So based on your calculation,
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So (12V - 2.5V)/11.5mA = 826 ohm
Do the math for 5V - gonna be dim!
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led will act like a 826 ohm resistor. So without the resistor I will have
5V/826ohm = 6mA . So it should be OK without an resistor?
and if you add additional 220
5v/(826+220) = 4 mA.
Am I missing something ?