Metal Detectors

Introduction

Metal detector circuits are very simple. There are a few variations of them. This application note will focus on the simple beat detector type.

The simple beat detector circuit consists of a simple oscillator circuit and a coil for searching. In a packaged oscillator, another oscillator will be present and the output signal from the coil will be compared to the second oscillator which is a reference oscillator. This will be combined to a single output signal which is amplified to a pair of headphones for the user to listen to the variations.

Now implementing the metal detector largely depends on the coil and interfacing with the UP1 board.

Coils

The metal detector will use a coil to do the actual detection of the metal. The circuit sends a current through the coil and generates a magnetic field. When the coil passes over metal, the metal will change the magnetic field, which in turn disturbs the current in the coil. When there is a disturbance in the output current from the oscillator, there is a difference with the current, the user actually hears this with his ears.

Now the number of windings and the size of the loop affects detection efficiency of the coil and the wiring used. Currently, we find in the lab that the coil only responds when metal objects are extremely close or in the coil. This is not very useful, now is it?

The recommended turns on a coil largely depends on the coil windings and the size. This information will be updated later this week as we complete more research in to this item.

Interfacing with UP1

Interfacing a metal detector circuit into the Altera UP1 board is very simple. You can take the output directly from the oscillating circuit through a voltage comparator and have the FPGA sample the beats for a particular time. Please refer to our project (Metal Rover) for more implementation details.

References

A simple metal detector circuit with documentation and circuit schematics can be found at this site:

Steve Shageman's Metal Detector

Good Luck and Happy Treasure Hunting.

Metal Rover Team.
Andrew Leung
Michael Cumming