Shake Bone

In this instructable we show you a project related to the decoration of Halloween, specifically we will show you the design and assembly of a coffin with a skeleton arm with movement. The main objective when building this project was to make the arm of the skeleton capable of moving the lid of the coffin where it was housed, as this would be the main movement to make, and we had to make it with a servo motor or a stepper to also achieve our second objective, a simple low-cost project suitable for everyone.

Firstly, we proceeded to locate a 3D model of the set of bones that form the arm of a skeleton that would be a reasonable size for a model, since we were not interested in creating a full-size model either, since this would increase the cost considerably, as well as limiting the torque offered by the servo motor. The assembly of parts that form the arm are designed in SolidWorks.

Once we had the 3D defined we started to design the coffin where it would be housed.
When designing the coffin, we had to take into account certain factors such as the dimensions of the skeleton, the dimensions of the prototype together with the Arduino so that all the hardware was housed inside it in order to obtain higher quality finishes. The design of the coffin was done using AutoCad as the idea was to make a wooden coffin so that it could be laser cut and look as real as possible. In addition, in the design of this one a series of laces were realized with the purpose of obtaining a casket in the shape of puzzle in order that everything was fitting perfectly and a double fund in order to lodge the whole hardware of the project, that is to say, the Arduino, the protoboard and the other elements that shape the project. We also decided to add certain drawings with a theme of terror to mark the wood with the laser in order to provide originality and personality to the coffin.

Here we show you the list of all the components and pieces needed to build your coffin to decorate on Halloween. All the pieces are standardized so they are easy to find both on the internet and in physical electronics and hardware shops.

Electronics:

Arduino Uno x 1

Servomotor Towerpro SG90 x 1

Sensor ultrasónico HC-SR04 x 1

Led (red) x 1

Resistance 220 Ω x 1

Protoboard x 1

Wires Jumper male x 6

Wires Jumper female x 4

Cable USB 2.0 x 1

Hardware:

Sheet metal screws (M3) x 4

3D printer filament (in case you do not have a 3D printer, there should be a 3D printer in a local workspace or prints can be made online for quite cheap)

Wooden board (600x800x5) x 1

Hinges x 2

Tools:

3D printer

Laser cutter

Drill

Silicon Pistol

Double sided tape

Decoration (Optional):

Spray can

Cotton

The necessary parts of this project had to be custom designed so they were designed in 3D with the SOLIDWORKS software, specifically the skeleton arm. These were printed in PLA. You can choose the colour you want but white is what makes this a bone similar to the real ones. Some of the pieces require supports as they have a complex shape with projections, however, the supports are easily accessible and can be removed. They are mainly dissolved in water, but the arm, as it has small bones, is complex, so you can use a cutter. While the pieces that make up the coffin were designed in AutoCad and laser cut in a 5mm thick pine wood. Below you can find the complete list of parts and the STLs to print your own version and the 2D designs for the laser cut parts. In total, there are 3 parts that need to be 3D printed, and X parts that need to be laser cut. The total printing time is about 4 hours and 30 minutes.

Once we have all the material and hardware prepared we are ready to start to mount the fixed and mobile connections. First we need to tape the protoboard and the arduino, with double sided tape, to the bottom part of the coffin, you have to check that is the side without drawings. Now we have to fix the servo motor, we need the silicon gun and 2 of the square pieces. We fix the pieces together and the motor, so it’s at the right high, and finally we fix the motor with the 2 pieces to the bottom piece of the coffin, making sure that the axis of the servo motor is lined with the center of de coffin and in the lowest part, without blocking the holes for the wall pieces. Now we have the servo motor in the right place and completely fixed. For the next step we need to glue the end, the “shoulder”, of the skeleton arm to the “L” shaped piece of the servo motor axis, making sure again it´s well centered.

The assembly of this circuit has no complications since all the elements are suitable for working with the voltage that Arduino works, since if we use other more complex devices we should make changes so as not to burn the Arduino motherboard. The pins and connections of the servomotor and other elements are defined in the code below.

Once we have the wiring and the fixed and mobile connections ready we can start assembling the coffin. So we have the bottom part ready, now we need to mount the side pieces correctly, so the drawing it’s outside. The placing is really simple, the pieces only fix in the correct place, you will notice quickly if doesn´t. When we are sure that everything is well placed we proceed to glue it with the silicon gun. Should look like this:

The next we have to mount is the false bottom part, the one with a rectangular hole. For that, first, we must put the square pieces in a vertical way inside the walls of the coffin, so the work as supports, and finally put the false bottom on top, no need to glue it because it’s really tight, but we have to make sure that has made stop with the supports. Then we take the sensor and we glue it in the remaining piece of the door, and them to the coffin like this:

The last step of the assembly is to mount the door to the coffin, for that we need the two
hinges and screw them in the side right wall, make sure that the door opens and closes before fixing it and the assembly is complete!

For the proper functioning of the project we decided to program this code in order to be able to move the degrees of movement of the servo, since depending on what material you use for the coffin you will need to give it a greater degree of opening, so that it exerts greater force in order to open the coffin. You can modify this value in the code, concretely in the angle variable, as well as the value of return of the servo. That is, if you want the hand to return quickly or with a certain delay, you can also modify it, specifically the angle value (-X). The bigger the value you write the faster it will return and the smaller the servo will return in a slower or smoother way. We leave the code below so that you can set up your own coffin.

Finally, once the code and all the coffin assembly are loaded together with the skeleton arm and all the hardware we check the correct operation of the coffin. We recommend varying the angle of rotation of the servo motor depending on the materials you have used in the construction of the coffin to make the arm capable of moving the top of the coffin. You can also modify the return speed of the arm, as we have mentioned in previous steps, as you wish. You could also experiment by placing a stepper instead of a servo motor or two servo motors to provide a faster opening of the coffin. Hope you enjoyed this Instructable and it has inspired you to build your own.

Happy Making!