The Yorick Project

I like to decorate for Halloween, including various talking skeletons that I’ve set up over the years. For Christmas 2015, my wife gave me a great 3 axis talking skull with moving eyes so I could upgrade one of the skeletons from just a moving jaw skull. Then a friend suggested that there had to be other applications for the rest of the year. This got me thinking, and when I saw the Alexa Billy Bass I knew what I had to do, and the Yorick project was born. I’m pretty happy with the result:

Now, if you put this project together from scratch, it’s pretty expensive, due to the cost of the 3-axis talking skull, but if you are looking to re-purpose one you have, or a similar device, then you may want to develop a similar project. The key elements are the talking skull, a Raspberry Pi and the AlexaPi software for turning the Pi into an Alexa client device, the audio servo controller for turning the output sound into servo commands for the jaw, and the servo controller for controlling the nod, turn, tilt, and eye servos of the skull.

Block diagram for Yorick

Bench testing the set-up

The AlexaPi software provides output to two GPIO pins, intended to light up LEDs as the Pi hears the wakeup word, listens to the input, gets the response from the Amazon Alexa service, and then speaks the response. All the directions for AlexaPi are on the creator’s GitHub site. For this project, we also linked the same pins to input pins on the Maestro servocontroller. The Maestro I used allows pins to be used as output (primarily for servos, but also for other purposes) or as analog inputs. Other models also have digital input pins. By reading the status of the input pins, we know which state to be in, as there is a separate routine of head motions for inactive, wake, listen, get response, and speak response.

The servo sequences are developed using the GUI-based controller software provided by Pololu, and then custom control commands are added using their stack-based scripting language. The partial script is included at the end of the article. The short first section that is provided is the control code I wrote, the rest (not shown) are the automatically generated subroutines based on the sequences I defined using their GUI driven software.

The skull motions for each state are predefined and fixed (the routines are looped as needed for the typically longer lasting states (get response and speak response). The one key tip is to slow down the servos in order to look more realistic. The Maestro controller software lets you limit the speed and acceleration of each servo, and with the exception of the jaw servo, which had to have quick response to changing audio, I set both the speed and acceleration values to 20.

The audio servo driver board converts the audio put out by the Pi into servo commands for the jaw, while also passing the audio through to the powered speakers. Others have developed their own software do drive motor (rather than servo) based devices such as Billy Bass and Teddy Ruxpin, based on the amplitude of the sound. I’m sure the same could be done to drive the jaw servo by extracting the volume of the sound, but I already had an audio servo driver board that is otherwise unused except for Halloween, so I used that. (Update: I’ve since developed my own Raspberry Pi based audio servo controller and there’s also the Jawsduino project that uses an Arduino.)

3-axis talking skull with moving eyes

Raspberry Pi with Kinobo USB microphone

Audio servo controller board

Mini Maestro Servo Controller board
















I put it all in a project box, and covered the threaded rod supporting the skull with a short length of PVC pipe, painted black, to produce the final result:

Rather messy (mainly with the 5 servo cables), but it all fits in the box.

Hardware Components

  • 3-axis talking skull with moving eyes. Mine is from
  • Powered speakers – The Raspberry Pi puts out very little volume from the audio jack, so you want amplified speakers, whether battery powered or from line current. I used an old pair of inexpensive computer speakers
  • Raspberry Pi 3 – Other models with WiFi should also work
  • Kinobo – USB 2.0 Mini Microphone – You need a microphone for input, and I was quite happy with the performance of this one, considering it only cost $5.25!
  • Pololu Mini Maestro Servo Controller – I used the 12 channel version, larger versions should also work.
  • Audio servo driver board – I used an ST-200 board from Cowlacious that I had for a Halloween project. That model has been replaced by the newer and improved ST-400 model, which should work fine. also sells a similar board
  • Misc., including project box, LEDs (optional), resistors (optional, if LEDs used), breadboard (optional, if LEDs used), PVC pipe, and jumper wires.

Software Components

  • AlexaPi open source client software for Amazon’s Alexa service
  • Pololu’s Maestro Controller Software – I used the Windows version, but they have versions for Linux as well.
  • Custom script for the Maestro servo controller to control the skull. An excerpt to give you a feel for the scripting language is posted below. The full script I used is posted as a gist on Git Hub:


begin     # Loop until input 9 goes high
  9 get_position # get the value of the red trigger, 0-255
  155 greater_than # test whether it is greater than 155 
  if      # Wakeup word detected, run wake movement sequence
    750 delay
    1 # flag for getting out of loop
      9 get_position
      10 get_position
      300 greater_than
      if     # If after wake, both inputs high, then getting reply
        9 get_position
        155 greater_than
        if     # if just input on 9 high, then speaking reply
          10 get_position
          155 greater_than
          if   # if just input on 10 high, then listening to query
          else     # when both inputs at zero, back to rest mode
            1 minus

### Sequence subroutines: ###
# Sequence subroutines removed not included for brevity

An unseasonal post: Automating Halloween Props

This post is more suited to Halloween then the coming Yule, but I finally got around to writing it, and besides, as I write this, there’s only 327 days left to next Halloween!  There are several options for controlling animated props, including prop controllers specifically designed for this purpose.  A recent addition to the market is the MonsterShield, an Arduino-based prop controller with open source code you can modify.  I haven’t tried one out, but thought it was work a mention.  In my props, I just use a handy micro-controllers, such as an Arduino, along with sensors such as a PIR.  The PIR (or pressure mat, or whatever sensor you choose) sends a signal to the microcontroller when it detects someone, and then your microcontroller can trigger a whole sequence of pre-programmed actions.  I had two such props in my Halloween display this year.  The first is a classic “Monster in a Box“.  I use a PIR sensor to detect when someone comes near.  When this happens,a Teensy sends out an output to a Power Switch Tail to turn on the power to a wall wart that delivers 12V power to the windshield wiper motor, as well as to a green light inside the box.  I use a Power Switch Tail so that I don’t have to worry about any safety issues dealing with house current directly.  The motor sequence has several stops and stop in it, of differing lengths, so that the action is more natural.  After it triggers, there’s a dead time, so that it doesn’t keep restarting while trick-or-treaters are standing in front watching it.

Closed Monster in a box, showing the box with the PIR and Arduino that triggers the box.

Inside view of the Monster in a Box, showing the wiper motor and irregular cam.


The other prop where I use a PIR and an Arduino is my scarecrow skeletons, who do a little talking and singing routine when triggered.  Here, an Arduino controls two Cowlacious audio boards that in turn drive the servo boards that control the jaw servo and lights in the skulls (also from Cowlacious).  I’ve had the audio boards for some time.  They work fine, and have a wide host of control options (including the ability to be triggered directly from a PIR).  However if I was remaking this prop, I’d probably just wire up some really cheap mp3 players. The video shows the two skeletons doing their routine. Sorry for the quality, my video editor is refusing to save the edited version.