How to calculate the resistor value for a led:
$\displaystyle R=\frac{(V_{power}-V_{LED})}{I_{LED}}$

Where:
R = Resistance specified in Ohms (Ω)
$\displaystyle V_{power}$ = voltage of the powersupply specified in Volts (V)
$\displaystyle V_{LED}$ = voltage drop over the LED specified in Volts (V)
$\displaystyle I_{LED}$ = the desired current through the LED specified in MilliAmperes (mA)

When specifying the current through the LED, never go above the spec from the manufacturer. It is possible to have the LED shine less bright by adjusting the current downwards.

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