The Arc AAA-P was such an amazing light when it debuted that even now, years later, it is a legitimate carry option and quite sufficient for a keychain. Even in a field of multi-level output lights, the Arc AAA-P was a contender for two reasons--its stellar construction and its anemic battery consumption. The Preon Revo, a recently discontinued light from 4sevens, is similarly designed but with many of the features that are essential in a modern light. Specifically, the Preon Revo offers multi-level output and impressive current regulation. So, if you can find one, buy it now. These will be hot commodities unless there is a successor light. There were two editions--a basic aluminum one, which is nice enough, and a stainless steel one, which, if I had a choice, would be the one I'd get. They both came in around $40-$50. Here is a shot of the SS version:
The big technological leap with the Revo was the use of current regulation instead of PWM (pulse width modulation) to dim the output of the light. To explain why this is a big deal, I am going to have to get a little tech nerdy. I usually try to avoid this for a few reasons, one of which is I have only a limited base of knowledge, so bear with me. Flashlights, other than those that are current regulated, don't actually put out less light in dimmer modes. Instead the light cycles on and off extremely quickly to deceive the eye and brain into thinking their is less light. So long as the cycles are very short, the trick works. Sometimes they aren't short enough and your eye picks up images jumping (like a moving object in front of a strobe light). Additionally, sometimes the cycling gives on a low level whining noise. Finally, PWM is not as efficient. LEDs are very efficient when using a little bit of power, but less efficient when using a lot of power (incans are the opposite), so when the very bright LED is going on and off quickly, it is still using the high setting and thus running inefficiently (relatively speaking, though, you can still get monster runtimes out of PWM lights). Current regulation is entirely different, it actually limits the amount of power the emitter can draw and thus dims the light more naturally. The benefits are threefold--no image jumping, no whine at low levels, and more efficiency and thus longer runtimes.
The Preon Revo is one of the few production lights that uses current regulation to dim the emitter. This is not just a cheap trick. It is PERFECT for a keychain light. It allows you to stretch out the time between switching batteries. The Preon Revo is truly the next generation of the Arc AAA-P. On low the Revo can run for DAYS (2.8 to be exact). It is also very small, even for an AAA light, smaller even than its brother the Preon I. I also like the look of the light better than the Preon, the knurling especially.
There are a few drawbacks--the light can't tailstand. It is probably going to be more expensive on the secondary market simply because its initial MSRP is a little higher than other lights in its class. It also lacks a pocket clip, but I think this is a keychain light first and foremost so that is not a killer for me.
Overall this is a really unique light and one that performs its given task--being a keychain light--very, very well. They aren't being made anymore, you can still find them just about everywhere for sale. Buy one now before they are gone and become mod fodder on CPF where the current regulation meets newer emitter tech and the light becomes insanely expensive.