Changing a homemade LED light (14V Battery Pack) to run off mains?
For one blue/white 10mm LED just use 3.3V or drop down with a Schottky diode to 3.0 for &lt;half brightness roughly . Use a series 1 ohm to test current for 20mV +/- 50% (or more)
Where you get 3.3Vdc is up to you.  E.g. 2V drop from 5Vdc / 20mA = 250 ohms. Higher V means bigger power resistor. [ohms &amp; watts]