Inductive Sensor

OSE Use Case

• OSE currently uses the LJ18A3-8-Z/BY-5V sensor. Sourcing - [1]
• This allows for a longer sensing distance, which is useful for larger machines (1 meter bed and larger)

Introduction

Inductive sensors are used to sense the Z height on 3D printers. For Marlin with RAMPS - use one that has a 5V output, as opposed to 12V, as RAMPS cannot handle 12V without using additional components.

Part numbers look like LJ18A3-8-Z/BY-5V for the 2018 D3D with LCD screen - where we use the 8 mm distance sensing to facilitate the sensor not hitting the print bed.

• 5V is the critical part - 5V input - so can be powered from the RAMPS board directly
• LJ18 - the 18 refers to the diameter of the sensor in mm (actually, M18 external threading)
• -8- refers to the distance of sensing in mm
• Z refers to DC (vs. AC)
• /
• B refers to normally open (vs. normally closed)
• Y refers to PNP (vs. NPN)

Tom's Guide 1

• Mentions 12v sensor with voltage divider. Video description sources the 5V sensor

Tom's Guide 2

• Comparison - essentially, all distance sensing capacitive sensors are quite good.

Transcript: https://toms3d.org/2017/03/02/autoleveling-3d-printers-9-myths-12-sensors-tested/

Samples

• From Aliexpress -
• 3 Wire Polarity: NPN Detection
• Voltage: DC 5V
• Ouput Status: Normally Open
• Part number: LJ18A3-8-Z/BX
• See part numbers and what they mean at Aliexpress - [2]

The BX vs. BY is NPN vs. PNP (both normally open) - wiring is different!

How to wire LJ18A3-8-Z/BX vs. Z/BY?

Notes

• For normally open, Z_MIN_ENDSTOP_INVERTING in configuration.h of Marlin should be set to false? See [3]
• NPN vs PNP - [4]
• See D3D BOM for sourcing.
• See D3D Height Controller for design

5V vs. 6-36V

As already mentioned, pick the 5V version of the sensor if it is available to you. The 6-36V version can be a drop-in replacement by powering it with 5V, but only if you have a sensor with 8mm distance sensing. However, the sensor will be less sensitive [5]:

#2 You can just run a standard 6 to 36V probe from 5V
Ok, this one is actually easy to test. Out of all the 6 to 36V probes, none worked on 5V except the 8mm capacitive one. And even then, it was much less sensitive than on 12V and only triggered against metal parts, while with the higher supply voltage and a voltage divider on the output, it reliably also triggered against printed parts and had a larger trigger distance than on 5V.

This approach has been tested with printers built in Ghent at January 2020 STEAM Camp.

The "proper" approach is to drive the sensor within the correct voltage range (6-36V) and to use a voltage divider circuit (eg., 15K and 10K for 12V) so that the pin that reads the sensor voltage gets no more than 5V (12 * 10/25 = 4.8). Schematic [6]