As you may have noticed, Halloween was this last weekend. This is the first year that I have been living on a street with lots of families and trick or treaters and so I decided that it was time to put my engineering abilities and love for doing funky projects to good use and make myself an animatronic motion-activated skull to provide that “trick” aspect of trick or treating that is often overlooked (don’t worry, I had plenty of candy to hand out as well). Ideally, I would have given myself more time to complete my project, but I completed the whole thing over the course of a few hours. Working Skull Video
I used the following materials (most of which were readily available online and at the local hardware store) to create my animatronic skull:
Arduino Uno Control Board
Arduino 4-relay board
Variable DC power supply
12V push/pull solenoid (I could have gotten away with just a push solenoid, but I ordered most of the materials online, so I wanted to give myself the ability to use either option).
Plastic Skull replica with working jaw and removable parts
Wood replacement shovel handle (for mounting the skull)
Below I have roughly outlined the steps and included some photos of the build:
Step 1: Secure the jaw and the skull so that they don’t move drill holes at a slightly smaller diameter than the threaded rod through the joint where the jawbone rotates against the bottom of the skull. Make sure that the rod you are using is small enough to be screwed down into the jawbone without becoming visible. I used some 3/16” rod that I had laying around. Trim the threaded rod so that it is as high as you can make it while still giving it room to move within the re-assembled skull. Since you are making a lever, you will want to be able to adjust the length and travel of the lever by moving the push rod up and down.
Step 2: Move the jaw portion out of the way and drill a slightly larger hole in the top of the skull so that the threaded rod will be able to move freely back and forth once it is screwed into the holes in the jaw bone. Make sure that the top of the jawbone will still have enough material to pivot against the skull and not go into it. I left about 1/8” or more clear on both sides of the holes to ensure that it worked correctly.\
Step 3: Use pliers to screw the threaded rods into the holes in the top of the jawbone
Step 4: Cut off the rounded top of the shovel handle and cut the bottom of the shovel handle to make it into a stake (You could also adjust the height of the handle or create a different platform that would allow the skull to sit on a table. You just have to make sure there is enough clearance for the jaw to move).
Step 5: Insert the shovel handle through the bottom of the skull (mine had a big hole in it where the backbone would connect) and screw a wood strip to the top of the handle so that it runs about the length of the skull. This will be the platform for the solenoid.
Step 6: Use glue or screws to secure the skull to the wood handle and solenoid platform. I used screws because I was short on time, but I did make sure to place them in inconspicuous locations.
Step 7: Fasten the solenoid to the solenoid platform. My solenoid had some threaded mounting holes built into it, so I drilled some small holes into the platform to fit the heads of the screws and then ran a zip tie over the top to secure the whole thing to the platform. This would also allow me to fine-tune the height of the solenoid, if needed, to push against the lever mechanism (the part that slides along the wood piece)
Step 8: Program and wire the Arduino control board. For the sake of not losing you all here, I have not included the code that I used to program the board, but it boils down to activating the skull with a motion detector and then setting a series of on/off delay times for the relays that control power to the solenoid. The photo below is a messy version of the wiring for the board. The motion detector is the plastic thing at the bottom right and the black and red alligator clips lead to the variable power supply that drives the solenoid through the relay switch.
I tried to include lights in the skull as well, but found that my soldering skills were not quite up to the task this year (and I ran out of time). Next year the skull will include lights and a speaker system to make it even scarier!
I hope you had a Happy Halloween and feel inspired to try to tackle a project like this yourself!