This idea (pwm for flashlight bulbs) is a REALLY GOOD one. It doesn't waste battery power, provides near constant illumination, and can vastly extend the bulb life if done properly. There is an easier solution than making a big pcb with tons of discrete parts - use an off the shelf PWM IC (maxim, unitrode, and a few other companies make them) - then have it drive a BHF (big honkin FET) to run the bulb. The result could be fit inside the tailcap of a lamp, as streamlight did on the sl-35 and sl-20 (x) lamps, complete with a dimmer circiut.
The only drawback is that extended runtime at low intensity (dim) will not allow the halogen cycle to re-deposit the filament metal on the filament itself, resulting in shortened life. So the USER is responsible for running it 80-90% of the time at full power, with occasional dimming. If you buy an SL-35x with the intent to use it in a planetarium, you've made a tactical error.
