Hall sensor vs. reed switch

justjed:
If I use the Neodymium Diametric Disc N35 6x2.5mm down at the bottom right of that page, it seems they’re plenty strong to activate the sensor. But with 8 of them around the perimeter of a 5" diameter drum, will the flux density at the midpoint between 2 of them be low enough for it to turn off? …

I’d be tempted to use the little cylindrical magnets at the top left of that page. They will have a more narrowly defined field. They can be attached by drilling little holes in the drum and inserting them in the holes. No need to worry about the field between the magnets. There will be a 0 point between each magnet. As an alternative, you could place them in in alternating orientation (N-S, S-N, …) as others have suggested, and use the polarity sensing devices, but I don’t think that would be necessary.

BillO: I'd be tempted to use the little cylindrical magnets at the top left of that page. They will have a more narrowly defined field.

I hadn't settled on which magnets, specifically. Those were handy as an example of the sort of gauss strength I was looking at, as a 1st pass. It looks as if I'll end up ordering from Digikey again, just because they have everything on my shopping list, as opposed to Tayda, e.g. But you're right that smaller will be easier for me to deal with.