Wiring Z-stop directly to hot end and aluminum bed / spacers So I am really fed up with inductive probes. The one I am using keeps getting shifted slightly every time I switch nozzles or run an oozy print. That means I have to autolevel again, then manually set a Z-offset (as I would have anyway if I didn't have an inductive probe).

On my Lulzbot Mini there is a different scenario. There are four washers at each part of the bed. The nozzle is "grounded" so that when the Mini touches the washers, a current is created that seems to act as the Z-stop. Surprisingly there isn't much out there for a DIY implementation of this.

Since I have an aluminum bed (and aluminum is conductive), I am thinking of doing the following:

1) Put one wire from the Z-stop ground pin to the aluminum bed. Make sure it is away from the wires for the heater / thermister (?)

2) Put one wire from the Z-stop 5V into the heating block of my nozzle.

When the nozzle probes the bed, a current will be created from the 5V heating block, through the conductive nozzle, into the conductive bed, to the Z-stop ground.

I'm always unsure when it comes to circuitry. Will there be any dangerous interference from this technique from, say, the bed heating circuit? I'm not sure what kind of protection circuitry are on each of the Arduino's pins, and I'd rather not fry my board if this sounds like a bad idea to someone.

I figure most people don't do this because they have sheets of PEI or some other non-conductive material on their bed. I can use PET tape but still leave holes in the tape for this autobed leveling probe. It would be really great if it worked and wasn't dangerous. 

I shouldn't even need the third pin?
Aluminium is conductive, but aluminium oxide is not, which is just so what there (unavoidably, since aluminium rapidly oxidises in air) happens to be a thin layer of on top of your bed. The coating is very thin, but it might foul your plans. It would work better with a sharp probe (that can puncture the layer) than with a 3D printer nozzle. You should be careful, because your probing method might be unreliable (which could cause the nozzle to crash into the bed).

Wiring the endstop 5V directly to ground will create a short circuit which will damage your printer. You should use the third (signal) pin and ground instead.