You need as followes: 1. headphones 2. a power supply (a battery) 3. an interupter (a buzzer) 4. 4 germanium diodes arranged in a rectifer bridge 5. Wire enough for two coils, one oval and one in a figure 8 shape about the same size wound inside the other The headphones are attached to the + & - outputs of the rectifier bridge, the oval coil is attached to the ac inputs of the rectifier bridge. The battery, buzzer and figure 8 coil are connected in series. The buzzer pulses the current flowing through the figure 8 coil. In one hole of the figure 8 coil the magnetic north is up in the other it is down thus it normally does NOT induce a current in the oval coil and cause a tone in the headphones. When metal is brought near the coil assembly it causes one side of the figure 8 coil to have a stronger (or weaker depending on the metal) magnetic field thus allowing a current to be induced in the oval coil producing a tone in the headphones. You could build something like this using transistors.