Here is a simple metal detector circuit using a CA3140 IC. There are many metal detector circuits or kits are available to built but mostly are very complicated but the circuit shown here can be built easily and can detect a metal object from 2 to 3 inches away.
The heart of the circuit is a
CA3140 BiMOS operational amplifier IC. A 2N3904 transistor is connected at the output of the IC to drive the peizo buzzer and an LED. Any similar NPN transistor will work here for example BC547, 2N2222 etc. For coil L1 I have used a guitar pickup coil but you can experiment with many different coils with the circuit. The performance may increase with a suitable coil.

After completing the circuit fit it in a suitable casing. To check the circuit, move the coil around any metal object if the circuit is built as described in the diagram you will hear a beep from the buzzer. The circuit can be operated with 9 to 12V DC.

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Simple Metal Detector Circuit Diagram