Possessed Pumpkin Project Problem

Hi I'm interested in making a pumpkin head rotate to follow someone.

Originally, I was thinking about modifying a solar tracker solution which uses 3 photoresistors and using PIR's instead.

But as I looked into this I don't think that will work because the PIR responds to change.. (I think)

Would PIR be the way to go for this, if so 3? If not, what would be better/cheaper? Thanks,

Problem with a PIR is that it is not very directional. Most projects of this sort use a video camera to detect the motion, processed by a laptop and then the results fed to an arduino to control the motors.

This one is good:- http://www.sarahangliss.com/spotted/stookybill

Look for motion tracking software, there are lots of free options like this one:- http://opencv.willowgarage.com/wiki/FullOpenCVWiki