Some devices are elaborately made to appear as if they operate on Faraday’s Law, when in fact the components have as much functionality as a pet rock
By Grant E. Griffin, dspGuru.com
All the rage a few years back, shake-to-charge flashlights employ a magnet passing through a coil of wire to create electrical energy, which is stored in a capacitor and used to power the light source.
Uh-huh. As Grant Griffin at dspGuru.com, who recently helped his son put together a school science project that employed shake flashlights as a power source, discovered (along with a whole lot of other unsuspecting consumers) that some (more than some?) of these devices are elaborately designed to appear to operate as claimed.
“My son's idea was to wear something on your feet that would charge via your normal body motions. Then you could use the charged gizmos to power your iPod. We decided to use the shake flashlight as the charging sources.
The idea seemed like a good one when my son first proposed using shake flashlights as a power source. But we immediately ran into quite the unexpected problem: It's impossible to find them in stores. We discovered a few on Amazon—all with some seriously negative reviews.
Realizing at this point that we might be in for problems, we nonetheless decided to buy a few different models from both Amazon and eBay. Altogether, we spent close to $50.
The first one to arrive, purchased on eBay, turned out to be one of the best. It appeared to be well built and had a nice strong light, even without any shaking. All the components could easily be seen through the transparent body, including a couple of coin cells that evidently were powering the thing out-of-the-box, the requisite magnet and coil, plus a little circuit board that looked like it had a large capacitor and a full-wave bridge rectifier built from four discrete diodes.
Expecting a regulator of some kind, I was surprised at how simple the circuit board was. But it had all the elements needed: a shake power-generating system and short/long term storage for the generated power. So far, so good.
Since the flashlight shone brightly directly out-of-the-box, we needed to run it down in order to get an idea of the kind of power source the shake-charging system would be. So, we left it on overnight. The next morning, I was amazed to find that it was still shining brightly. Twelve hours later, it was…still shining brightly.
With the project deadline fast approaching, it seemed to make sense to just disconnect the batteries. Initially, I thought that we might have to cut a wire, although I was reluctant to do anything that was hard to reverse. However, it turned out we could just pop the coin cells out from what amounted to a built-in battery holder.
Now we could see more of what was going on. Each shake created a burst of light that seemed about as bright as what we had experienced before. However, it was very brief. True, we had taken the coin cells out, but wouldn't the capacitor keep the light going for just a little while?
Our measurements indicated that we would need two flashlights to get enough voltage for my son’s project. When the second flashlight arrived a few days later, we followed a similar procedure. However, after we took the coin cells out, we found that it didn't do anything. Upon close examination, we determined that the coil was not connected, and the slug inside wasn't even a magnet!
Beginning to feel ripped-off, I started looked at the packaging to see who we bought it from , thinking that we ought to write some kind of appropriate comment about the seller on Amazon or eBay. However, the company that sold it had the word "toy" in its name. Arguably, a shake flashlight sold by a toy company doesn't have to be any more real than a phone sold by a toy company. I decided to let it go.
At that point, everything was beginning to make sense: Shake flashlights aren't really flashlights that you charge with a shaking motion: they're disposable flashlights powered by coin cells! In that sense, the toy flashlight was the real deal.
All the discussion of magnetic induction is clearly technical mumbo jumbo, intended to impress and reinforced by a transparent body that allows one to see all the working elements (even the fake ones!).
That also might explain why shake flashlights aren't available at retail anymore: As the online reviews suggested, retailers must have experienced lots of complaints and returns after the coin cells (finally) ran out. So, selling them is a very bad idea.
But the fact that shake flashlights are really disposable flashlights, which ultimately stop working after a while, explains it all.”
Grant Griffin is the founder of dspGuru.com, where he originally blogged about his adventures with shake flashlights.
I purchased 10 or more shake flashlights at an Electronics Swap meet. They actually work quite well albeit the single LED is not real bright, but then how much energy can you get by shaking a flashlight. Curiously the smaller size flashlight works much better the full size shake light - so my review is abt the smaller. Anyway it just takes a few shakes to brighten it up for minutes. It holds a charge a long time so it almost always works at first without shaking. I tested several of the lights and they work. One has a flaky switch though. As an emergency flashlight, it is perfect.
I actually have a shake flashlight - that actually worked! I won't say it worked well, though. The magnetic slug was repelled by magnets on either end to minimize impact - good thinking. However, one of the cushioning magnets eventually fell off inside the tube and stuck to the slug. It is now a toy. What impressed me was the sealed nature of the light. There is no way to disassemble the light. Even the switch was sealed inside. It was a reed switch inside activated by a small magnet on the slider outside.