This circuit is on oscillator with LI being a 4` diameter coil of 35 turns of #26 magnet wire. Metal in proximity to LI will cause the oscillator to shift frequency. An AM transistor radio is used to detect the frequency shift.
The transistor is the MPS5172 or use a similar.
Simple BFO Metal Detector - Improved Circuit Design
HOW TO BUILD THE GOLDFINDER XTR METAL DETECTOR - PART 1 (Subscribe
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