This simple Linear circuit provides continuously variable regulated current (~25-400mA) from a 4-6 Volt source. I chose a linear design for simplicity, reliability, ease of repair, and to avoid switching EMI in my Cave Radios. The circuit requires only 0.2V headroom above the parallel LED Array voltage to provide regulation at maximum current. The headroom stays low until the LED's are extinguished at about 0.75V/cell for 4-cell packs. End of life for alkalines is usually considered to be 0.9V/cell. My HDS 24-LED array requires 2.9V at 25mA and 3.5V at 440mA.
The circuit can be scaled for larger or smaller arrays, and should actually handle several Amps with a larger heatsink. With 4 AA Alkalines, life should be 3 hours (or more) at maximum setting for a 24-LED array, but ~60-80 hours at minimum setting, which is bright enough for many activities, including reading, if the mounting bracket allows the lamp to pivot downward like mine does. In a two week test, I found that the white even light made caving easy. The "rings" and sharp cutoff of halogen lamps are absent. I found that I could usually get away with 200mA when moving, and much less when stationary (surveying, resting, eating), getting up to a10 hour trip from 4 AA's. The real beauty of white LED's is that the light remains white even at the lowest settings, compared to halogen light which shifts rapidly to infrared when dimmed.
The battery pack can be 4 AA, C, or D size Alkaline, Ni-Cad, NiMH, 1.5V AA Lithium cells, or a 6V Lead-Acid. 3-cell C or D Alkaline packs should also work, but they will not be discharged as deeply. "Dead" cells from Halogen lamps, GPS, etc., should give hours of "free" light at lower current settings. My latest pack is two 3V, 8AH lithium cells removed from a military surplus BA-5598/U battery (5 cells/battery). These are available from Fair Radio Sales at http://www.fairradio.com at 2 for $6.50 plus shipping.
My headpiece is an old waterproof "Easter Seals" Lexan "Roosa"...