How Does a Metal Detector Work

A metal detector is a specially designed device that can detect metals lying deep in the ground or water. Since its invention, the device has found a wide range of applications in the human society. More commonly, it is used for security screening, and for locating mines.

In the industrial sector, metal detectors have found applications in the food, pharmaceutical, textile, chemical, plastic, and packaging industries. They play an important role in identifying the presence of metal shards in food. However, you can find many people using these devices for treasure hunting and coin shooting as well.

It works on the basic principle that, when electric current passes through a coil, it produces a magnetic field around it. The device contains an oscillator, which produces an alternating current (AC). When the alternating current passes through the transmit coil present in the device, a magnetic field is produced around it.

Now, if an electrically conductive metallic object is present near the coil, then eddy currents will be generated in the object, which will produce another magnetic field around it. A metal detector contains another coil in its loop, called receiver coil.

The receiver coil can detect any changes in the magnetic field due to the presence of a metal or metallic object. The modern-day metal detectors usually use one of the three technologies, which are known as VLF or very low frequency, PI or pulse induction, and BFO or beat-frequency oscillation. These three technologies are briefly explained below.

VLF metal detectors contain two sets of coils - transmitter coil and receiver coil. The transmitter coil is the one through which an electric current is sent to create a magnetic field that constantly pushes down into the ground and then pulls back.

The magnetic filed generated in the device interacts with any metallic or conductive object that comes in its way. If such an object is encountered, then eddy currents and subsequently, a magnetic field are created around the conductive object.

The receiver coil on the other hand, is shielded from any influence of the magnetic filed produced by the transmitter coil, so that it is affected only by the magnetic filed of the conductive or the metallic object.

An electric current runs through this receiver coil, whenever the device passes over a conductive object that is producing a weak magnetic filed. The coil in turn, amplifies and sends the frequency of the current (which is same with the frequency of the magnetic filed generated by the metal) to the control box for analysis.

In this way, the device can detect metals, and determine the difference between different types of metals and the depth at which they are located.

In this technology, a single coil is used, though sometimes two to three coils can also be found in the device. The device basically sends short bursts or pulses of current through the coils, each of which generates a short magnetic field.

At the end of each pulse, the magnetic filed generated reverses its polarity suddenly and then collapses. This creates electrical spikes that can last for a very brief period of time.

When the electrical spikes and the magnetic field of the pulse collapse, another current, known as reflected pulse runs through the coil. The reflected pulse lasts for an extremely short period of time.

But when the device encounters a metallic or conductive object, the reflective pulse lasts for a longer time period. This is because, the pulse sent by the metal detector produces an opposite magnetic filed in the object, and this magnetic field causes the reflective pulse to last longer.

The device also contains a sampling circuit that closely monitors the spikes and the reflected pulses, and sends these signals to the integrator. The integrator reads, amplifies, and then converts the signals to direct current (DC). The audio circuit then produces a tone, which indicates the presence of a metallic object.

Like VLF technology, the BFO technology uses two coils of wire. One coil is present in the control box of the device, while the other one is located in the search head. The coil which is in the control box is usually smaller than the one present in the search head.

Both the coils however, remain connected to the oscillator that sends thousands of electric pulses in a single second. Radio waves are created when pulses pass through each coil of wire, which are then collected by a receiver located within the control box.

The receiver then creates audible tones on the basis of the frequency of the radio waves. But, when the device encounters a metals or metallic object, the electric current running through the coil of the search head creates a magnetic filed, which in turn generates another magnetic filed around the metallic object.

The magnetic field created around the metallic object interferes with the radio waves created by the coil in the search head. This brings about a change in the tones produced by the receiver, which helps detect the targeted object.

So, these are the technologies used in modern-day metal detectors. These devices can not only detect the targeted metallic or electrically conductive objects, but can distinguish the different types of metallic objects and find out their location as well.