LL Bean Crank Flashlight - Disassembled

I recently received an LL Bean wind'n'go flashlight. This is a flashlight that doesn't require batteries (so they say, we'll find out the truth soon enough), but instead relies on a small hand-cranked generator (motor) to power the flashlight. They advertise one minute of cranking yields 30 minutes of light. I've yet to verify that statement. The light also has the nice feature of being able to run in high or low modes, which correspond to powering all three or just one of it's white LEDs.

Being the naturally inquisitive sort that I am, I found it to be pretty darn cool, but also wanted to know how it worked. So, I took it apart. It's amazingly simply actually. The crank system has a set of 4 gears that increase your turning power, so that for every crank of the handle, you are able to turn the generator many more times (and thus produce more electricity). The handle gear has 40 teeth, the first gear is a 12:40 ratio (meaning that turning one tooth on the small part of the gear will turn 40/12 or 3.33 teeth on the large part of the gear, so we get a 3.33 increase in speed), the second gear also has a 12:40 ratio and the third gear has a 12:43 ratio. The final generator shaft has 16 teeth. Multiplying all our ratios out, we see that we have ( 40 * (40/12) * (40/12) * (43/12) ) / 16, which comes out to 99.24. So the gear system provides a 99 times increase in turning efficiency. In other words, for every 1 crank of the handle, the generator shaft is turned 99.24 times. That's pretty good!

Now on to the internals of the beast! Here's a picture of what the flashlight top and bottom look like. It's relatively small and easily fits in your hand and cranks nicely.

There are four deeply recessed phillips screws on the bottom that hold the flashlight together. If we remove the screws we can pop the top off and here's what we get.

The generator and circuit board simply lift off of the bottom part of the case where the gears are located.

Now onto the circuit board! It's amazingly simple as far as electronics are concerned. No fancy electronics or even any type of regulation. The most complicated we get is one transistor. Also, we can clearly see in the following photos that the flashlight contains a 3.6v Lithium Ion battery (model LIR2450). I found a datasheet for the battery. It has a nominal capacity of 100mAh, which doesn't seem like much, but the LEDs probably only draw about 20mA, so on the low setting and with a fully-charged battery, it could theorhetically last about 5 hours. The charging specs indicate a max charging current of 100mA and 4.2V. It seems to me that the generator must be supplying more current than that to the battery in order for one minute of cranking to provide 30 minutes of light. Assuming the LEDs are only using 20mA and the generator is providing 100mA of charging current, that's a 5:1 ratio and so it would be more like 1 minute of cranking would provide 5 minutes of light. Something to look into... Another important thing to note on that spec sheet is the cycle life. It's rated at 500, which means an average battery can be charged and discharged 500 times before failing (or at least getting worse). That's probably plenty for most people.

Let's take a look at the circuit again. It's shown in the following 3 figures and then the bottom circuit traces are shown. I also created a circuit schematic for the flashlight using EagleCAD. That's shown at the bottom. The generator (motor) produces an AC voltage. That is rectified into a DC voltage using a full wave rectifier made from four 4001 diodes. A 25V, 100uF capacitor is placed across the power rails here to help smooth out glitches and average the power. The battery is charged thru an SC8050 NPN transistor. A 4.3 zener diode at the base of the transistor assures we have adequate voltage before charging. A 470 ohm resistor is used to control the current to the base of the NPN. Note to self: Need to measure the current that is being supplied to the battery while charging.

The control of the LEDs is done completely with a switch. The switch has one input (from the battery) and two outputs. One output goes to all three LEDs, the other goes to just one LED. The switch can be in one of four states, where the next state is entered by pressing the switch momemtarily. The states loop around.