A simple circuit for metal detecting

Dear Friends,
I need to a simple circuit for metal detecting.
The detection object is coins in the depth 10cm.
I need this circuit for a small robot. So it's very good if i can connect alarm output of circuit to a microcontroller instead of a buzzer or LED.
It's important that i know the details of circuit parts.
Thanks

Not sure of the distance of detection, but this link shows what could be made into a tiny PCB for your small robot:
NEW:: picoDetector : How to detect metal with a PIC

Gary (Chemelec) had few of 'em too:
**broken link removed**

Lots of ranges from simple to complex from this page:
**broken link removed**

Dear Mickster and Vizier,
Very thanks. They was very useful.
Good Luck!