Metal Detectors – Part 1: Introduction

Each one of us as a child is dreamed to find the golden treasure. Many people are lost their minds until they are searching gold.

We will see the detecting devices only as part of the hobby and with an entertainment goal, not as some kind of madness.

Firstly, let’s talk about the theory and try to explain Hobby and beginner’s level Metal Detector’s principle of work.

After that, we will continue to talk about the metal detectors and will list some interesting members from the huge market.

Searching the metal items into the ground is a hard work.

There are many aspects and many troubles to resolve by the Metal Detector’s manufacturers, in a long process of developing such devices:

The ground mineralization needs to be “Ground Balanced”- in ground detecting activity,
the saltwater – when are using the waterproof metal detector for underwater searching also need to be balanced as “as relative and basic level”,
and, many other stress factors and situations – as false detecting of rusty ironic trash – because most sands and soils can contain a significant amount of iron.

Precisely because of these reasons, metal detectors use the different principal of work – combining both or part of them in the more professional devices.

To use well the Metal detector уou don’t need to know the principal of work, but for more curious readers, we try to explain the basic theory.

Dependently of the principal of work in years of developing, the developers are separating devices in few groups:

Frequency Measurers Counters: In their basics and their principal stay the simple LC-Generator. For Explaining we will use the Thomson’s equation, which said: Oscillation frequency f = 1 /(2PI SQR(LC)) – see SCH. 1.

with C is signed The capacitance of The Capacitor [in Farads]

with L we Sign the Inductance of the Inductor [in Henries]:

lc-generator-1-hadrware-pro

SCH. 1 Schematic example: The Hartley LC Generator

Link to LC Circuit Theory:

In the lack of metal objects near the inductor, we calculating or measuring the frequency and get it for the Basic or relative frequency.

Example: Relative frequency is 7 KHz.

When we approach some kind of metal object near to Inductor {remark by the author: Inductor Realized as an inductive coil with an open magnetic field structure – as a frame for example }, its Inductance is changing.

fd-1-hardware-pro

PIC. 1 Example of Searching coil in BFO design { BFO – Beating Frequency Oscillator }

The sign of that change is a function iron content and can be as positive, and, as negative.

It is depending on the using metal characteristics:

For Paramagnetic metals or alloy – without iron in their content – we have the decreasing of the inductance – which means higher frequency than relative. Example: f _Param. = 7.001 kHz

For Ferromagnetic metals or alloy – iron-containing, we have the increasing of the inductance – which means Lower frequency than relative. Example: f _Ferro. = 6.999 kHz

Measuring the frequency and calculating the difference to relative frequency brings the kind of near situated object.

The Measuring can be done by mixing the LC generator frequency and high stability fixed etalon frequency generator and measuring the “beating” result frequency. The used principal is named “BFO Metal detectors”.

The “Beating” resulting frequency is suitable to be applied to a detecting – diode-based circuit { for visual control and showed as an analog value} and attached to an audio amplifier – to be heard with headphones for example.

Benefits: Simple Sensor and schematics construction.
Disadvantages: Little sensitivity, weak metal kinds separation.

BFO Related Video: