I found a very cheap (19 cents) hall effect sensor that literally perfectly meshes with ramps. You just stick the sensor in one end of a servo cable and plug the other end into the ramps board as is, and it works perfectly. much cheaper than switches or optos and very reliable. I wrote it up on my blog.

[notanumber.net]

The only issue I found is that there is a slight hysteresis in the sensor, meaning that the field level at which it turns off and that which it turns on are different, each is sharply defined and accurate but you gotta make sure your firmware chooses the right one. sprinter seems to do the right thing automatically at least, backing off until the sensor turns off then advancing til it triggers so always choosing the on->off edge. I don't know whether that was by design or accident so can't speak for other firmware so you may want to test it to make sure. In any case, manually moving the axisess a few cm away from the endstops so they don't start out fully activated before homing works just as well too.

Looks neat. The backing-off of the stop and advancing is on purpose, as microswitches have hysteresis too.

So the total cost is 19 cents plus one magnet. Does it work well with cheap ceramic magnets?

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It should be fine, it will just trigger closer and you will have to make sure it is aligned. Seeing as how it can sense the really weak rubberized refrigerator magnets.

For a reference, it triggers at about 15mm away for my strong rare earth magnets.

What it actually senses is the amount of magnetic flux perpendicular to the face of the sensor, meaning the iron bars are going to affect it some (magnetic flux likes going through iron more than air). It doesn't really matter as you still get consistent results, but it did puzzle me for a bit when the trigger distance was slightly different in my hand and on the machine.

I can also very super highly recommend replacing bottom z-stops with top z-stops when doing this mod as nophead described in this post
[hydraraptor.blogspot.com]

so, so, so much nicer than bottom z-stop. like, it took a good 20% of the calibration hassle off the entire build. It really should be done for all new designs.