This is an arm that is modeled after some pretty cool things that can be found by searching up "skeletonics". They're completely mechanically powered as they simply transfer movement. I originally made this as a Halloween costume but I thought it was cool enough to keep around.

Supplies

~ 30 feet of 1/2" pvc-pipe
1 or more led strips and "appropriate" power supply (I used 2 AAs and a 9-volt in series for one 12 volt led strip).
35: 1/4" 20 nuts and bolts (15: 1.5" long and 10: 2" long)
6(+): 1/4" fender washers
1/2" Plywood
Foam
18: wood screws
6 : 1/2" pvc endcaps
3: 1/2" pvc couplings
2: 5/16", 2.5" long carriage bolts
Glue
Saw, most will work.
Heat gun or other heat source
~1 foot 1/4" vinyl tube (optional, I found that using small cuts of it can be used as nuts
Soldering iron and solder
Really any switch (I used a rotary switch because it felt cool).
Battery mount
Wire (I didn't set up my LEDs efficiently and ended up also using alligator cables)/p>

Step 1: Processing the Pvc

Cut the pipe to length and drill 1/4" through holes at the meeting sites. These measurements were specific to me, for reference I am 5'11" and 23.5" from shoulder to wrist. Although multiple people have been able to wear it, it may be best to scale the sizes. Cut the ends of the 14.5" and 9.75" sections and use a heat gun and a board the same thickness as your pipe to make slotted ends. The 7.25", 6.5", and 17.25" sections are only slotted on one end. I used three halves of some pipe to make arm holders, they should match the size of your bicep, forearm, and wrist.

Step 2: Making the Base

Using a jigsaw, I cut some 1/2" plywood to a 8"x13" section (roughly the area of a backpack). Then, I drilled six 1/4" holes to secure pipe to, (these should receive the 2" long bolts). The main pipe extends 17.5 inches above the board and should have a good amount secured in order to remain stable.
I used 1/2" pvc for all of the arm, but a thicker pipe could provide for a sturdier base. A 20.5" section is bent into a C-shape and attached with 2 5/16", 2.5" long carriage bolts. The ends are angled at ~40° down to make a more natural position. For the "straps" I bent some pipe into a sickle/hook shape, these should be long enough to go over your shoulder and past your armpits. The non dominant strap has a steel bar running across under the armpit for better weight balancing, this most definitely should be padded (I ended up padded it with some cardboard and tape which works surprisingly well)

Step 3: Assembling the Parts

To make the pivoting joint at the base, take 2 drilled endcaps and thread a 1/4" 1.5" bolt through. Then attach the ends to a 6.5" section. Put washers on the ends to make a smoother motion. The part that is worn on the arm should be scaled to fit. Double joints are needed at the elbow and at the connection. *The four joints on the big arm need the 2" long bolts.

Step 4: Final Tips

I tried using spray paint on the back of the board to match it with the pvc-pipe but it ended up blotchy and didn't hide the wood very well. For a better color I should have used a brush on paint or put on more layers.

Over time, the LED strip started to tear at the joints and I ended up having to solder a connection at quite a few areas. I suspect that I didn't give enough leeway in the length of the strip so they snapped when the joints revolved too much. It would be best to give the LEDs more space or to put in physical stops on the arm to prevent overextension.

If there is squeaking or groaning when you move the arm you can use wd-40 or another lubricant on the joints to quiet the noise.

Keeping the jointed parts parallel is very important to getting good movement.