Analogies are often dangerous... Regarding the 220 vs 120, well as long as the resistor is bigger than the value you calculated, you're safe and the lower current will safeguard the led, which will just not be as bright.... up to a point of course, too big a resistor and the led will be too dim to see.
I understand that the bigger resistor is the lower current will but I miss some explanation about how to choose the resistor's value.
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