|Two White LED Flashlight|
To make a flashlight with multiple LEDs, you must take into consideration the voltage drop across the LEDs. For the white LEDs which typically drop 3.6 volts each, the most you can use in series is two, because 3.6 * 2 = 7.2 which is less than 9 volts, whereas 3.6 * 3 is 10.8 which is greater than 9 volts. To calculate the series resistor needed, use the same equation as in the single LED case, but take into consideration the voltage drop across both LEDs:
There is a great web site called OtherPower.com that has a good explanation of using multiple LEDs.
Ready for more fun with LEDs? Try these out: