What Light Through Yonder Flashlight Shakes?
I recently helped a lad put together a school science project that involved the idea of using "shake flashlights" as a power source. The idea seemed like a good one when he first proposed it. But when we began to implement it we immediately ran into a quite unexpected problem: it's impossible to find shake flashlights in retail stores.
I hadn't even anticipated that possibility. Last I remembered, they were available at the local Target in the flashlight aisle alongside their cousins, wind-up flashlights. After we struck out at Target we tried Home Depot and various other flashlight Meccas. Wind-up flashlights were still available but not a single shake flashlight could be found.
No matter - To the Internet, Watson! First stop: Amazon. Yup, Amazon had a few but each one had some seriously negative reviews. Sensing from that we might be in for problems, we decided to buy a couple of different ones from both Amazon and eBay. Altogether, we spent close to $50 on shake flashlights.
The first one that arrived, from eBay, turned out to be one of the best. It seemed to be well built and had a nice strong light, even without shaking. It contained a couple of coin cells which evidently were powering the thing out-of-the-box. It contained 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. The body was transparent so all that could be seen even before taking it apart. I was expecting a regulator of some kind so I was surprised 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 it shone brightly out-of-the-box, we would need to run it down to get some real idea of what kind of power source the shake-charging system might provide. So, we left it on overnight. I was amazed to find that it was still shining brightly in the morning. So we left it on all day. It was still shining brightly that evening. The project deadline was looming ahead so at that point, it seemed to make sense to just disconnect the batteries. Initially, I thought that we might have to cut a wire, and I didn't really want to do anything that was hard to reverse. However, it turned out that they coin cells could be just popped out from what amounted to a built-in battery holder.
With the coin cells out, we could see more what was going on. It turned out that each shake would create 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 to get enough voltage for the project we would need two flashlights. It turned out to be a couple of more days before the second flashlight arrived. With that one, we followed a similar procedure to that above. 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! We were beginning to feel ripped-off so I started looking at the packaging to see who we had bought it from, thinking that we should write some kind of appropriate comment about the seller on Amazon or eBay. However, the company that sold it had the word "toy" in their name. So, 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.
With that, though, The Mystery of the Vanishing Shake Flashlights 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 we got was actually a genuine shake flashlight. It's notable that each shake flashlight we received had a transparent body that allows you to see all elements - even the fake ones.
That also explains why shake flashlights aren't available at retail anymore: retailers must have experienced lots of complaints and returns after the coin cells (finally) ran out. So, selling them is a very bad idea.
And it explains another thing that I didn't originally understand: cranking a crank flashlight is pretty hard, whereas shaking a shake flashlight seems almost effortless. Crank flashlights really do work: you can crank one that's completely dead and it will shine for a while. So, although crank flashlights typically have three LEDs rather than a single LED in a shake flashlight, the comparison of energy input-versus-output of the two systems never seemed to make sense.
But the fact that shake flashlights are really disposable flashlights - that ultimately stop working after a while - explains it all.