Sears used to sell a kit for their garage door openers that prevented
the door from coming down if a child or pet was blocking a beam across
the bottom of the door opening. It consisted of an emitter on one side
and a detector for the other side. The detector side had a led that
would indicate when it saw the beam. This was used for aiming the
emitter at installation. It would even work on the double-wide doors.
This kit is now required by our local building code for new constuction
doors.

Van Gardner