The LJ18A3-8-Z/BY proximity sensor will detect large metal fields at 8mm distance with great precision. This allows you to accurately measure the distance to an (aluminum) heated bed for example. If you are using a glass plate as your print surface, you can use aluminum foil right below the glass, which will also be detected by the sensor (mirrors however are not detected). The sensor is equipped with a small red detection LED, indicating a nearby object. The sensor has an M18 threading and includes a nut for easy mounting to your 3D printer or CNC machine.
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.