Also (5V - 3V) / 2700 ohm = 74mA, not 7.4A

NO, the higher resistor value does not mean the LED can take a higher current. The LEDs have a specification that lists maximum current. The resistor is to protect the LED from drawing too much current.

A higher resistor means that the LED can have the same current at a higher voltage. Current = Voltage / Resistance.

Follow ashimoke's instructions.

Even if he is trolling you, you're posting useful information for people like me who don't know anything about electroncis.

NO I'm certainly NOT trolling, I'm just sitting here and desperately trying to figure out why there are 2,7k ohm resistors metioned in trash80's shematic, that's all!

ok, do this:

1) find LEDs you like
2) find their voltage (for example 2V) and their current (like 30 mA)
3) open the link I posted
4) fill in the red (5V), green (LED voltage) and blue (LED current)
5) Click "Click to calculate"
6) read the "Calculated Limiting Resistor" (Yellow)
7) write it down

Now you have the resistor value. You buy these resistors and add 1 before every LED.

...but I guess you are just trolling me anyway.

okay , i tried to change the formula to fit the specs i got from the drawing of the instructables cause i think it would be better to have 1 resistor for 1 led instead of 1 for all cause if that one fails, all led will be destroyed.

so the formula is:
(source_voltage - led_voltage) / resistor = (led_current)

5V - 3V = 2
2V / 2700 ohm ~ 7,4A

that means that the LED can draw up to 7,4 ampere, right? so basically, the higher the resistor is, the safer it would be for the led?

You can buy LEDs of any voltage, as long as the supply voltage is greater than the LED's forward voltage. Just make sure the resistors for each LED are big enough. You should use a resistor of at least (source_voltage - led_voltage) / (led_current). For example, a 3V LED with a maximum forward current of 20mA powered by a 5V source needs (5V - 3V)/(0.020A)=100 ohms.

edit: grammar mistakes

Thank you very much for the Link!

quote from the arduino site (about the nano which i want to use):
"Each of the 14 digital pins on the Nano can be used as an input or output, using pinMode(), digitalWrite(), and digitalRead() functions. They operate at 5 volts . Each pin can provide or receive a maximum of 40 mA and has an internal pull-up resistor (disconnected by default) of 20-50 kOhms."

so i i understand it right, 5 Volt Led's should do the job, right?

i believe only one pin does (13 if memory recalls), the very early arduino boards didn't at all though i'm not really sure if that feature is standard now or not.  basically...  its set up so you can do the "Blinky" (or whatever) tutorial with just the LED and pretty immediately...

There are no built in resistors - you have to add them like in trash80's schematics. Use this to calculate the resistor value.

i tried to find the right specs for the led's online but i didn't find any infos regarding the voltage. that's why I'm asking here.
do the digital pins of all arduino modells have a built in resistor? otherwise it would destroy the led if i use them without a resistor.

That's what the resistors are for. But you can use 5V without resistors. Just buy some leds and try it - costs few cents. I'm not answering no more questions about leds - you can easily find all info you need online. Do that next time please.

I stilltrying to figure out why I should get 3V LED's. Don't get me wrong, if you tell me i should use 3v, i will use 3 v. But the digital pins of the arduino run at 5v as far as i know, so wouldn't it be better to also use 5V LED's?

Any kind of a technical explanation regarding this would be really helpful cause i don't want just build something, i also want to understand the technical aspect behind it

it's best to use 7V to 12V because of the 5V regulated internal supply being linear-regulated.