I'm looking for a simple short range metal detector.
I've looked everywhere and I can find only complex metal detectors circuits to find metal at several cms of distance, most over 60cm, they are an overkill for my purposes. What I want is a simple circuit to detect metal presence at no more than 1 cm of the search coil, I have an Attiny MCU available on the board where this should go, if that helps.
I was thinking into pulse the coil at some frequency, and measure the frequency at the other side of the coil. From what I've read, if I put a metal object near enough the inductance on the coil and the metal would make the frequency change at the receiver/measuring side. But i'm not sure how exactly implement that.
Answer
Use the simplest LC generator, where the inductor is implemented as a searching coil.
Use frequency in the range of 100..200kHz
Measure the frequency, using the MCU. If the metal is ferromagnetic (iron, nickel, etc) it will cause decreasing of the generated frequency. If the metal is not ferromagnetic (copper, aluminum, gold, etc.) it will cause increasing of the frequency.
Notice, that some temperature drift of the frequency is unavoidable, so provide the proper (automatic) zeroing procedure by the software.
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