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?
Rich
In 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.