Circuit diagram for metal detector using Transistor

Hi Rahul,

A Metal Detector requires a air cored inductor. The simplest and quickest way to build a metal detector is by using a 555 Timer. You can find the circuit diagram and explanation here

https://circuitdigest.com/electronic-circuits/simple-metal-detector-circuit

Once you understand the circuit you can also use a transistor in place of 555 if you do not need the beeping sound 

Surprising enough I found many metal detector circuits that work only with a help of a single transistor like the once shown below

This circuit uses a single BC547 NPN transistor. The circuit is claimed to be tested, but the difficult part in building this circuit is with winding the tapped coil. If you looking for a simple easy to build circuit you can also try the circuit given by aswinth.

Do note that these circuits are not foolproof, if you serious into building a good metal detector then this below link will be very useful  

http://www.talkingelectronics.com/projects/200TrCcts/MetalDetectors/MetalDetectors-1.html