Proximity Sensor Wiring Incorrect

I've been having bad luck getting my proximity sensor working. I have 2 to choose from, both chosen after reading many posts about the individual sensors.

The first is a LJ12A3-4-Z/BY NO PNP (orange tip). Second is a LJ18A3-8-Z/BX NO NPN (orange tip). I do have an aluminum heat bed and a piece of glass with PEI mounted so optimally i'd like to use the 8mm sensing distance (even though i'd love to use the smaller form factor of the LJ12A3-4-Z/BY).

My first question isn't really related to anything i've been able to find. I have an interesting problem. I'm breadboarding the voltage divider using a 9v battery and i'm finding that I CAN get the voltage change on the NPN (LJ18A3-8-Z/BX) however the wiring should be, from what I read, brown (12+, in my case either 9v+ or 12+ from printer), blue (to negative, either 9v- or 12v- from printer), black output (zmin on printer), but in my case it is not. Nothing happens if i wire this way, but I get normal operation if I wire Black (12v+), blue (12v-), brown output. Has anyone run in to this?

My second question I cannot really find an answer to-but my gut says I may just need to buy another LJ12A3-4-Z/BY NO PNP to test. This sensor I cannot get a voltage change when wired in any fashion. I basically just always read ~1mv when sensing or not sensing metal. Am i correct in thinking this sensor may just be faulty?

Finally, i'm am curious if there is an issue, also, with the LJ18A3-8-Z/BX because i have specifically chosen the 15k/10k ohm resistors but when measuring voltage (when testing on 12v) it has not pulled down enough-still sits at ~8v and ~6v. Note I have yet to actually connect either sensor to the zmin because i have not gotten voltage close enough to 5v (don't want to burn it up).

Any help would be greatly appreciated!

When a NO sensor switches you will not see a voltage change, but only the resistance between signal- and gnd wires become "0". That's why you need a pullup resistor on the ramps side and no voltage divider...

Awesome! Thanks for the reply. So from your suggestion I should test standard wiring (with the NO LJ12A3-4-Z/BY), no voltage divider, and enable the z-min pullup in marlin then?

BTW, another update, late last night I was attempting to bread board a few things with the LJ18A3-8-Z/BX. Again, first bench testing with 9v battery. I went away from the voltage divider route and tried 2 things. The first was a NTE960 5v positive voltage regulator. I noticed some interesting behavior (i'm a software engineer and not super familiar with a whole lot of electronics) that i couldn't quite figure out. I first tested the voltage regulator directly to the 9v battery; was spot on at the 5v. I then wired up the sensor and if the voltage regulator was grounded (middle pin) i would see ~3.3v when not triggered, and ~2.2v when triggered (weird). With the voltage regulator not grounded I would see ~8.88v not triggered and ~7v triggered. Needless to say i couldn't bring myself to test it on 12v when it was misbehaving on 9v. I moved on to my second, and final success with that sensor, which was to nix the voltage divider and voltage regulator altogether and use 1 rectifier diode on the output. Everything worked on the breadboard at 9v so i enabled the pullup resistor in marlin and wired to the printer and all functions works smashingly

Hopefully after testing your suggestion with the LJ12A3-4-Z/BY i will have two choices of sensor and then it just becomes which is nicer to mount. I have already modified (my previously modified) Ultimate Gregs Wades Extruder body to have a very nice, close and sturdy, mounting point for the smaller diameter LJ12A3-4-Z/BY, but am going to toy around in AutoCAD with designing something similar for the larger diameter LJ18A3-8-Z/BX.

Just tried, again, testing out the LJ12A3-4-Z/BY and i really thing it must just be faulty. If i wire it standard i get no detection light and no readings. If i, instead, wire black as 12v+ and brown as output i get the detection but it is still not triggering when combined with the marlin pullup resistor on zmin. I may just order a new one just for sanity checking. At very least I can start working with the 8mm sensor and getting it an available mounting point.