I wish I had seen those $3 ones before I spent $20 for a set of two of the "as seen on TV" lights.

The large one is supposed to be waterproof and can't be taken apart, but the small one's bezel can be unscrewed and the guts come out. My toddler son did this three times and I was miraculously able to find all the parts each time before I finally glued the whole thing together permanently.

Before I did that, I took a look at what makes it tick. The two wires from the coil connect to a bridge rectifier, which charges a .1 Farad 5.5v supercapacitor. The LED connects to the cap through a mechanical switch. There is no current-limiting resistor but the cap has significant internal resistance that takes care of that. Very stingy minimalist design. Enough to work, but not very well.

While the electronics could be made better, the real limitation of this device is the magnet & coil generator. It is just very poorly matched to the load and source of power (shaking). The magnet is not perceptibly slowed as it passes through the coil, which means very little power is being generated during each shake. A 50-cent motor from a toy car, coupled to a crank or lever through a gear train makes a far superior generator.

It serves its purpose for keeping a toddler busy until he falls asleep, but it would be very frustrating to use for some critical task in an emergency. It is actually worse than nothing, as someone might depend on one of these instead of buying something much better such as a $1 keychain led light.