how to make metal detector

how to make a metal detector?/how does a metal detector work?/what are the components required for making a metal detector?/how to make a cheap and affordable metal detector?

Description: this circuit  most useful for security checking.Metal detector available in the market are quite expensive 

This metal detector can be used to detect slightly big size metallic objects.It uses a sensing coil.This coil should be kept near metallic objects for detection.Input of circuit is a weak clopitt's R.F. range oscillator. Sensing coil forms parts of tuned oscillator.

When coil is brought near a metallic object magnetic energy is absorbed and oscillator fails to work. then Final transistor conducts and buzzer is activated. use a 9V battery, after connecting battery, adjust 4.7K preset till circuit just stop sounding  

                          circuit diagram 

Parts list:-                     

Resistors          

R1,R4     -    56K

R2         -   3K3

R3         -   22K

R5         -   2K7

R6         -   2K2

R7,R9      -  680E

R8         -   15K

P1         -  5K 3386Trim

Capacitors  

C1,C6    -    1PF (104) 100 KPF

C2       -    1KPF (102) 001 PF

C3       -   220PF

C4       -   270PF

C5       -  12 KPF / 100V

C7       -  100/ 25 V

Transistors 

Q1,,2,3,4   -  BC 547 PH

Diode

D1,2,3,4   -  IN 4148

Miscellaneous 

LED     -  5MM red LED

coil     -  metal detector coil

buzzer  -  VK 27 CT (s)

supply  -   9V snapper 

PCB    -    VK 557 PCB