I can't find a tutorial on this anywhere. I'm new to this and want to get it right. Let's say I'm powering my Arduino off a 12V DC plugged into the wall (here in the US). If I hook up three LED lights to three different I/O pins, how do I calculate the resistors I need for those LEDs? Is the outgoing power on each arduino I/O pin the same or does it drop with each use? I've read tutorials on calculating resistors for LEDs but they only talk about when it comes from the same source, not even talking about Arduino.
Power is stabilized at 5 volts, so every pin will give you 5 volts, and deliver an absolute maximum current of 20 mA. (*) That's all there is to it. 
*: With a max. total of 200 mA for the UNO.
does it drop with each use?
No.
Thank you. This helps so much.
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