DynaFlex Dynamo

About a year ago a friend of mine introduced me to the DynaFlex wrist exerciser. I was intrigued by its simple design and clever use of angular momentum, but it didn't seem useful for much. Then, out of the blue, it occurred to me that this device would make an excellent mini-generator. Since the rotor spins at such a high rate (200-300Hz), the magnets and coils could be small. And unlike the lever-action handheld generators, this would have very few moving parts--no pesky nylon gears to wear down. I'd done a decent amount of reading about wind turbines, and I thought I had a pretty good understanding of how dynamos worked, so I decided to give it a shot.

I found a pack of neodymium magnets at Wal-Mart that looked like they'd fit inside the spokes of the rotor, and I bought some 26-gauge wire at a local electronics store. The magnets fit inside the rotor spokes, and I temporarily wedged them in using folded paper. This worked well enough that I would never bother to glue them in. Four magnets could be placed inside the spokes on each side of the rotor, giving eight magnets total. I arranged the magnets so that their polarities alternated every quarter turn. 

Next, I used a power drill and a screwdriver handle to wind a pair of test coils. The coils were connected in series, taped to the top and bottom of the generator, and the rotor was spun up. Sure enough, a sine wave appeared on the scope.

After experimenting with different numbers of winds, I determined that a coil with 320 winds would yield between 3 and 5 volts, depending on how fast the rotor was spun. I wanted a pair of coils that could easily drive a simple linear 5v regulator for USB charging, so I figured two of these in series should work. I made a simple rectifier/5v regulator circuit and scalped a female USB connector from something less interesting.

The coils themselves only output 1.9 Watts at maximum rotor speed (0.25A at 7.5V), but I think this could be greatly improved by someone who actually knew what they were doing. A more efficient switching regulator and stronger, better-placed magnets would be great improvements.

Kindle Case Update

Though I was more than happy with the way the first Kindle hardcase turned out, there were still several improvements I wanted to make. First, I was really dissatisfied with having to route out the kindle insets by hand with the router on top of the board. The process was tedious, messy, and altogether unpleasant. I thought I might be able to keep the router in the routing table and make some sort of bounding frame that would allow me to just  slide the case half around on the table. This was a little tricky to set up, but ended up working really well. The resulting insets were cleaner, straighter, and much faster to cut. I don't have a picture of this setup at the moment, but I'll get one soon.

The first case was pretty heavy, so the second improvement I wanted to make was to route out both insets a little deeper, allowing thicker foam. This ended up reducing the weight from 1.4kg to 1.1kg with the Kindle inside.

Another major improvement was cutting the Kindle inset big enough for foam around all four edges.

I also wanted something that would keep the Kindle from falling out of the case when it was open. I settled on brass turn buttons and routed out insets for them in all four corners using a dremel. This ended up looking really nice, but it's a little inconvenient to have to move the top turn buttons to turn the thing on and off. Also, if I leave them engaged when I close the case, they keep the foam on the front side from doing any good. I'm sure there's a simple solution for this, but I haven't come up with anything decent yet.

As a nod to this xkcd comic, I wanted to put a Hitchhiker's Guide graphic on the cover somehow. I printed it out, taped it to the cover, and then outlined it with a thumbtack. 

After the paper was removed the little holes were visible enough that I could score the design using a small, flat tool. Unfortunately, the design didn't end up being quite as visible as I would have liked.

I found a guy that does laser engraving, so I'm going to give that a shot. All in all I think this case is a major improvement over the first one. It's still a little heavy, and the hinges aren't aligned quite right, so there's still some improvements to be made yet, but I think I'm almost done here. A friend of mine has commissioned me to make one for her dad, so maybe this project will even pay for itself!

Puzzle Crate

I have always been fascinated by puzzle boxes. When I was little I used to draw up very simple designs, but lacked the tools and know-how to actually build them. I got distracted with high school and the idea wandered off into the back of my mind where stayed for about a decade....That is, until it resurfaced a week ago. 

I had come into possession of a box (a shipping crate, to be exact). It was clear to me from the moment I laid eyes on it that this was no ordinary box, that I loved it, and that a man could do great things with a box like that. I wasn't quite sure what I would do with it, but I knew it would be good.

My first idea was to spray-paint a chessboard on the top. After a mask had been made out of an unreasonable amount of painter's tape, the board was sprayed and there was much rejoicing.

