PRINCIPLE OF METAL DETECTORS:

     
A coil and capacitor are electrical components, which together can produce oscillations of current. An L-C circuit behave just like an oscillating mass-spring system. In this case energy oscillates between a capacitor and an inductor. The circuit is called an electrical oscillator. Two such oscillators A and B are used for the operation of common type of metal detector . In the absence of any nearby metal object, the inductances L1 and L2 are the same and hence the resonance frequency of the two circuits is also same. When inductor 2, called the search coil comes near a metal object the inductance L2 decreases and corresponding oscillator frequency increases and thus a beat note is heared in the attached speaker. Such detector are extensively used not only for various security cheaks but also to locate buried metal objects.

Circuit of Metal detector