How do I determine what magnet to use for a reed switch?

I'm looking to buy this reed switch but I don't know how to choose a proper magnet. I would like the optimal distance to be ~1 inch.

• Wave a magnet around and see how far it works from... if not enough get a bigger magnet. Commented Jan 13, 2015 at 2:55
• The rule is simple - the magnetic field lines must be closed through (along) the reed switch. Commented Jan 13, 2015 at 6:17
• What do you mean by "optimal distance"? If you want the switch to close when the magnet gets to within one inch of it, that's one thing, but if the switch is closed because the magnet is close to it and you want it to open when the magnet gets farther away from it than one inch, that's another. Yet another is the hysteresis of the switch if you want it to close and open as it approaches and recedes from the magnet. What are you trying to do? Commented Jan 13, 2015 at 8:57

Here's a data sheet that lists the spec of the Philips 10JK coil that is referenced in the reed switch spec, http://www.cotorelay.com/downloadables/Application%20Notes/Reed%20Switch%20Tech%20&%20App%20Section/Reed%20Switch%20Technical%20&%20Applications%20Info%20(Entire%20File).pdf

The above data sheet also gives some general info on magnet actuation of reed switched.

In most cases the newer high power magnets (Neodymium types) should have no trouble operating this reed switch. Though at a distance of 1" you may still need a fairly good size magnet, (maybe a cylindical type about an inch long by a 1/4 inch dia. - just a guess). A simple ceramic magnet may work up close but might not work at all at the distance you need.

See K&J Magnetics ( http://www.kjmagnetics.com/ ) for some ideas on small powerful magnets. Don't go crazy with a huge magnet, you may have trouble getting the switch to turn off. With a huge Neodymium magnet you'd likely have a lot of other problems too. Be sure to read the warnings that are on the K&J web site.