[eugenebrooks]

We use the Banner QS18VN6LV sensor.
It is a red light sensor, so you can see the beam.
You put a patch of retro-reflective tape on
the other side where the beam hits.

Brown wire goes to 12 volts,
blue wire goes to ground of the digital input
black (or white) wire goes to signal.

The sensor costs around $60 last I checked.

Eugene