the principle of a metal detector is quite simple Inside the machine are wire coils A transmitter sends an electronic current through the coils the current flow goes from clockwise to counterclockwise and back to clockwise again this current creates an electromanetic field the direction of the current flow reversed several thousand time every second After a few seconds this current or pulse slows down and disperses Any metallic object like keys coins a influence even a cell phone which happens to enter this electromanetic field will influence the speed of the current If the pulse to go away this delay sets off an alarm