This is a very simple but *very* sensitive normally closed (NC) touch probe. When the tip is moved by any amount it opens the switch contacts.
You should be able to find the parts that aren't printed in your toolbox or spare parts box.
Parts not printed
>Screw with a flat (not countersunk) shoulder 3.3mm diameter and a smooth shank longer than 18mm. Mine was from a PC heatsink
>Washer that fits the screw loosely but has an OD of 9.4mm
>Coil spring 4mm OD and about 10mm long (extended). Mine came out of a squirt bottle trigger/pump
If you don't have these parts it is easy to find similar things to fit, like using a pop-rivet head or a 3.3mm rod and glue a nut to make the head.
I have included .stp files so you can adapt the parts you have to work for this design. Failing that, these are very easy to design, even in Tinkercad and its really only the "Guide" that will need to be changed.
If you have something very close in size but too big, PLA will easily soften if you warm the metal part up and push it through before it gets too hot and warps everything.
How it works
The probe is attached to the z axis (or fixed to the focus lens in the case of a CO2 laser where the bed moves). When the probe tip touches the bed the screw "sensor" moves up against the spring pressure and no longer touches the washer.
Because wires are attached to both the washer and the spring, this movement causes normally closed contacts to open.
When the bed moves away from the probe the sensor can move back down and close the switch contact.
This action is very accurate and repeatable if the screw shank is a snug fit to the "Guide" hole and the mounting to the laser does not flex.
The device is completely passive and the tip does not retract. It does not need power to work, it is simply a switch that opens when the sensor tip touches a surface.
It is probably NOT suitable for a 3D printer bed leveling system because it could touch the model you are printing. If you added a removable cap to protrude past the print head then you could use it but there are many available options for bed leveling that don't need this.
Update Mar 2022:
Added a version that screws directly to an ER11A spindle in place of the compression nut.
Also changed the design a bit making the front removable. It uses spring pins in place of the springs and wires. It also shows how to add indicator LEDs. Look at the [ER11A Zero Probe](https://www.thingiverse.com/thing:5332260)
Klave
