... but how do I work out what size resistor I need to use when working with an LED? I have looked around on Google but I can't find a formula / answer?
Please can someone advise?
I'd go with a 330 ohm resitor in series with the LED. The color code for it is Orange, Orange, Brown with Gold or Silver in the fourth band.
Or, if you feel like being extremely lazy check this out:
It does the same thing as the formula, except it is automatic and a calculator.
*******This is the correct answer**********
Wow, how did I miss that posting?!?!?
Thanks for posting that ctc, great ref material.
Thanks to OB as well, excellent material!!!
We cannot answer all questions about your specific situation, because we do not know the voltage drop and the highest safe current in your LEDs. Voltage drop 2 V and safe current of 20 mA are ballpark figures. LEDs are dirt cheap, go with these numbers. Or better yet: learn how to Google datasheets for your parts. And then show us your datasheet and maybe we can teach you how to read those.
And then you really are no beginner anymore!