It is incredibly simple to build and contains only a few components
      - Two PNP transistors
      - Two resistors
      - Two capacitors
      - Search coil
      - And Android smartphone

Simplicity is due to the use of the smartphone, that processes the signal and displays the result on the screen. Let me mention that this is another great project of "" web, where you can download the Android application. A signal with a certain frequency that is defined by the input L-C circuit is inserted into the microphone input of the smartphone. When the coil is located near a metal object, the oscillatory frequency changes and it is displayed on the screen as a detected metal.

The detection coil consists of 90 to 120 windings of insulated copper wire, wound to a diameter of 8 to 10 cm. By changing the capacitor C2, the oscillatory frequency of the L-C circuit also changes, which should be around 9-12 KHz. In my case the value of C2 is 67nF. Depending on the number of turns and the diameter of the coil, the value of C2 is determined experimentally. In this case the aim was to demonstrate the ability to discriminate against different metals. However, to conclude that although it is extremely simple to build, this metal detector has relatively good features and can detect a small coin at a distance of 5 cm.

   Schematic diagram: