Okay, Nym's had his shot.
There are several LED flashlight schemes. The crudest connect 3 series cells to the LEDs, relying on the cells' series resistance and the LEDs resistance. Some add series resistors. If you parallel enough LEDs they can handle 3xAAA in series, but as soon as one LED fails, the rest rapidly die.
Boost converter schemes apply either constant voltage, constant voltage from a current mode boost, or current mode boost, in order of sophistication.
More sophisticated schemes modulate the converter, for multiple power settings.
None use 555s AFAIK.
-- Cheers, James Arthur