Is Halloween coming up? Has it already passed? Doesn't matter! Any time is the perfect time for your very own robotic human skull. When you finish this Instructable, you'll have a skull with articulating neck and jaw, and glowing movable eyes that can change color between red, green, and blue.
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Time Required: ~4 hours
Total Cost: ~$70
Step 1: Carving the Cranium
The skull we bought has a completely empty brain cavity, which will be perfect for our servos, lights, and controller! However, we still need to do a bit of reconstructive surgery.
Before hacking away at the skull, grab your piece of PVC and your wood piece and spray paint both pieces black. These can dry while you're working on the rest of the skull.
Undo the clasps, lift the top of the skull clear, and set it aside - we won't need it until everything else is installed. There's also a loose tooth on the right side of the jaw that you can hot glue back in place at this time.
Now let's start making room for the eye servo motors. Take your dremel and use a cutting bit to slice away the top of the eye sockets, as per the diagram. You want to cut so that the small TowerPro servos can fit on the top shelf of "bone", with the axle of the servo sticking partially forward into the eye socket.
At this time, you can also unscrew the machine screws holding on the lower jaw, and take the springs off - you won't need these anymore. Using a drill, widen the screw hole on the right side of the skull so we can fit some wire through it.
The jaw has probably fallen off by now; let's take a moment to fix this by wiring the right side to the skull. Take the jaw and pop off the silicon cartilage on the right side. Use the drill again to put a hole in the rightmost point underneath where the silicon was, then drill a small hole through the top of the silicon. We'll need one more hole where the jaw connects to the skull, so go ahead and drill one there. Run some mounting wire through this series of holes (with the silicon piece in place) and tie it at each end so that the jaw is fixed to the skull, but loose enough to move up an down.
Now let's wire up the other side of the jaw. Take some more mounting wire, tie it to the left jaw hole where the spring was attached. Cut a length of wire roughly a foot long and run it wire up through the hole in the skull (also where the spring was attached). We'll want to attach the end of this wire to our jaw servo later.
Finally, we'll need some eye material. Cut out the ping pong materials into a shape that would fit in the eye socket, kind of like a contact lens. This might take a few tries, but once you get a good shape you can lay that piece on top of your other ping pong ball and use it as a template.
Step 2: The Eyes
Now we're finally ready to get started with the circuit. Grab yourTowerPro servos and your RGB LEDs, and let's make the eyes!
The two servos should come with a servo horn that looks like the one in the picture. Take an LED and solder a four-wire strand of ribbon cable to it, keeping note of which lead is the ground if you're using a common-cathode LED, and which is power if it's common-anode. This ribbon cable should be extra long so it can be free to move with the servo.
Attach these LEDs to the Arduino, using the above schematic. We chose to keep the color of both lights consistent, and shared the same RGB lines for both LEDs, but you could also use a separate set of Arduino pins to drive each.
Next, grab your hot glue gun and glue the LEDs to the servo horns, with the head of the LED just past the center axle of the servo and the leads extending along the flat of the servo horn. Try to get the ground lead of both LEDs on the same side, as otherwise the green and blue colors may not match up and your skull will get "derp eyes".
Attach the servo horns to the servos, then use more hot glue to fix the eye servos to the top of each eye socket. Try writing a few servo commands to each, and blinking them - they should be able to swing 20 degrees from center in both directions without major problems. You can modify our starter code to do this - use the look_to() function.
Now try popping the ping pong balls into the eye sockets. Be patient here; it takes a few tries to get them seated nicely. When you turn on your lights, they should project an eye-like circle onto the ping pong balls!
Step 3: The Mouth
The mouth comes next. Mount your HiTec HS-311 servo towards the back of the skull, with the axle on the right side. Attach a servo horn - it doesn't really matter which one - and fasten the right-side jaw wire to the outermost point of the horn such that the mouth is closed when the servo has that point fully extended away from the front of the skull.
You can use mouth.write(MOUTH_OPEN) and mouth.write(MOUTH_CLOSED) to test out your mouth servo. If it doesn't open all the way, look for snags on your wire or obstructions of the jaw. If it doesn't close all the way, your wire might not be taut enough or your servo may not be pulling the wire far enough back.
Adjust the wire and servo positioning until you're content, then let's move on to neck control.
Step 4: The Neck
The neck is pretty simple - we just want a servo in the pipe that mounts to the board and turns the whole head.
Using a cross horn and some wood screws, mount the last remaining servo into the middle of your spray painted wood piece. Then take your black PVC pipe section and fit it over the top, so that it rests on the servo. Likely it'll tip to one side, so let's fix that.
Grab your 1"x1.75" wood piece and hack off a section roughly 5" long. Using your jigsaw, cut out a pocket in the bottom that's shaped like your servo, then stick it on top of the servo and glue it in place. Fit the pipe over the wood and onto the servo, and it should now stand relatively straight. If there's still some issues, take some duct tape or gaffer's tape and wrap it around the wood piece to thicken the wood and give the PVC something to grip.
Now fix the skull on top of the PVC using some hot glue, mounting wire, and tape. There's no real method here - just try to get your head on straight!
You should now be able to lift the head+PVC assembly off the servo, then feed the servo wire through the PVC and up the spinal hole in the center of the skull. Feel free to test the neck with your Arduino at this time using the neck.write() function.
Step 5: Arduino and Power
Now that all servos and LEDs are mounted, finish wiring everything up according to the schematic we showed earlier. We had a custom board laying around that made the job easier for us, but it should be simple enough to solder or route the servo/led leads onto the Arduino. If you have any female header sockets, you can solder the servo/LED wires to them and connect to the Arduino without wrecking its pins.
Finish the circuit by chopping the tip off the 5V wall wart. Connect the red and black wires on the cable to VCC and GND on the Arduino, respectively.
It's highly recommended to use a multimeter at this point to check to see if all of your connections are sound. Definitely try probing power and ground to ensure there are no shorts.
Now it's time. Load your code onto the Arduino, and plug the wall wart into an outlet!
Here's our code on GitHub
Step 6: That's It!
If all goes well, your skull should be rotating, looking, eye-coloring and mouth-moving happily all on its own!
We originally built this as a figurehead for our home automation system, but it turned out to be great for Halloween and was the perfect chance to use a bunch of servos we had laying around. A skull like this would make a great prop for a play, a fun practical joke, or just a nifty office accessory. We'd love to hear how you use yours!
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