I just wonder, why I need to put one resistor in front of each LED and not simply use one R between LED GND and Arduino GND? Wouldn't that have the same effect?
The heart of the problem with the one resistor circuit is that the more current there is through the resistor, the more voltage drop you get in that part of the circuit... so the brightness of the LEDs changes when you go from one to two to three LEDs lit.Long answer:http://www.arunet.co.uk/tkboyd/ele1led.htm
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