[HH] Opening a skull (on topic, honest!)

[Mark Komarinski]

Ok, so here's the background.

After many years of software and Linux tinkering I finally got into 
Arduino for hardware tinkering.  As I usually do when learning a new 
software language, I pick a 'big' project and give myself plenty of time 
to work on it - the thinking is that starting big I can break the 
project down into small parts that I can figure out separately.

My Arduino project is a Halloween skull.  Ideally, I'd like:

- motion sensor to activate when someone is nearby and cause the jaws to 
open and shut quickly
- LED eyes that glow brighter the darker it gets (I already have this 
working)
- sounds (optional, probably add a sound shield)

My problem is the first item.  The jaw is held in place using two 
springs (no other connection exists between the jaw and rest of the 
skull).  The tension that's required to open it is too much for the 
servo that I have.  I was thinking a linear actuator, but those appear 
to be way more expensive than I'd want to spend.  Different springs may 
work, but I want the jaw to snap shut thus requiring a good bit of 
tension.  I've already got a few PIR sensors I'll use to know when 
there's nearby motion to activate.

Any thoughts?

-Mark