Question about wiring LJ18A3-8-Z/BX geeetech pro b

I have a LJ18A3-8-Z/BX NPN NO proximity sensor (orange tip) that I am wiring to a GT 2560 board. Right now I have the blue lead to fan 3 connector gnd, brn lead to fan 3 pos (12 vdc), and plan to hook the blk lead to z min sig connector. When I test the probe, I show .078 vdc when open (no led) and 4.7 vdc when closed (led on).

Now for what has me confused. First, I thought that NPNs needed resistors and PNP were the ones that automatically reduced voltage to a 5 vdc signal. I see several posts on here that say the orange tipped sensor is PNP but it definitely states that it is NPN. Second, my board has 2 pins for the Z min end stop. When I check with my meter, the lower pin shows 4.7 vdc and the upper pin shows 2.8 vdc. My schematic shows that the upper pin should be Bat+ and the lower should be Signal. Why would the signal pin have positive voltage with nothing connected? Am I right in assuming that the pin with lower voltage is signal even though it doesn't match the schematic? I checked all the other end stop connections and they all show the same voltages with nothing connected. When I back probe the connector the voltages remain the same. When I manually close the z axis end stop both pins show 4.7 vdc as expected. Just a thought, when I mod the firmware, would that reverse the pins so that the correct pin is signal? Lastly, does the way I plan on wiring the sensor sound like it should work? I tested the sensor and it responds as it should. Thanks for any help on this. I have a couple of days to wait for my longer extruder fan screws to arrive so I can mount the sensor. I would like to try and get the firmware and hardware up and running in the meantime. Is there firmware out there for the Geeetech prusa I3 Pro b or what would be the easiest way to configure the machine for auto level. I've seen references to programs that assist in setting up firmware but no links to the program.Thanks Again.

I have a NPN NO probe too, but it switches the signal to GND when triggered. It has a blue cap.
Maybe your probe has a wrong sticker on it? ( even underpaid chinese childslaves can make mistakes )

Your probe is 12V powered, so the signal should also switch to 12V instead of 5V?! Are there any external components attached? ( eg. Voltage divider, made from two resistors )
Also the input pin of the GT2560 should show 4.7V instead of 2.8V when the endstop_pullup resistor is enabled in firmware...

Thanks for the response. There are no voltage reducers attached. The board output (Zmin end stop disconnected) is 4.7 VDC on the sig pin (according to the schematic) and 2.8 VDC on the + pin (according to the schematic). The + pin goes to 4.7 VDC when the end stop is activated. That is the same on the other end stops as well.. All pullups are set to false in firmware. I will see what changes when I make firmware changes for autolevel.

Edited 1 time(s). Last edit at 03/31/2017 09:50AM by searcher 205.