I need to use 5 white LEDs connected to an Arduino Duemilanove
I'm absolutely new to electronics and have no idea which resistor value I should apply to each LED. Looks like 200 ohms or 270 ohms is fine for the LEDs provided with the starter kit but for the LEDs I intent to buy, I'm not sure. See info below.
? LED Ø 5 mm
? Intensité lumineuse >10cd (10.000mcd)
? 50cd@ 3,6V / 75mA
? Maxi: 50cd @4,2V
If someone could tell me how to calculate that value that would be great.
You'll need to wire up a separate resistor in series with each of those five white LEDs, i.e. five LEDs, five resistors, 18 Ohms each. You can't connect all five LEDs in series (not enough voltage) nor can you connect them in parallel with a single resistor (forward voltages won't match).
If you want to control the LEDs indepenently, then you'll need five separate Arduino output pins.
Will it work with this kind of LED and 18 ohms resistors only if I use 5 output pins or do I need a transistor?
You will need 5 transistors. The reason is an Arduino output pin can only source or sink a certain amount of current before it will burn out. 75ma is too much current for a output pin.
So, if I use this kind of LED (18 cd @ 3,1V / 20mA - Maxi : 22cd @ 3,6V), I don't need a transistor (below the 40 mA) and can only use 95 ohms resistors.