Which resistor for a white LED 3,6V / 75mA ???

Forum 2005-2010 (read only) Hardware Interfacing
2

5V supply
3.6 volts across the LED
therefore 5 - 3.6 = 1.4 V across the resistor.

If this resistor has to carry 75 mA then it needs to be:-
R = E/I = 1.4 / 0.075 = 18 Ohms.

Two problems:-

  1. You can't get that current out of the Arduino's output pin so you will have to use a transistor to drive it.
  2. White LEDs are best driven not with a resistor but with a constant current drive. But you might get away with just one LED per transistor.