The first time I use FM receiver because you have here amplification and tone is present via "interference" in the FI section. The circuit of emission I put near the radio.
The second time, invent a system of "interference" (the tone in the receiver). Here I was "liberated" of the need of locate the circuit of emission near the radio. The loops creates the necessary "interference". I bought this detector to a man who wish this. He brought me a silver spoon located withs this, and later he found more things as roll of copper wire, etc. Don't know more of this man.
The third time I use a milivoltmeter based on 741 (classic simple circuit), but when I raise, this beeps. So, here you must reduced the sens. and stay more critic. Rapidly found a washer of copper. I leave this for the problem. I'm not sure to use low noise ICs in this application for example.
You measure changes in audio, because simple audio I use. Regarding audio, Alonso told me once: "The phenomenom is present in a piece of wire. The matter is to convert in sound." Great words of this genius!
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