Hello Hello I really need some help. please Help me... :'(

(Each led = 20 mA + 1,6 V - 2 V)

If I have 3 leds in series on one pin without transistor, would that be fine? if so, what would be the resistor needed? If I have 3 leds in parallel on one pin without transistor, would that be fine? which resistors do I need?

and If I have 20 leds in parallel on one pin using a transistor of 1A, how can I know what would be the resistors needed? Can I use a 9V battery? or a 5v power supply?

I'm sooooo confused... please help me.. i would really appreciate it a lot... :-[

The arduino uses the Atmega 168, you should not drive that many LEDs as it's too much current.

Use the proper transistor and drive them like this

Yes each LED needs it's own resistor, calculate it's value from VCC which is 5v The total current the arduino can make is under 800ma give or take so if you're going to be driving a lot of LEDs you'll have to use a separate supply.

The 2N2222 does have a limit on the number of LEDs it can control. If you need more controlled from one pin use a bigger transistor, if you use enough LEDs that transistor would need a heat sink or you could use one 2n2222 to drive several 2n2222 transistors each driving a string of LEDs

You'll have to google 2n2222 transistor specs to find out it's max current.