I don't remember how I arrived at the idea, but about six months afterward it occurred to me that I could modify one side of the box to have a hidden sliding panel which, when moved, would allow the whole side to open out. I drew a fancy picture on my desk to cement the idea:

I was a little worried about tearing into my most beloved box, but the idea seemed simple enough, so I promptly began ignoring the upcoming week's midterms and began construction. 

The hardest part of the whole process was actually removing the panels from one side of the box. Shipping crates don't look terribly inspiring, but this one was really well-built. Some of the nails could be pried out, but many were buried in the wood and I didn't want to risk splitting the planks with a crowbar. I ended up having to use a hacksaw blade between the joints to cut most of the nails holding each plank on. This took some time. There was also a 2" thick sheet of foam insulation glued to the inside of off three planks, complicating the process.

Once the planks were removed from the box and separated from each other, things went pretty quickly. I drilled holes in the middle board for the pins, and cut mortise grooves in the other two boards using a drill and dremel.

The three planks would eventually act as a door, so to give it some rigidity I connected the upper and lower planks using two 1x4's. This allowed the door to stay square while sandwiching the center plank in place. I found some concealed cabinet hinges at Home Depot, but they didn't travel outward quite enough and I had to remove some material from the inside edge of the box so the door would open completely.

Unfortunately, the door won't open with the center plank fully translated for the same reason I had to remove material in the inside edge. I considered removing material from the adjacent side to make a recess that the overhang of the center plank would fold back into, but my friends informed me that this was lame since it would be very obvious that the box had been tampered with. Instead we decided that it would be okay if the center plank was slid out once to release the bolts and open the door slightly, and then slid back in to open the door entirely. This was the only real design decisions I wasn't sure about, but in retrospect I think we really made the right decision here. If anything, the added step adds to the box's charm, though you have to be careful not to yank the door open with the center plank still pushed out.

I added a section of 1x4 to the center plank so that bolts could be attached at the top and bottom of the door instead of just the center. I chose to use spring-loaded door bolts so that the door could be opened and closed freely if the bolts were retracted.

The bolts were installed.

The box was closed and opened repeatedly.

And there was much rejoicing.   \o/


(Forgive my creepy breathing, I was a bit sick at the time.)

Kindle Hardcase

About a year ago I was fortunate enough to receive a Kindle for my birthday. I also received a protective neoprene case to keep it from getting damaged. The case seemed like it might protect the Kindle against short drops and light direct pressure, but I never felt comfortable throwing it in my backpack or leaving it out. I looked online for a good, protective hardcase for the Kindle, but found only neoprene sleeves and moderately protective leather covers. None of these things seemed to offer the amount of protection I wanted for a $350 gadget with a very thin and delicate screen (which cannot itself be replaced). I wanted something more like a Pelican case, something capable of withstanding high static pressures. The closest Pelican case I could find to what I wanted was designed for a laptop, but that seemed way too big to be practical. So, having been failed by capitalism, I endeavored to build my own.

Initially, I had grand schemes about making the case out of fiberglass and integrating a solar charger into the cover. I even drew a fancy picture:

I put a lot of time into trying to make fiberglass case halves, but this ended up being mostly messy and difficult without the proper tools. Someone with more (read: any) experience with molds and fiberglass could probably knock it out pretty easily, but my feeble attempts using wooden molds just weren't as clean as I'd hoped they'd be.

I felt like I'd gotten something that, with a little more effort, time and money could have been refined into being usable, but I was discouraged by the general messiness of fiberglass and decided to try a new approach. I wanted to look into building a solid wood case, so I drew another fancy picture and began construction.

Although I had no prior experience working with hardwoods, I think the results turned out just fine. Here's the first iteration of my mesquite Kindle hardcase:

Some will say that two spring-loaded clasps is overkill. They will say this until their singular clasp accidentally unlatches, their case pops open and their Kindle falls out & onto the ground. To protect something that costs several hundred dollars, an extra clasp makes sense to me.

The edge of the kindle is surrounded by high-density foam to protect it from short drops. I cut the inset for the Kindle a little too short, so there wasn't space for padding along the top edge. Future versions will have foam on all edges.

You'll notice there also isn't anything holding the Kindle inside the case. This is definitely something that needs to be fixed, and in future versions I think I'll add brass turn buttons to all four corners. I also still want to integrate a solar charger into the case, but that will take some more time.