Don’t trust the internet (and how to add an Inductive Proximity Sensor to your 3D printer the proper and easiest way)

Every now and then I still fall for it. Whenever I’m trying to accomplish something I’ve never done before, I start a Google search to find some nice video’s or blog posts that explain how I can accomplish this specific kind of task. That in itself is perfectly fine, however… one should not stop using its own brain!

This week I wanted to add an Inductive Proximity Sensor (LJ12A3-4-Z/BX) to my 3D printer as touching the printer bed while measuring the distance towards the printer bed has an actual effect on the measurement.. duh. Especially when you have a round aluminium printer bed that has supports at three “corners” only.

Side note (1): The main reason I own a 3D printer is that I’m into electronics  – I like to built and program my own IoT hardware – and wanted to be able to print my own custom cases.

So let me refer you to some contents on the internet that explain how to install this proximity sensor:

They all tell you to work with a voltage divider as these sensors are powered by 6V to 36V, which means that their signal wire can be at that level to. Most 3D printer control boards however are Atmega based, which have max input of 5.5V. My MKS Base V1.5 printer board, is indeed based on an Atmega 2560 and suffers that same limitation.

This all seemed very legit, so without thinking, I started to built a voltage divider, following their samples. I started out by combining 6.8kΩ and 4.7kΩ as this does stay within the standard range of “pull-up” values which is between 4.7kΩ and 10kΩ.

Standard voltage divider
Voltage divider v1

Tested my voltage divider with a 9 volts battery, and it all worked out fine. So let’s tear down my printer and add the proximity sensor. Installed the sensor together with the divider and… nothing! I then tested the voltage divider in combination with the sensor (don’t ask me why I didn’t test it with the sensor the first time) and noticed that the output, which should be around 5V, was around 2.5V only. OK, that might be an indication that the supplied current is not enough to maintain the “high” voltage when we are “pissing” away 2.5 mA towards Gnd. That is a bit weird as 2.5 mA is not that much, but let’s give it a try by using some higher value  resistors. I don’t have the 15kΩ and 10kΩ mentioned in the video at hand so let’s take a 68kΩ and 47kΩ!

Voltage divider with very high resistance
Voltage divider with very high resistance

I was amazed as well, but indeed, this seemed to be the solution as the measured voltage now was 4.5V. Still to far off in my opinion, but it is high enough to pass as a logic high so let’s continue. Connect the sensor, configure the printer firmware and see what’s happening! Ehmm… nothing. Even though the sensor’s LED lights up when I approach it with something metal, the printer software still says NO.

Let’s take the digital multi meter at hand again and see what’s going on. We first measure the sensing pin on the printer board. I’m pretty sure I disabled the pull-up in the firmware so we should measure either a floating pin, or something close to GND.

Side note (2): In electronics we do not like floating pins. We either pull them softly, but fully, up towards Vdd or down to GND.

But wait! It reads a high 5V! That’s not good. Verified my firmware and the pull-up is indeed disabled, so did they add some hardware pull-ups?

Please tell me they didn’t. Because if they did add a standard hardware pull-up between 4.7kΩ and 10kΩ , I will indeed never be able to pull it down to LOW through the 68kΩ resistor in my voltage divider. OK, getting a bit tired of it but, never give up and search for the schematics on the printer board then.

You can find some sort of design of the board on GitHub, but that is not detailed enough. Luckily I found someone with the same issue that I had and he had a solution! Hurrah, all I needed to do was to remove some resisters from the printer board and then it will all work. So I prepared my soldering iron to attack my printer board and then……

NOOOOOH, of course not. It was just then when I finally decided to use my own brains. I should be able to solve this myself! Serious? I almost got medieval on the ass of my printer board with a soldering iron.

So let’s use our brain and look at the schematics of these type of sensors.

Sensor_ wiring_NPN_PNP
Sensor wiring NPN PNP

The sensor type I’ve got is NPN NO which means that you measure the BLACK line and:

  • when it is not sensing anything inductive nearby, it should be open and thus floating
  • when it is sensing anything inductive nearby, it will be connected to GND

Hmmm… floating. Remember when I said we don’t like floating pins in electronics? We would like to have either all or nothing. So in this case, the following will most probably be true:

  • when it is not sensing anything inductive nearby, it will be pulled up to Vdd
  • when it is sensing anything inductive nearby, it will be connected to GND

So let’s draw a logical design on how an NPN transistor with a pull-up would look like.

Logical NPN with pull-up
Logical NPN with pull-up

We should be able to measure that with a digital multi meter. Just measure the resistance between BLACK and BROWN. I did and guess what? I indeed measured an almost perfect standard pull-up value of 10kΩ.

