Based on "DIY Arduino Based Metal Detector"
Follow the circuit diagram in the link above, use BC547 instead of 2N2222 (both are working, however the author mentioned some problems with 2N2222).
The most important and critical part is the search coil, I used shielded wire 60 rounds around CD cover, I will try the Spiral form, some bloggers say that spiral form is more efficient.
The current sensitivity is set to 2000.0 (download Arduino code from http://dzlsevilgeniuslair.blogspot.dk/2013/07/diy-arduino-based-metal-detector.html)
See it working on youtube:
