Tobbens Adding Inductive Proximity Sensor Log

Introduction

This page documents adding a Research_On_Inductive_Proximity_Sensors#LJ12A3-4-Z.2FBX LJ12A3-4-Z/BX inductive proximity sensor to a Folgertech 2020 Prusa i3. Printer assembly up until this point is documented in Tobbens_Folgertech_Kit_Unboxing_Log and Tobbens_Folgertech_Kit_Assembly_Log. Not documented there is only the addition of an aluminum plate on top of the heat bed, like shown in the pictures below:

Note that the sensor also triggers on the heat bed, so I'm not sure the top aluminum plate is necessary for bed probing yet. Will be investigated closer...

Theory

The sensor if of NPN type and normally open. It is not drivable with 5 V, so we will feed it with 12 V. We expect the following two states if we wire it in directly:

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The Z_min pin is part of the RAMPS' logical circuitry and will be damaged if it is exposed to 12 V, so we need to protect it. For that, we use a diode and internal Z_min pullup:

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The diode stops current to flowing into the Z_min pin with harmful 12 V pressure, but allows the pullup voltage of 5V to flow down to ground.

Implementation

I'll be using an IN4004, MIC salvaged from another RAMPS board.

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Feed 12 V directly from power supply.

Updated Marlin in repo to auto bed compensate.

Posted video of it all working...