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Borderlands style jewelry box research thread
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Youkai:
Hello everyone. New member here; thank you for having me. I'm pretty new to this type of thing and I have no real electronics background. If you would like to get an idea of my current level of knowledge you can take a look at this youtube playlist of a previous project I attempted (it failed). https://www.youtube.com/playlist?list=PLBDD0A3B6E5C15862 That was pretty rough but I learned a fair amount while working on it.

I'm currently endeavoring to build a motorized jewelry box inspired by the Borderlands treasure chests. Attached are two pictures of a rough model I have created. Blue is the lid which would hinge at the back where the blue cylinders would match up with the grey cylinders on the back of the base. Purple are a couple of trays which would rotate forward and out. Purple cylinders would sit on top of the grey cylinders they are above.

The dimensions of the final product will be approx: 20cm wide, 16cm deep, 12cm tall. All of the cylinders are 4cm in diameter. The lid will rotate up 90 degrees. I'm thinking the trays will rotate 180 degrees probably. I'm still very early in the design/planning phases so if the electronics dictate I can change some of the dimensions.

So my questions are how to make the items move. In my previous project I used servos and that worked fine but I was watching some videos on youtube later and learned that stepper motors are a thing that exists. Would one of those be better than the other for this application or is there some other type of motor that I'm not aware of which would be good? I also need to consider the size of the motor. It either needs to fit in the 4cm pillar or I'll need to find some way to adjust the design to hide it's bulk inside the body of the chest.

Whichever type of motor I decide to use is there a way for me to "know" it's position or will I need to have sensors/switches of some kind to tell me when the parts reach certain positions?

Finally I'm likely going to use and arduino board to control it unless someone knows of a better way.

Thank you all for your time!
Ian.M:
With a small design change so the rear cylinder pillars merge into the grey hinge cylinders, most of the mechanism could go into the base.   You'd need a vertical shaft in each of the front pillars to swing out the trays and a vertical shaft in one of the rear pillars to drive a pair of bevel gears to operate the lid.

I see little point in using steppers or even servos - they are bulky and only needed if you want to be able to stop at arbitrary intermediate positions.

Put a large cam in the base with three tracks machined into its face, one for each swingout tray and one for the lid.  Drive the vertical shafts via cranks from pushrods with roller ends to fit the cam tracks.   Drive the cam with a belt on its rim (so the mechanism slips if its jammed rather than destroying itself) from a tiny 3V motor.   The cam will need limit sensors or switches, and possibly an extra one for the lid open but trays not yet swung out position.   Run the whole lot off 3x AAA batteries for a nominal 4.5V dropping to around 2.7V at end of life, in the base (or 6x AA if you want to add lighting).   PWM the H-bridge driving the motor to maintain 3V as long as possible as the batteries discharge.  Alternatively use a 18650 LiPO - get a dollar store single cell USB powerbank and hack it for its cell and charger/boost board.  Run the Arduino straight of the cell, and disable the boost circuit by removing the inductor if you don't need 5V for anything.
Youkai:
Wow there is a lot of information there. Thank you Ian.M. A fair portion of it I don't understand though or have questions about.

I get the bevel gear to operate the lid.

I think I understand what you are saying about the cam in the base. Couple questions though. I originally thought I would have the front face of the two drawers be part of the front face of the chest. Then I realized if I just have a flange on the lid that covers them I don't have to have such tight tolerances on them to prevent gaps in the outside wall. So the lid will need to open a little before the drawers start swinging out. I think you cam idea will allow for this correct?

Also this design is incomplete. I could leave the main body of the chest as a big cavity but I was thinking about having a tray under the drawers that would swing up/out after the drawers are out of the way. I'm not sure if this feature would be possible with the cam idea. Possibly a second motor? I guess since it's my first project I could scrap that as too complex.

There obviously needs to be a button on it somewhere to activate the mechanism. Additionally I was thinking about having some LED on the inside of the lid's flange to light the inside of the chest when it's open. Also a few LED mounted to the bottom of the chest to light up the surface it's on would be cool. The LED would be RGB LED's possible a short section of an LED strip so I could do cool things with the colors using Arduino. I don't think either of these has much influence on your comments.

I get the sensors or switches for the cam. I could have a very light trigger switch that the cam runs into to tell the Arduino when to stop the motor.

After a quick google search I see that the H-Bridge will allow me to run the motor in reverse. Will applying PWM to that really save a noticeable amount of battery? What would you set the duty cycle of the PWM to?

Also how do I insure that it doesn't move/wobble in the open position? I was thinking there would be hooks in the lid to hang necklaces on the inside. So there will be minimal amounts of force applied when it is resting in the open position. Will the motor/cam/band hold it steady enough?

Finally I don't understand too much about what you were saying with the LiPO cell and disabling the boost circuit, etc. Do you have some links to blogs/tutorials that cover that topic? The arduino runs off 5v so couldn't I just plug the battery pack directly into the arduino using a USB cable to power it? Actually to power the whole thing I was thinking since it's going to be sitting on a counter top/desk anyway maybe I could just use one of those USB wall outlet charger blocks and plug the arduino in using USB that way. Then if I can power the motor and LED off of the arduino (possibly PWM outputs for the motor?) then I can power the whole thing with only one plug/power source. Thoughts?

Thank you for your time and help!
Ian.M:
Here's a face cam connected to a crank that partially rotates a wheel.
For your application it would have more than one track and follower, but the tracks would only go a third of the way round the cam.   The relative sequencing of the push rods with respect to the cam rotation is set by how  each track changes radius as the cam turns.



Additionally, it would need to have something to trip a limit switch at the ends of the cam's 120 deg rotation so the Arduino knows its reached the fully open and fully closed position, and probably an extra switch detecting the intermediate position with the lid fully open, but before the swing-out trays move.

You could prototype the mechanism on a much much larger scale than is going to go into the box, using a disk of 1/2" plywood for the cam, cutting the tracks with a wood router.  The final version would need to be machined from aluminium or brass or cast from glass fibre reinforced epoxy (in a mould that forms the tracks) as I doubt the easy option of a 3D printed cam would be strong enough.

As long as the cam track is at a constant radius at (and close to either side of) the fully open position, so pushing on the lid cant turn the cam, the lid will be as rigid as the slop in the linkages and geartrain allows.  However you may want to include a spring in the linkage so forcing the lid doesn't break the mechanism.
Youkai:
A appreciate the help and video Ian.M but I think I'm going to stick with servo's. I have used them before and I think they will be more dependable than my hacked together cam setup.

I did a little research and found this thread http://www.instructables.com/id/Servo-Feedback-Hack-free/ about how to hack a feedback loop in a hobby servo. Also I found that analog feedback servos are a thing that exist so maybe I can get those. https://www.robotshop.com/en/analog-feedback-micro-servo-metal-gear.html That servo and this one that I have some of already https://www.servocity.com/hitec-hs-55-servo are both small enough to fit within the 4cm pillars. If I can check the position of the servo's then I can do multiple moving parts without worrying about the pieces colliding.

So I think 3 servo's will handle the moving parts. Now to look into momentary press button switches and RGB LED strips that I can use to light the thing.
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