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An IB metal locator is usually far more complex than the design shown here. The reason for the simplicity is that I have dispensed with analogue circuitry, and instead used a digital transmitter and receiver. As the search coils pass over metal, only digital signals of a certain amplitude break through to a peak detector (IClb). Since these are in the audio range, they are immediately transferred to the piezo sounder or headphones. On testing the sensitivity of this design in air, with optimal tuning and using a 25mm-diameter brass coin, it gave a clear signal at 150mm, and a 'screaming' signal at 110mm. It was also able to detect a pin at 30mm.

Circuit Diagram: https://www.dropbox.com/s/fo4oxe1j7cn...

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