dc42:
Qsilverrdc:
Just thinking out loud… :~A round clear plastic rod could effectively transmit light from the circuit board in bottom to top.
Place a spiral trace (coil) flat on the upper surface of the board with the led in middle (traces on back side). The trace coil would then align with the plane of ring.
Could this function as a capacitance, eddy current detector?
RichIn principle, that could serve as an eddy current detector (and I like the idea of a transparent rod lit from the bottom). However, the inductance of a spiral trace is low, so he would need to run the coil at a high frequency. A coil of fine wire just below the surface that the rod is attached to might be more practical, because it could run at a lower frequency.
dc42, I THINK I'm following your thinking here, but if you had a moment to explain a bit further, I would appreciate it. Since I read your post, I've been investigating the (sadly EOL, but still around) CS209A chip from ON SEMI. There's a sexy way to do it metal detector style, I'd just have to be very aware of how the coil was made.
I'm now thinking more in line of metal detection vs optical, and thinking that perhaps a linear hall effect sensor just inside the tube testing against a weak magnet put somewhere off toward the rim of the base? I will have to test later, but perhaps there is enough of a repeatable difference there? meh.