Add Motion Sensor to Halloween Prop

Hi,

I have a Halloween prop which has sound activation and is not sensitive enough. I would like to add motion detection activation to it. It is powered by 3AAA batteries. What might be the best way?

I would say PIR sensor.

I'm using a Zilog e-PIR sensor which has a range of about 5 meters and is $8 from Digikey. Futurlec.com has some PIR sensors as well. If you want a more reliable source, Sparkfun has a couple:
SparkFun Electronics - PIR Motion Sensor