Looking for circuit that can detect a nearby tuned circuit...

It all depends, as they say ... Checkout "Grid Dip Oscillator", or more probably these days "FET Dip Oscillator" for simple devices that respond to a tuned circuit.

Alternately, there are some 'electronic' pet doors that do exactly this - the cat wears a small resonant circuit and the door opens to it (on a good day).

Dave

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Something like that used in the Schlage/Westinghous access card system should be just the ticket.

You can use a single oscillator, if you have the time to characterise the circuit.

Make your rocks around 3 different masses of an inductive material. When you drop each rock through the 'detection coil' (which should be part of the tank on your oscillator), they will perturb the output frequency and amplitude, but each will perturb it differently.

For best detection, integrate the output of the oscillator and watch the signal amplitude (now as a DC signal).

This is the basis of certain electronic coin scanners.

Cheers

PeteS

Investigate those security labels they stick on expensive good in some stores. They are tuned circuits that are detected when they go through the checkout.

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