A pulsed induction metal detector induces a current spike in its search coil, and times the decay of the resulting oscillation. The coil is part of a resonant circuit with a capacitor, and any ...

This one differs by using an Arduino to read from the metal detecting coil. The circuit starts with an oscillator that produces a signal of about 160 kHz which is constantly measured by the Arduino.

Consumer metal detectors have a round head known as the "search coil," which is the part that actually does the detecting.