For high power leds
While you can buy hi-power torches that will run off 1.5V batteries, they are built with IC's designed to do the work. While you can get close, a simple circuit using a joule thief has trouble, even using a couple of high gain transistors. It is possible if you were to use 2 x 1.2v NICAD batteries, or if a 3.7v lithium battery was used with the circuit slightly updated.
