Diy Arduino based metal detector.

UPDATE: Try using BC547 as transistor people have reported problems with 2n2222.

Me and my son decided to go treasure hunting with a metal detector that we have but we couldn't find it anywhere. Being proper makers we decided that it would be more fun to build one ourselves rather than keep trying to find it. 

Most metal detectors uses a search coil that act as part of an oscillator circuit. When metal is put in proximity of the search coil the frequency of the oscilations changes.

Many metal detectors (including the one we can't find) uses another more stable oscillator BFO (beat frequency oscillator) to act as a reference for the frequency of the search coil. Usually the frequency of the BFO is adjusted to exactly match that of the search coil oscillator when no metal is present near this.
The signals from these two oscillators are then fed to a, usually analog, circuit that create an output proportionally to the difference in frequency og the two. This may be either an audiable tone and/or some meter reading.
Another device that are really good at detecting minute frequency changes is a microcontroller. We decided to swap the BFO approach for a microcontroller and came up with following simple circuit:

The oscillator circuit feeds a around 160kHz signal to pin 5 of the Arduino. The Arduino sketch then measures the frequency of this pin very accurately. When the 'NULL SW.' button is held this frequency is stored. Any deviation from this frequency is then represented as a series of 'geiger counter' clicks on the piezo. The rate of the clicks increases as metal approaches the coil.

We tried different search coils and found that around 30 turns of wirer around a 15cm. plastic bucket worked well.

All we needed then was to tie it all to a discarded Ikea lamp and hey-presto off to the beach to find treasures.
The metal detector has excellent sensitivity and by changing the SENSITIVITY value in the Arduino sketch you are able to tune it for both small and large objects.
Here is the source code if you want to build one yourself.
Happy hunting!