But wait a second… that’s why our voltage divider gave such weird values! What we thought was this:

Standard voltage divider
Voltage divider v1

Actually was this:

Actual voltage divider
Actual voltage divider

And that’s why it all didn’t work. Once we started to create a voltage divider with insane values it started to work as the 10kΩ did not have that high of an impact any more.

We can now also answer the question of how we actually should connect our NPN sensor to our printer board. We do need a voltage divider, but one of the resistor values is already given. We can use the following formula to calculate the required value of the second resistor R2:

Voltage divider formula
Voltage divider formula

Or navigate to if we are lazy.

The output should be 7142.86Ω.

To be on the safe side we pick a standard value which is less so we end up with 6.8kΩ. Our final solution just looks like this:

Final solution
Final solution

Indeed, all we need to do is add a single 6.8kΩ resistor between BLACK and BLUE and all should work fine.

Once the single resistor was added, I verified all voltages with my digital multi meter and it all worked perfectly fine.

Don’t forget that you do need to change your firmware because we still need to invert our end stop. It is HIGH when not triggered and LOW when triggered.

const bool Z_MIN_ENDSTOP_INVERTING = true;

So after all I got it working in the easiest, cheapest and most logical way one can, simply by using my own brain.

The moral of this story is to never stop thinking yourself and to not blindly trust all stories on the internet. Use your brains!

That being said! Don’t just do as this posts says! First you have to verify which type of sensor you’ve got. If you have a NPN type of sensor, you should be able to measure the 10kΩ pull-up resistor between BROWN and BLACK. If you have a PNP type of sensor, you will most probably(I don’t own one) be able to measure the 10kΩ pull-down resistor between the BLACK and BLUE. In case you’ve got a PNP type of sensor, the very first voltage divider should indeed be used.

Standard voltage divider
Voltage divider v1

One thing is sure. IF you do connect your sensor to 12V (which you should as that is within the specs!) you should not go without any voltage regulation, even though many people suggest you should. It doesn’t matter which type of sensor, you should not connect them as is to your signal wire if the sensor is connected to 12V.


Boris had a perfect comment on using a diode instead. And, believe it or not but, using a 1N4148 diode was my first solution. So I did test using a diode and that also works perfectly fine, simply because the direction in which the 12V current would like to flow when the sensor is open will be blocked by the diode. The internal (or hardware) pull-up still pulls the Atmega’s sensor pin to HIGH however.

Current flow when the sensor is open
Current flow when the sensor is open

But as soon as the sensor is closed, the diode would allow the 5V current to flow in the opposite direction towards ground, which pulls the Atmega’s sensor pin to LOW.

Current flow when the sensor is closed.
Current flow when the sensor is closed.

So both the proper voltage divider and a diode can do the job. Which one you prefer… well, the diode might actually be safer to use (they often call it a protection diode with a reason ;-)) if installed correctly, but it also depends on what you have at hand and even what you prefer I guess. The reason I preferred to explain the voltage divider in the first place, is that one cannot wire the resistor in the wrong direction. If you do that on the diode, you will permanently damage your printer board.


While I was watching referrals to this blog post I noticed some Swedish(?) web site. When I looked at that blog post (after translation) it showed an ever better way of connecting the sensor to the printer board by using an opto coupler. I know a lot of people don’t have one of these lying around, but if you do… you can’t be any safer. Check it out here.

25 Replies to “Don’t trust the internet (and how to add an Inductive Proximity Sensor to your 3D printer the proper and easiest way)”

  1. Good job, smart done and clear explained.
    May I ask you something. I’m fighting with MKS Gen-2z v1.1 board on 3D printer. It has a standard hardware pull-up resistor. Now trying to use NPN NO capacitive sensor (LJC18A3-H-Z/BX) for autobedleveling. Already played with voltage divider from Tom’s guide etc. Got floating values, as you did. After reading your article I measured resistance between brown and black and it is infinite. The resistance between black and blue is 1.15 kOhm. Made double check: it is NPN NO sensor. Any ideas?

  2. Update: I used 7.3 kOhm resistor (just did not have other closer to needed value) as you described. In firmware I have disabled pull-up resistor. Now sensor state is reproducibly shown as Low when not triggered and High when triggered (checked using M119 and G31 commands). By the way it shows 5.37 v and 120 mcA when not triggered and 3 mV and 1 mA current when triggered.
    I thank you SO MUCH for your investigation! If you don’t mind I will post links to your blog at the places where other lucky owners of MKS Gen-2z boards with built-in pull-up resistors will probably look for the solution: groups of Sunhokey printer owner at, facebook,
    Thanks again!

    1. Nice to hear it worked out for you. Please feel free to share this information in any form you like. You can even copy the contents to your own blog if you like. As long as the information is shared and you don’t charge people money for it, I’m fine with it.

