Tony, if your LED is actually rated to operate with a 12v source it means it already has a resistor built in or included, probably around 500-600 ohms. All LEDs need a resistor to limit the current to around 20mA. If you are sure that your LEDs already have a resistor bulit in you can still use them with 5v but will be a little less bright. Careful, if there is no resistor in series with your LED you will blow it up applying 5v (or 12v) directly.
Andras
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