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UPDATED:.21:07 21 October 2013


An LED gives off light which can be of almost any colour. They come in a wide range of shapes and sizes.

Like all diodes it must be connected the correct way round to conduct.

5mm LED

A wide range of colours
and sizes

In most circumstances (unless specifically stated by an IC specification), an LED must have a resistor in series with it to prevent it destroying itself.*

For voltages between 6 to 12v, a 470 ohm resistor will be fine but to get maximum brightness, without damaging it, you should work out the correct value.

This will depend on

  1. The forward voltage (VF) of the LED
  2. The forward current (IF) of the LED
  3. The voltage Vin.

To work out the resistor do the following:

  1. Subtract the forward voltage (usually 2 to 4v) from the Vin,
  2. Divide the result by the LED current.

So if Vin =12v, VF = 2v and IF = 30mA then R1 would be

= (Vin - VF) / IF

= 12v - 2v) / 0.03 = 333 ohms, or 330 ohm resistor

*You can match the LED voltage exactly thus eliminating the need for a series resistor, but this is a risky practise keeping the voltage exactly right, especially if driving a display of LEDs. The outcome can be catastrophic.

For information about wiring up multiple LEDs, please refer to the Wiring cinfiguration in
LED Display Tutorial page

Most LEDs have a long and a short leg. This shows which is the anode (+) and the cathode (-) If the wires are cut there is another way to tell which connection is which. The cathode (-) has a flat section in the rim above the wire.

Cathode lower leg

Standard 5mm LED

A = Anode
(long leg) +

K = Cathode
(short leg) -

Written by Phil Townshend - 2008
www.edutek.ltd.uk - Working Electronics For Students & Teachers