May I suggest you use COTS PIR sensors from the alarm industry? They are
small, cheap, modular,
and readily available. Many can be set to close a contact immediately on
motion, or after 2 or 3
'beams' have been passed by. 12V at 50 mA for power makes them easier to
use than most
 'security light' types.

Many have 'range' pots so you can dial down their sensitivity, and by
aiming them correctly,
at the floor at 3' distance, you prevent false triggers from targets
farther away than you desire.

Many also come with interchangable lenses with differing numbers of focal
elements, giving you more
or fewer detection beams. You can mask out the beam pattern with opaque
paint to get exactly
the sensing box you desire.

Given their range, you could mount them on the ceiling in a store, to get
them out of harms way,
while giving you exactly the trigger box you desire.

They also have the advantage of KISS, where a contact closure triggers your
playback/ animation,
so no fancy controller required.

Robert
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