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[Update -- WEDNESDAY, OCTOBER 16, 2013
Since I wrote this post, I've seen evidence to suggest that there may be more to LED flashlight flakiness than meets the eye, and that I may have been mistaken in my diagnosis of the cause of it. I'll leave the post in place below as I originally wrote it, but be aware that it may be fallacious.]
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I'm a big fan of LED flashlights; I have a bunch of them that my son got me for Christmas two years ago. I think they're the neatest thing since the Ford 8N tractor.
However, as with all electro-mechanical devices, LED flashlights can exhibit misbehaviours. Two of my small flashlights had the annoying habit of dimming, or going out altogether now and then. A rap against a hard surface would get them back to full brightness, at least until they'd randomly dim or go out again. As it turns out, there's an explanation for why they were doing that, and there's a fix.
Here's a view of the two troublesome flashlights.
The key to this is the manner in which the head-end of the flashlights is assembled; the clear 'lens' is a 'snap-fit' part that retains the little LED array circuit board assembly inside, and establishes its electrical connection to the metal shell of the flashlight. Using a 3" length of 3/4" copper pipe as a 'punch', the whole business can be dismantled with a tap from a light mallet, like so:
Note two things -- the three spots of solder at the periphery of the circuit board, and the un-anodized (bright, uncoloured) ledge inside the flashlight's red shell. That's where the battery current circuit is made between the LED array and the shell of the flashlight. If the circuit board is not pressed tightly against the naked metal ledge, the flashlight will be flaky.
Anyway, so much for 'show-and-tell'. The upshot of all that is that there's no need to dismantle one of these little flashlights in order to fix it. All it takes is to punch the LED array down tighter, like so.
Strike the periphery of the lens lightly and evenly all around to firmly seat the LED array's circuit board against the naked metal ledge inside, and the flashlight will work reliably.
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Update -- SUNDAY, APRIL 21, 2013
The trouble recurred on the red flashlight. These things have a design flaw -- the battery contact at each end of the battery is a coil spring, applying its force in exactly the direction likely to unseat the LED circuit board from its contact ledge. I punched the lens down again, and that got the flashlight working again, but I'll have to give some thought to a way to correct the flaw permanently.
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