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With few incidents, I think I made what is called a BFO metal detector, which actually works very well over a large distance. 

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By: Mehdi Sadaghdar

0:00 Wrong Way of Detecting Metal
1:48 Theory Behind How Metals Effect Inductance
6:10 More Fake and Real Metal Detector Video Tutorials
7:32 Designing a “Simple” Metal Detector Circuit
15:48 Testing the DIY Metal Detector Circuit