[Andy A.]

If the lighting around the object remains consistent, a photo resistor might work. It has the advantage of being excedingly cheap, if the environment allows for it.

As an aside, many 'motion activated' toys and novelties really just have a photo resistor. When you move by or wave you're hand in front it will cast a shadow and set it off. They are surprisingly sensitive and rarely cause a false alarm. Slowly changing lighting does not set it off, only abrupt changes. The 'range' is entirely dependent on the lighting situation, as that changes the length of the shadow the objects would cast.

It's fun working in a toy store when turning the lights off at the end of the day. The whole place starts squawking with every other toy thinking someone walked in front of it. It's a little creepy the first few times.

-Local