With the help of my friend Roger, I built a skull with 3 servos attached: one to make the head turn side to side, one to make the head nod up and down, and one to make the mouth open and close.

The overall prop consisted of the skull, a CD player to play it's speeches, a "switch" to signal when the candy bowl was picked up, and a servo that held the string to a spring-loaded window shade. When the servo rotated 90 degrees, it released the string, which allowed the spring in the shade to roll it up fairly quickly.

The overall skull prop had so many things to monitor and control that I needed more than the 13 I/O pins available on a PIC16F628. Rather than delving into new territory and buying a part with more pins, I just connected two controllers together. MCU one controlled the skull (3 pins), the FF button on the CD player, the digital potentiometer for the LED eyes (3 pins), the shade servo, and the spotlight LEDs. MCU one also monitored the left channel of the CD player (see below). MCU two controlled the stop and play CD buttons, the candy bowl switch, the candy bowl indicator, and the LED eyes. Both monitored the same standby and run signals coming from the master. When the run indicator from the master turned on, MCU one ran sequence one (the greeting). If, while that signal was still on, the bowl was lifted from its switch, MCU two sent a signal to MCU one, telling it to run sequence two (the shade release). As a result, MCU one used all 13 I/O pins, while MCU two used another 8.

I made the speech for the skull by recording my voice on my computer using CoolEdit '96 (yes, it's old, but it does the job for me, so I don't see the need to upgrade). I added some effects (lower pitch, echo, etc.), also using CoolEdit. I made two separate tracks: one for the greeting, and one for when the shade is released. I burned these two tracks, along with a track containing 15 minutes of silence onto a CD, and the CD was loaded into a CD player that I had rewired so that the play, stop, FF, and rewind buttons could be "pushed" by a microcontroller. However, since these CD players wrap from the last track to the first when the FF button is pressed, I did not use the rewind button.

Since CD players are not consistant in the amount of time it takes from pressing FF to the time that you actually hear the next track, I set up both tracks so that the speech was only in the right channel. The left channel was silent, except for a 20ms tone a fixed amount of time before the speech began. The left channel was fed into an AC (i.e. LEDs in both directions) optoisolator. MCU one monitored the status of the optoisolator, and when it went low (it used a pull-up resistor), that was the signal that the sound was about to start. With this setup, the timing between audio and motion was perfect every time, regardless of the seek time delays of the CD player.

One of the comments I received last year was that it was too dark. It was too hard to see things. Since I definitely wanted the skull to be visible, I bought some ultra-bright yellow LEDs and used a couple of them as a weak spotlight on the skull. MCU one turned these on when it was running either of the sequences. It didn't make a huge difference, but it did help.

The bowl switch is the same one I used two years earlier. It simply consisted of two wires woven into the table cloth that was on the table the candy bowl sat on. When the metal bowl was set down across the two wires, the circuit was closed. When the bowl was picked up, the circuit opened, which caused MCU two (the one monitoring the bowl switch) to send a signal to MCU one. This signal told MCU one to turn the shade servo, which released the string attached to the shade. The spring in the shade caused the shade to wind up quickly and somewhat noisily. The skull then turned to look at it and recited a short speech.

Below are a few pictures of the skull's mechanism from various angles. Unfortunately, I didn't take any pictures of the side of the skull with the mouth servo before I started converting the prop for next year. That's why the skull is partially green in the last picture.

I used my brother's video camera and video digitizing equipment to make a video of the skull before it aquired a costume. The video is about 700KB.

Halloween 2003 technical details
Halloween 2003 pictures

Page last modified 12/14/2008