I'm looking for some sort of sensor to use on a center pivot irrigation system. These are outdoor conditions with water, dust and a range of temperatures. The sensor needs to close a circuit to activate a 110VAC relay. It needs to detect something like a flat metal plate or maybe some sort of reflector. The sensor would have to work even if the sensor to activator alignment varies. The alignment might vary as the depth of the pivot wheel track changes through the season. Operating range of maybe 2 feet. It should not activate because a corn plant or weed happens to get in front of it. I did a search and found retro reflective and diffuse sensors. The proximity sensors I found didn't seem to have enough range. Thanks for any guidance or comments.
Dean