Farooq,

It seems you are searching for a ready made and tested design.

In my personal opinion, a good and sensitive metal detector can be built without 8051.

My search on google for "Metal Detector Design" shows some 128,000 pages. The first one I got is

http://home.global.co.za/~trh/#Introduction

You can insert 8051 into any metal detector.

Good Luck

IA