I am building a 3DR,

I am building a 3DR, and was wondering if you can use a hall effect sensor (just the sensor with no additional circuitry) as an endstop

There are hall effect switches that put out a digital signal instead of an analog one. You might be able to use those with only the controller’s internal pull-up, or you could certainly use them with a simpler circuit than the analog one’s that need to have a threshold set. See here: http://www.thingiverse.com/thing:24189

I believe there was also talk at one point about support for using the analog hall sensors with an analog input on the main controller. I’m not sure if it was ever implemented, or if it was, if the changes were ever pulled into mainline Marlin.

I use Allegro A3144 type hall effect switches (matched with little neodymium disc magnets) as homing/endstop switches in a couple of things that move. They need power (anything between 4.5 and 24v) and ground supplied in addition to the sense wire, which can sometimes be annoying, but are otherwise just the same as using a mechanical NO switch, in a much smaller package, without the impact-related issues.

Hobby servo extension cables are great for wiring them up, the sensor ICs fit right into the connectors. Instillation example from my Shapeoko: http://pappp.net/wp-content/uploads/2013/05/SH7-6.jpg where they’ve been sealed onto the cable with heat shrink to avoid conductive contaminants. The sense magnets are just packing taped to the makerslide where I wanted the stops.

They’ve been depreciated in favor of the A110x series for a decade now, it looks like the new ones are finally cheaper and easier to find. I’ve usually been able to find them for something like $0.40/pc.

BTW, reed switches are also an option. Simpler to wire up, but they might be more expensive.

If you do use reed switches be careful not to use overly strong magnets. You can end up making the contacts magnetised themselves and they’ll stick to each other. Or the reed to magnet distance at which they’ll close will change.