Slightly over three years ago, I acquired my first 3D printer, a UP! Mini. I didn’t know it at the time but as a beginning printer it was perfect for me. Two years later I was familiar with the weaknesses of my printer and wanted more. I studied reviews, I put various printers on my Amazon wish list but never actually committed myself to one.

I fell in love when Prusa 3D introduced their i3 MK2. My fate was sealed when Make magazine rated it number one in their annual review of 3D printers. About a week later I placed my order, just in time to enter a queue of all the other people that read the same review. I bought the kit because I had great confidence in my ability and I wanted to save money. Call it a case of penny-pinching and overconfidence.

Six weeks later, I received a very complete kit. No problems putting it together but getting it to print was another story. Everything pointed to problems setting the PINDA sensor. I found a tool on Thingiverse that makes adjusting the position trivial. It helped but not enough. When I found out I was setting the live Z adjust way too small everything came together.  For the next two weeks everything was great and then disaster struck.

Somehow, in the middle of a print and deep in the night, my PINDA sensor broke off and got raked over my extruder nozzle. Melted insulation all over the place.

Prusa has since improved their design to make setting the PINDA easier and based on the stress fractures I saw in my broken PINDA mounting, I was partly to blame for the failure. Still, I needed a new PINDA.

Prusa will sell you a new PINDA and at a very reasonable price, about $10 but it’s not listed. You have to ask one of their sales agents and then you have to pay postage, lots of postage, $60 in my case.

I don’t like paying $60 postage for a $10 part. I certainly don’t like the fact that my open source printer is not quite open source. Searching the Internet for information, I found very little telling me exactly what the Prusa sensor was. Cutting the remains of my sensor off the printer, I started making measurements. Here;s what I found.

  • 8 mm in diameter, 37 mm long
  • 2 mm sensor
  • 5 volt operation
  • Normally Closed (NC) operation
  • NPN

After searching a number of vendors, I had to admit that Prusa seems to have the market cornered. At  37 mm, they are the shortest in the market. I was able to find a 42 mm sensor, an E2B-S08KN02-WP-C2 2M by OMRON. This sensor would fit but it operated at 10 to 30 volts and was out of stock at Digikey and Mouser.

Looking at Amazon, my favorite shopping spot, almost all of the proximity sensors available are 6 to 36 volts and approximately 70 mm long. While the sensors will fit in the Prusa PINDA mount, the odds are they won’t operate at 5 volts. It’s close enough that some of them might but you can’t depend on it and reported results very.

I felt like I was at a dead end until I found the Pololu 12V step-up voltage regulator. The Pololu U3V13F12 ($3.95 at Pololu) is a tiny voltage converter. Put in 5V and 12V comes out. Perfect for the sensor and small enough to place in line with the cable but what about the Prusa controller?

I don’t believe it’s by design but the NPN structure of the sensor makes using this voltage converter easy. Because the sensor’s output is open collector, I can use the step-up regulator to operate the sensor at 12V and the output to the Prusa is exactly as if the sensor were operating at 5V. See the diagram below.

I used an LJ8A3-2-Z/AX ($8.62 from Amazon) with the U3V13F12 for my experiments.

Did it work? Yes and no. All indications are that my PINDA is working. I have excellent adhesion to the platform but now my filament feed is giving me problems. I may have deformed my Teflon tubing feed. Sigh, another tear down this weekend.

One last note. If you look in the literature on the OMIRON sensor, it’s rated for 70 degrees C. My print bed is usually set at 100 degrees for ABS. I don’t know what the Prusa PINDA is rated at but this could explain why so many people notice a difference when the sensor is near the print bed before it starts measurements. Just a thought.

PS: The MK3S eliminated a number of problems with the PINDA but introduced a totally different failure mechanism. If you own a MK3S make sure you read Prusa MK3S, Built in Self Destruct or Swiss Cheese Theory of Failure before it happens to you.


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