Phew, I dodged that bullet/rabbit hole, I had my finger on the order button, and somehow was able to control myself this time. Knowing that you’re on it will allow me to delay that impulse purchase further. Thanks 
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HAHAHAHA. Well, @WillAdams put of the link with several DIY Options and I knew from the way he was talking that @fenrus wasn’t going to be able to resist the challenge. So I am wondering if I will be able to build a probe before he can get the code working.
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ok I figured I should post a quick update on where things are
ebay toy arrived. I managed to do all the soldering and crimping to wire it correctly to the carbide controller board. I wrote a pile of javascript and I can now home the machine and move the spindle(probe) around, and even probe.
So far so good.
There’s something wonky with my probe (I crashed it at some point) and I might need to take it apart and see if I can fix it. The probe is not centered, and sometimes gets sticky and keeps signalling to the board that it’s closed.
This is pretty much what PCB “board leveling” functionality in the various gcode senders does.
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I never used board leveling, but I was under the impression that it probes a predefined matrix of points, coming in vertically at each point then retracting, which takes a long time if high resolution is required (not the case for PCB-related board leveling). While this gadget is intended to detect lateral contact using a small lateral increment and then moving up and laterally until it contacts again, etc… ?
By the way @fenrus how do you manage inflexion points ? I can picture the code for following an upward slope, but once reaching the top of a “hill”, I guess you need some specific logic to detect that no edge is detected even after moving laterally a certain amount, then go back to the last known contact point, and from there probe laterally and downards ?
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ok so you just made me think of a 2nd strategy.
the most naive thing I am planning to build is to just probe in a grid (but do it slightly smart, so that the grid is finer in more interesting places)
but it’s indeed also possible to probe similar to how you do 3D carving, e.g. in a zigzag.
the key is that you can probe in 2 directions; you can probe down until the probe goes from “off” to “triggered” but you can also probe upwards until the probe goes from “triggered” to “off”.
so assume you are in location 1 and on that hair edge of just triggering. you can go 1mm to the right. now if the probe is no longer triggered, you need to probe down. (deals with your inflection point). If the probe is still triggered, you need to probe up. Both probe distances will generally be short so pretty quick… and at the end of this algorithm you’re back in the starting point of “hair edge of just triggering” so the algorithm will repeat well.
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