The LJ18A3 is a PNP Normally Open (NO) type of proximity sensor. This means that the signal lead will output a positive voltage (same as the input voltage) when a metal mass is detected. When nothing is detected, the signal is equal to ground.
Most modern 3D printer firmwares have some sort of auto bed leveling procedure available, but you will need to enable it. You can use this sensor as the z_min_endstop for example, replacing your current end stop. Most firmware will also use the min endstop as a probe, but some allow a separate probe to be defined.
This sensor operates at 6-36V, so as the signal voltage is the same as the input voltage, also the signal will be 6-36V. You will need to down-convert the voltage to acceptable level for your electronics. Please see the linked schematic on how to connect this sensor to the endstop pins of your electronics board.
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