On Tuesday, July 14, 2015 at 4:07:23 PM UTC-7, wrote:
I bought today a "hands free" garbage can for the kitchen. .... The problem is
that when slid under the counter the motion detector detects the
underside of the counter and the lid opens and stays open.

Two solutions: redirect (use a mirror or prism) the line-of-sight of the sensor
so it sees cockeyed. Or, use a circular polarizer sheet (like old CRT
monitors were sometimes equipped with, to keep contrast high).

Easiest way to do the prism thing, is to get one of those whole-page Fresnel
lenses, and snip a few inches off the edge. You can experiment a bit to
see if it works before you damage your dollar-store magnifier.