The basic idea is that they are using an Arduino to monitor ambient audio levels and then drive a small servo-motor proportionately. And since the servo is hooked up to the jaw of the prop skull ...

Pretty much every project I've done just lately has required me to test out some servos and experiment with their positions ...

Making a servo tester using an Attiny85 micro chip, Attiny45 also works for this project. This diy servo tester is as good as ...

He built this neat little device from an Arduino Pro Mini, a battery, a small servo, a couple of power control boards and a cheap RF link from SeedStudio, all glued onto an iPhone case.

But it's possible to go even smaller, as the makers of a new Arduino-compatible product called "Microduino" show. At 1" × 1.1", it's nearly as small as a quarter.