Shake Flashlights are a Joke

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On Fri, 12 May 2006 16:35:17 -0500, Richard J Kinch

I dunno about the shaking ones, but the one I've got that runs off a hand-crank works pretty well.
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definetely batteries, my friend did a autopsey on one, standard button hearing aid batteries. my friend put the batteries in a battery tester they checked out nearly fully charged
the better emergency flashlight has a crank for charging.
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wrote:

I have one of those. It's a radio too.
Now when I hear a radio show where they say "wind up your radios". I'll be able to.
--
Mark Lloyd
http://notstupid.laughingsquid.com
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Mark Lloyd wrote:

I got two of those at Costco -- Durapro, 2/$19.98 or thereabouts. I wound mine up a couple of months ago and it's still bright; I just turn it on for a few seconds at a time to look for something under the desk, but still... OTOH, the one my Mom has runs down in a day -- every time she uses it (mostly to look into her pantry) she has to wind it. It achieves full brightness with a minute of cranking, though. And she can use the exercise.
3 white LEDs, no radio. There's also a flasher with a couple of flashing red LEDs, which seems kind of silly.
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The Real Bev writes:

It appeals to the rubes. "Oooo, pretty!"
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Kinch wrote:

If they made one with a decent magnet and coil and electronics one can get good output. Let's consider say, one foot-pound per second, which I consider fairly achievable from shaking. That's about 1.35 watts. A "1 watt" "Luxeon" LED actually is being given about 1.2 watts when "fully powered". Check out the brightness of battery-powered flashlights that have "1 watt" "Luxeon" LEDs. However, this "ideal" does not account for runtime being longer than shaking time.
- Don Klipstein ( snipped-for-privacy@misty.com)
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snipped-for-privacy@manx.misty.com (Don Klipstein) wrote in

If they used alkaline button cells,they would need THREE to power a white led(needing ~3.5V). The CR2032 are lithium,output 3v per cell.
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Don Klipstein writes:

This is way too optimistic.
Didn't you ever have one of those generators on your bicycle as a kid? What a chore to crank, and you had the advantage of using your legs on pedals.
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Kinch wrote:

I had those. They put out 3 watts, and were said to be 30% efficient, with the biggest loss being in the area where the tire contacts the part that the tire turns. However, I did once use two - one for a headlight and one for a taillight with a headlight bulb.
A younger adult biker in good shape can crank out 1/4 horsepower - about 185 watts.
Now there are bike generators that have a larger roller get turned by the centerline of the tire and losses are much less there.
Now, there are flashlights with hand cranks, and they seem to do better than many of those shake flashlights.
Meanwhile, I don't think it takes that much effort to lift a 5 pound dumbbell 1 foot and put it back down once every five seconds. That's one foot pound per second.
- Don Klipstein ( snipped-for-privacy@misty.com)
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Don Klipstein writes:

You mean for long periods, right? Simply climbing stairs is a lot more power than that.

Still too optimistic.
At least half of shaking is wasted right off (having to muscle the inertia to a stop). And you are shaking mostly dead weight of the flashlight *and* your arm, not just the magnet or whatever is the magnetic flux item (starting to sound like Doc in _Back to the Future_).
Most people would be quite uncomfortable just holding a forearm out steady for more than a few minutes. They used to punish us in grade school with that.
Shaking is a truly dismal generator principle. It just seems clever to a casual, first impression.
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It's not meant to be efficient,it's meant to be reliable;the light is "supposed" to never need batteries and always be ready for use.

Unless you use a bridge rectifier,the return stroke of the magnet induces current in the opposite direction,and you would need a diode to block the reverse current,the diode has a forward voltage drop (~0.4v if you use a Schottky diode),and you lose the energy from the reverse current with a single diode.
FYI,I took apart my $3 "shaker" LED light,and the "magnet" was just a piece of unmagnetized plain steel rod,the coil wires were tucked under the lower battery terminal,*not soldered to anything*,and the batteries were a CR2032 and a thinner CR2025,for a total of 6 volts.They evidently use the (CR2025?)cell's internal resistance to limit LED current.
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And why not?
Nick
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Jim Yanik writes:

But any number of more effective designs like a crank would be more reliable. Shaking is just a gimmick to impress the rubes.
Even bartenders learn that arm-shaking is not a good mixing method; you pivot the wrists.
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Not necessarily. A crank necessitates an opening in the case, which means you can't make it very waterproof - at least not cheaply. By keeping all mechanisms internal, you can effectively seal the case for even underwater use. For a flashlight in a survival kit, the shake method would have some advantage in the reliability department.
Terry & Skipper, Clearlake Texas
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On Mon, 15 May 2006 20:51:48 GMT, Mys Terry

Mine works no matter which way you move the crank. So if you want to use it underwater, you could stick it in a 1-gallon ziplock, and move the crank back and forth, instead of around and around.
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Mys Terry writes:

Yes, brilliant analysis. I am now reprovisioning my offshore survival kit with Chinese flea market trinkets.
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Good for you. Dont forget to pack plenty of WD-40 for when you get thirsty, crackpot.
Terry & Skipper, Clearlake Texas
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I saw one of those at KMart,it had 3 5mm LEDs,they wanted $20 for it. IMO,not a good deal.

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Jim Yanik
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Addendum;interestingly,they used the same circuit board for the fake,and the holes and copper traces are all there for 4 diodes(a bridge),a resistor,and the capacitor.With the addition of those parts and a strong magnet,the shaker-generator would be functional.I may shop around at Skycraft Surplus and see what those parts would cost,just for grins.
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A LED is supposed to need SOME series resistance to work properly.
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Mark Lloyd
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