LEDs

LED connectionsLED stand for Light Emitting Diode and as you might guess it is a p-n junction diode which emits light during the recombination of charge carriers in the semiconductor material inside the diode. LEDs emit light at different wavelengths depending on the band gap in the silicon, in this way the LEDs achieve different colours.

To control an LED you need to know the forward voltage and the forward current of your LED. Most LEDs are good to 20 mA so a safe number if you don’t know the forward current is around 10-15 mA. If you don’t know the forward voltage then I’d hook up my multimeter to measure current and with no limiting resistor on the LED ramp the voltage up from 0 volts until you reach 15 mA and make a note of the voltage and use that as the forward voltage.

The polarity of the LED is denoted by the length of the LED leg, the long leg is the Anode or the positive connection and the short leg is the cathode or negative connection.



Calculating LED Limiting Resistor

You need to use a current limiting resistor to limit your current to 15 mA if your supply voltage is greater than the LED forward voltage. You can calculate the resistance using the tool here or using the following formula:

Resistor Value (Ω) = (Supply Voltage (V) – LED Forward Voltage (V))/Desired Current (A)

It is pretty common to get 470 Ohm resistors with LEDs when you buy them from ebay sellers as that is a high enough resistance to ensure you are below 15 mA at 5 V for most LEDs.


Beginners Guide to LEDs