      1. Here are recommendations about using diode. I don’t quite understand how it works. Solution with resistor looks safer for me, because diode failure is much more probable than resistor failure 🙂
        Quote from there:
        James Brown The NPN sensor drives low when activated, if you have a voltage divider, it may not work properly. The voltage divider will itself form a divider with the on-board pull-up, preventing the signal going fully low; depending on resistor values it might or might not work. A better solution is to use a reverse biased diode, when the sensor drives high, the diode blocks the signal (too high a voltage for the board), when the sensor drives low, the board pin is pulled down to ~0.7V (one forward diode drop).

        1. Thanks for your reply. The diode was my first solution but I eventually went with explaining the divider as this allows me to actively pull-up instead of relying on the hardware or software pull-up. Besides that, the resistor cannot be wired in the wrong direction as can be done with the diode, which would damage the printer board.

          But, to prevent confusion on what to use I decided to update the post to reflect the diode option. It makes it more complex as a whole, but at least, it is now complete.

          For those people that are wondering if you could combine the two: “Yes you can!”. But the resistor would be completely redundant until the diode somehow shorts out, or is installed in the wrong direction. In the latter case you would not damage your printer board, but the head would crash into the bed as its hardware or software pull-up current will be blocked from flowing away to ground by the diode. But I sure hope you at least test the “switch” before you start a bed level.

    1. It indeed looks like the Omron E2BM18KN16WPC12M 16mm NPN inductive sensor does not have an internal pull-up and thus can be used safely without any voltage regulating at all.

      This sensor does not seem to contain an internal pull-up resistor which pulls the black line up to the brown's supply voltage.

      Did you test this with a digital multi meter yourself?

  3. I tried the optocoupler 4N25 as described in the Swedish blog post – it didn’t work. I’ve seen other articles where people are applying 12V and a 1KOhm resistor to the cathode, and the signal pin to the anode … anyone have a capacitive NPN NO sensor working with a 4N25 or optocoupler ?

  4. I tried the ***UPDATE 2 from the Swedish site. Unfortunately 1K ohm can only drop voltage to 2.5V and controller still accepts is as HIGH. After replacing the R with a 2.2 ohm. I got 2 Volts and it was enough for the controller for returning LOW. I tried it with 12mm NPN and 18mm NPN (both inductive) and 18mm Capacitive all worked but best was capacitive one.

  5. My solution was just to change the signal pin to a not used one on my mks gen 1.4. I took pin 44(aux 2)and in the ramps pin file changed the z min endstop pin to 44. Reflashed marlin and it worked.

    1. Going through the datasheets I did not found this pin to have a higher voltage rating. Can you point us to any reference that states that 12 Volt on this pin is supported?

      1. I did not feed it with 12 V. I used the 5 V and the gnd directly from the endstop connection. So the braun wire of the LJ12A3-4-Z/BX Inductive Proximity Sensor Switch NPN no connected to 5V, the blue wire connected to gnd and the signalbwire to pin 44.

  6. I also have a prusa i3 with a rumba board where I just connected the 3 wires off the lj12a3-4-z/bx(npn no) directly on de z min endstop connection. Only the endstopconnection of the rumba is not fitted with pullup resistors that one works straight away.

  7. Thank you very much, it helps!
    I use a schottky barrier diode instead of 1N4148 for it has a faster reaction.
    uncomment PROBE_DOUBLE_TOUCH in marlin, it now has a tolerance of less than 0.05mm

    1. As it is a PNP NO sensor, you should measure either floating or GND when not sensing anything nearby or input V+ when sensing anything nearby on the BLACK wire.

      Input V+ should be between 6V-30V. Based on the input voltage you use (12V?) and any pull-down resistors (internal or external) attached to the BLACK wire to prevent it from floating, you should calculate the required voltage divider.

      I would recommend you to start by measuring the resistance between BLACK and BLUE wire. This will give you the value of an internal pull-down resistor if any.

  8. I just had the same issue and i had come to the same exact conclusion, funny how i also came across the guy that gave advice to remove the onboard pullup resistor lol anyways thanks

  9. Hello, can you please redraw a final version with a diod for dummies (people who do not have experience to read schematics)? Like blue wire does here, brown there etc.

    Also i saw alot of people hook up old end stop sensor as backup measure, to prevent events when proximity probe failed. That switch have 3 nodes, i cant understand how to wire the electronic trigger chain (com, nc and something i cant recall)

  10. Thank you very much! This was really helpful and solved my problem!

    However, for safety puroposes, I first tried the higher resistors first (47K on signal and 32K divider) which did’t work despite outputting the correct voltage.

    Finally I tried the simplest solution with the 6.8K resistor divider (I actually used 4.7K + 2.2K since I didnt have the 6.8K) and it worked out just fine.

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