I wanted to see if cheap inductive proximity switches could be reworked to output a voltage proportional to displacement. This is what it looks like on the inside after I ham-handedly snapped off the fragile copper coil (inductor). Some strands of the coil are visible on the right trapped in potting epoxy.
On the inside they have this PCB connected to the coil and three pigtails that are Vs, GND, and a high/low signal indicating the probe is some distance from a conducting target. The LED lights up when you’re in range of the target.
I tried to reverse engineer it and came up with this circuit (possibly wrong, and I’m not sure where the coil attaches – it’s made of very delicate wire and tore off when I was trying to remove the potting epoxy):
Resistors values are all visible, don’t know what the caps are, all transistors are NPN type. I think it’s probably an oscillator that excites an LC or RLC circuit, but my circuit analysis skills are insufficient to make heads or tails of it now. Eddy-current or inductive proximity probes are a very common vibration sensor in industrial turbomachinery, but are way outside the hobbyist budget. The premise is very simple so I’m curious why they get so expensive.