Hi, and thanks for the compliment.
It is a regular opto-switch, I salvaged it from a relatively old inkjet printer. Actually the printer had 3 or 4 of them, some were easier to desolder and use than others.
As for precision, it is good enough.
I mean the nail is not 100% flat on its head and it does have a rotational movement while going up and down sometimes, so there
might be a factor of 0.Xmm of inaccuracy.
But all in all, this beats me having to calibrate it after each print or couple of prints.
Take a look at the cylinder printed, the elevation was as shown in one of the pictures, and that print came out very nice..