How much resistor does an LED lamp need?
Forward voltage drops for LEDs range from about 1.7 v (red) to 3.0 v (blue). If you plan on arranging the LEDs in series then you will need one resister calculated as follows. Assume you will power the LEDs with 9 volts of DC power. Then the voltage drop from 3 red LEDs would be 3 X 1.7 volts = 5.1 v. If you are powering this from, say a 9 volt battery, you will have 9 - 5.1 = 3.9 v that would need to be dissipated by resistance. Generally you should supply no more than about 20 to 25 mA of current to the LEDs. Anything above 30 mA will likely burn them out. Therefore the resister you will need above (assuming 3 red LEDs and 20 mA current with a 9 volt battery) would be R = V/i = 3.9/20 mA = 195 ohms. SO, in general for a series circuit of n (red) LEDs, V volt battery the resister calculation would be (V - 1.7n)/2010^-3.  (20 mA is 20^10^-3 A). If you have a blue LED substitute about 3.0 for 1.7 in the above calculation.