts).
LEDs in a Transistor Circuit
The most common LEDs are 5mm diameter. They glow (commonly red) when current flows through them (current can only flow if the anode is more positive than the cathode).
The LED has a forward voltage of 2V, i.e 2 Volts is dropped across the LED when it is in operation. It requires a maximum current of 30mA. The Resistor in the circuit is included to limit the current to around this value.