Posted: Tue Aug 07, 2007 7:42 pm
I wonder if you could use a compass based detector?
A quick test with a very crappy compass and a 5/8"D rare earth magnet shows that the compass will deflect ~45 degrees from north with the magnet to the west at 8" from the compass.
If you took a compass and oriented its base vertically it would point down when it crossed over the buried magnet (Or up if the magnet was pointing in the opposite direction.)
Might need two compasses, both mounted with their bases vertical (instead of the normal horizontal) and their two bases at 90 degrees to each other. As the 'bot drives around at least one of the compasses will point north (the needle will be horizontal). The other compass will be confused when it is pointing east/west since it can't rotate in the proper direction. You need two compasses so that at least one is always aligned properly with north no matter which way the 'bot is pointing. When the compasses pass over the buried magnet both needles will point down (or up) instead of at least one needle being horizontal. If you use orienteering compasses then both the top and bottom of the case are clear plastic. So, you could use a phototransistor to "see" the needle when it is pointing down (or up), or you could "see" when both needles are not horizontal.
A more "high tech" approach would use a solid state compass module like those found in some cars and other navigation systems. Digikey sells a compass module that might work;
Honeywell HMC1055 COMPASS SENSOR 3-AXIS KIT, $36
http://www.digikey.com/scripts/DkSearch ... 42-1036-ND
The PDF of the catalog page is http://dkc3.digikey.com/PDF/T072/P2114.pdf
The robotic hobbyists already have "fairly cheap" modules that should work for detecting a magnet (anything that can detect the Earth's magnetic field should be able to see a strong magnet at several inches.) You might want to take a look at http://www.superdroidrobots.com/shop/it ... 8&catid=35, for $52.
Parallax (the microcontroller people) also sell a compass module for their Basic Stamps ($40 from Mouser Electronics, http://www.mouser.com/catalog/631/46.pdf)
The Lego Robotics System has a compass module, $47 from http://shop.lego.com/ByTheme/Product.as ... 1034&cn=17
You can probably rummage around on the net and find info on this sensor's specifications and how to hack it for other uses.
Is there anything in the description of the problem that says which way the magnet will be pointing? It'll make a huge difference if the magnet is oriented with the poles pointing up/down vs. side to side vs. "the direction the poles point will be random". Any decently thought out challenge should specify one of those three conditions.
A quick test with a very crappy compass and a 5/8"D rare earth magnet shows that the compass will deflect ~45 degrees from north with the magnet to the west at 8" from the compass.
If you took a compass and oriented its base vertically it would point down when it crossed over the buried magnet (Or up if the magnet was pointing in the opposite direction.)
Might need two compasses, both mounted with their bases vertical (instead of the normal horizontal) and their two bases at 90 degrees to each other. As the 'bot drives around at least one of the compasses will point north (the needle will be horizontal). The other compass will be confused when it is pointing east/west since it can't rotate in the proper direction. You need two compasses so that at least one is always aligned properly with north no matter which way the 'bot is pointing. When the compasses pass over the buried magnet both needles will point down (or up) instead of at least one needle being horizontal. If you use orienteering compasses then both the top and bottom of the case are clear plastic. So, you could use a phototransistor to "see" the needle when it is pointing down (or up), or you could "see" when both needles are not horizontal.
A more "high tech" approach would use a solid state compass module like those found in some cars and other navigation systems. Digikey sells a compass module that might work;
Honeywell HMC1055 COMPASS SENSOR 3-AXIS KIT, $36
http://www.digikey.com/scripts/DkSearch ... 42-1036-ND
The PDF of the catalog page is http://dkc3.digikey.com/PDF/T072/P2114.pdf
The robotic hobbyists already have "fairly cheap" modules that should work for detecting a magnet (anything that can detect the Earth's magnetic field should be able to see a strong magnet at several inches.) You might want to take a look at http://www.superdroidrobots.com/shop/it ... 8&catid=35, for $52.
Parallax (the microcontroller people) also sell a compass module for their Basic Stamps ($40 from Mouser Electronics, http://www.mouser.com/catalog/631/46.pdf)
The Lego Robotics System has a compass module, $47 from http://shop.lego.com/ByTheme/Product.as ... 1034&cn=17
You can probably rummage around on the net and find info on this sensor's specifications and how to hack it for other uses.
Is there anything in the description of the problem that says which way the magnet will be pointing? It'll make a huge difference if the magnet is oriented with the poles pointing up/down vs. side to side vs. "the direction the poles point will be random". Any decently thought out challenge should specify one of those three conditions.