IC 555 has tons of applications in day to life and that’s the reason it is still a significant IC chip. This metal detector circuit uses IC 555 chip along with an inductor was to build a cheap and simple metal detector at our home.
The inductor of 10mh in the above circuit serves the purpose of metal detection and 555 serves the multivibrator section of the above circuit. The IC 555 vibrates at specific frequency in normal state which in turn drives the output buzzer for the purpose of indication. You can also add a LED along with the buzzer for indication provided sufficient power to drive both the components.
Inductor develops a magnetic field when fed with clock pulse. This field when brought near a metal or magnet its inductance decreases which alters the frequency at which the IC 555 operates. Difference in frequency of IC 555 will make buzzer to sound different than usual. Therefore this helps the user to find the metal buried beneath the ground.
So summing up things, Normal sound from buzzer indicates no metal whereas abnormal sounding indicates a metal is detected. By this way the above detector assist user in detecting the metal that is beyond our sight reach.