Hi! This will be my seventh update for this challenge. In this update I will cover the key picking mechanism. In the sixth blog (Fingerprint Skeleton Key - Slider mechanism part 2 - Design for a Cause Challenge - Blog Post #6 ) I constructed the key holder and the spring holder mechanisms. The key picking mechanism will go on top of it and will chose the right key for us using RFID which I talked about in the third blog (Fingerprint Skeleton Key - RFID Module - Design for a Cause Challenge - Blog Post #3). I will be constructing the whole mechanism using widely accessible and low cost parts. First, let's begin with the design.




The primary goal of this part of the project is to choose the right key for us, depending on the RFID tag. But we can't stop there. What if the tag falls off, or someone takes it? What if the program has a glitch or the battery runs out? We need it to have a fail safe mechanism so it can work without power. This all means that the connection between the device that will choose the key and the key itself must have some flexibility to it. After thinking about all the solutions I ended up with magnets. The design will consist of 2 big rails going down each side of the device where another set of smaller rails will glide on them, with a small key picker gliding on them. Put like this it sounds weird, but essentially the mechanism is like a big plotter. The way I plan on choosing the key will be with a small servo motor attached to the Arduino. A small magnet will be attached to the Arduino, and another small magnet will be attached to the gliding key picker. This means that when the key picker is in the back position when it's not used the servo will be able to shift it left and right depending on what key we need, but since we are using magnets, if we want to use a different key regardless of the key that is currently picked, we can still do that. To make this we need some sort of rails system. First, I planned on using ordinary screws with drilled through plywood or something along those lines, but of course the system wouldn't have been smooth and would generally have a lot of trouble functioning. But then I got an idea from a common and really useful tool, a telescopic magnet. So, I went out and got 2 of the smallest ones I can find, and here is all of the material we get from them.



We get 6 tubes from each of the telescopic magnets as well as a magnet, the magnets are a little bigger than I would like them to be, but they are pretty strong and should work perfectly for the project. When I dismantled it and took the small things on the bottom of the second picture out, which were holding the tubes from separating, the tubes glide incredibly well over each other. So now all that is left is to cut, drill and glue everything together and attach it to the slider mechanism we made. I mentioned in the last blog that this mechanism will be playing a roll in giving a bit more rigidity to the key holder and here is how it will do it. Some of the smaller tubes will be hot glued on top of the key holder, on top of the key slots. By doing this we add a lot of strength to the construction, while also having a guide for our key picker. The guide tubes on the top will also be used to lock in the key when being used. I will cover more of that through the build phase.



Most of the build will consist of the tubes shown above, so I needed a way to connect them. I wanted to try going tidy so I tried going with solder, I could actually get it to stick, but the progress was too slow, so rather than that I went with a little bit messier approach, hot glue. I had to begin from the smaller parts and slowly go towards the bigger rails. The tubes have pretty thin walls so I had to be careful when cutting, I used my rotary tool which I clamped into a vice and just went slowly rolling the tubes over the spinning disk. In the end I ended up with these:

I will now go into assembly, just want to point out a few things before that. Most of the dimensions to which I cut depended on the previous piece I put together, and the technique I used for getting sturdier bonds was to actually fill the tube with hot glue before connecting it to the other one, since that gave a lot more strength. And the only important thing when assembling all of this is to keep everything as parallel as possible.


Rail System

{gallery} Rail system assembly

First Piece: I didn't do the smallest part first, since it depended on the distance between the next level rails. For the size I aimed so I can comfortably cover the area needed to access all of the keys, so it is just a little bit wider than the key holder mechanism. When we have this put together we can go from there and assemble the rest.

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Magnet and key pusher: Here I added the key pusher, which is one of the tubes cut in half and the magnet that will later on work together with the servo, next on we needed the guiding hook.

Guiding hook: This hook will make much more sense when the top structure is installed, but simply said, it will be used as a guide and as a way to lock the key in the extended position when using.

Different angle: While it does look a bit messy it works incredibly smooth and the hot glue is holding everything in place extremely well.

Rail system: And this is the complete rail system for the key picking mechanism, all that is left to do is mount it to the part we have from the last part.


Back Plate

Last time I finished up the key holder and the spring holder build. But said that new reinforcements will come with this part of the build.



The big tubes will lay on top of the upper screw and the key holder part but we need something in the back to hold it. So in the end I ended up with the simplest solution that will solve some other problems as well. The solution is a small back plate with 2 screws that will act as supports for the big tubes. One more thing this will solve is it will act a barrier between the magnets. The magnets are really hard to separate when they are directly in contact, but work really good with a piece of plywood between them. I will also try mounting the servo into the back plate later on.



The 5 mm screws fit pretty loosely inside the tubes, which is actually what I wanted because similarly to how I connected tubes to tubes, I filled the tubes with a lot of hot glue and just slid it down the screw. The end product is extremely sturdy, but could manipulate it enough to get the tubes as parallel as possible when mounting to the rest of the build, which is the last thing we need to do with this part. As I said, I used the top screw from the key holder and hot glued the tubes to its ends and hot glued the back plate to the spring holder part and ended up with this.



Unfortunately since the deadline is approaching fast, and the makerspace was closed during the summer, I didn't have time or some of the tools I needed to fabricate this to look better. If I were doing this again, of course I would have gone with 3D printing for the most part and would change the springs to either a different type or design a different mechanism making everything much much smaller. But even considering all of that it really performs great which I was aiming towards! Here is a video of how it works and where the magnet from the servo will go.


First test



Summary and what's next

This is the part of the build I was most worried about when it comes to functioning. I didn't really know how smooth the system itself would be, or how much strain could the little tubes handle, but by filling them with hot glue and using that method all around it proved to be extremely durable, while still working great. In the next update I will be finishing the slider mechanism, and after that all that's left is mounting all of the electronics and programming it. The things left to finish for the slide mechanism is mounting the servo with the magnet in the back and determining at what angles the key pusher will align with the keys, as well as top and bottom plate. The bottom plate will be just a simple piece of plywood, while the top plate will have more tubes, and that's where the guiding hook comes in. The plan is to mount precisely cut and places tubes on top of the key holder part so that when the key pusher is in the back that the end of the hook is near the entrance of the tubes. And on the other hand when you want to use a key and you extract it, you would move the slider a bit to the left so the hook can hook into the side of the tube keeping the key extracted until you pushed forward and right releasing the hook. Time is running out but I should be able to finish with a fully working prototype!. Thanks for reading the blog, hope you liked it!