How Metal Detectors Work

Mention the words metal detector and you'll get completely different reactions from different people. For instance, some people think of combing a beach in search of coins or buried treasure. Other people think of airport security, or the handheld scanners at a concert or sporting event.

The fact is that all of these scenarios are valid. Metal-detector technology is a huge part of our lives, with a range of uses that spans from leisure to work to safety. The metal detectors in airports, office buildings, schools, government agencies and prisons help ensure that no one is bringing a weapon onto the premises. Consumer-oriented metal detectors provide millions of people around the world with an opportunity to discover hidden treasures (along with lots of junk).

In this article, you'll learn about metal detectors and the various technologies they use. Our focus will be on consumer metal detectors, but most of the information also applies to mounted detection systems, like the ones used in airports, as well as handheld security scanners.

Stabilizer (optional) - used to keep the unit steady as you sweep it back and forth

Control box - contains the circuitry, controls, speaker, batteries and the microprocessor

Shaft - connects the control box and the coil; often adjustable so you can set it at a comfortable level for your height

Search coil - the part that actually senses the metal; also known as the "search head," "loop" or "antenna"

Most systems also have a jack for connecting headphones, and some have the control box below the shaft and a small display unit above.

Operating a metal detector is simple. Once you turn the unit on, you move slowly over the area you wish to search. In most cases, you sweep the coil (search head) back and forth over the ground in front of you.

When you pass it over a target object, an audible signal occurs. More advanced metal detectors provide displays that pinpoint the type of metal it has detected and how deep in the ground the target object is located.

In the following sections, we will look at each of these technologies in detail to see how they work.

Very low frequency (VLF), also known as induction balance, is probably the most popular detector technology in use today. In a VLF metal detector, there are two distinct coils:

Electricity is sent along this wire, first in one direction and then in the other, thousands of times each second. The number of times that the current's direction switches each second establishes the frequency of the unit.

Receiver coil - This inner coil loop contains another coil of wire. This wire acts as an antenna to pick up and amplify frequencies coming from target objects in the ground.

The current moving through the transmitter coil creates an electromagnetic field, which is like what happens in an electric motor. The polarity of the magnetic field is perpendicular to the coil of wire. Each time the current changes direction, the polarity of the magnetic field changes. This means that if the coil of wire is parallel to the ground, the magnetic field is constantly pushing down into the ground and then pulling back out of it.

As the magnetic field pulses back and forth into the ground, it interacts with any conductive objects it encounters, causing them to generate weak magnetic fields of their own. The polarity of the object's magnetic field is directly opposite the transmitter coil's magnetic field. If the transmitter coil's field is pulsing downward, the object's field is pulsing upward.

The receiver coil is completely shielded from the magnetic field generated by the transmitter coil. However, it is not shielded from magnetic fields coming from objects in the ground. Therefore, when the receiver coil passes over an object giving off a magnetic field, a small electric current travels through the coil. This current oscillates at the same frequency as the object's magnetic field. The coil amplifies the frequency and sends it to the control box of the metal detector, where sensors analyze the signal.

The metal detector can determine approximately how deep the object is buried based on the strength of the magnetic field it generates. The closer to the surface an object is, the stronger the magnetic field picked up by the receiver coil and the stronger the electric current generated. The farther below the surface, the weaker the field. Beyond a certain depth, the object's field is so weak at the surface that it is undetectable by the receiver coil.

In the next section, we'll see how a VLF metal detector distinguishes between different types of metals.

How does a VLF metal detector distinguish between different metals? It relies on a phenomenon known as phase shifting. Phase shift is the difference in timing between the transmitter coil's frequency and the frequency of the target object. This discrepancy can result from a couple of things:

Inductance - An object that conducts electricity easily (is inductive) is slow to react to changes in the current. You can think of inductance as a deep river: Change the amount of water flowing into the river and it takes some time before you see a difference. Resistance - An object that does not conduct electricity easily (is resistive) is quick to react to changes in the current. Using our water analogy, resistance would be a small, shallow stream: Change the amount of water flowing into the stream and you notice a drop in the water level very quickly. Basically, this means that an object with high inductance is going to have a larger phase shift, because it takes longer to alter its magnetic field. An object with high resistance is going to have a smaller phase shift.

Phase shift provides VLF-based metal detectors with a capability called discrimination. Since most metals vary in both inductance and resistance, a VLF metal detector examines the amount of phase shift, using a pair of electronic circuits called phase demodulators, and compares it with the average for a particular type of metal. The detector then notifies you with an audible tone or visual indicator as to what range of metals the object is likely to be in.

Many metal detectors even allow you to filter out (discriminate) objects above a certain phase-shift level. Usually, you can set the level of phase shift that is filtered, generally by adjusting a knob that increases or decreases the threshold. Another discrimination feature of VLF detectors is called notching. Essentially, a notch is a discrimination filter for a particular segment of phase shift. The detector will not only alert you to objects above this segment, as normal discrimination would, but also to objects below it.

Advanced detectors even allow you to program multiple notches. For example, you could set the detector to disregard objects that have a phase shift comparable to a soda-can tab or a small nail. The disadvantage of discrimination and notching is that many valuable items might be filtered out because their phase shift is similar to that of "junk." But, if you know that you are looking for a specific type of object, these features can be extremely useful.

A less common form of metal detector is based on pulse induction (PI). Unlike VLF, PI systems may use a single coil as both transmitter and receiver, or they may have two or even three coils working together. This technology sends powerful, short bursts (pulses) of current through a coil of wire. Each pulse generates a brief magnetic field. When the pulse ends, the magnetic field reverses polarity and collapses very suddenly, resulting in a sharp electrical spike. This spike lasts a few microseconds (millionths of a second) and causes another current to run through the coil. This current is called the reflected pulse and is extremely short, lasting only about 30 microseconds. Another pulse is then sent and the process repeats. A typical PI-based metal detector sends about 100 pulses per second, but the number can vary greatly based on the manufacturer and model, ranging from a couple of dozen pulses per second to over a thousand.

If the metal detector is over a metal object, the pulse creates an opposite magnetic field in the object. When the pulse's magnetic field collapses, causing the reflected pulse, the magnetic field of the object makes it take longer for the reflected pulse to completely disappear. This process works something like echoes: If you yell in a room with only a few hard surfaces, you probably hear only a very brief echo, or you may not hear one at all; but if you yell in a room with a lot of hard surfaces, the echo lasts longer. In a PI metal detector, the magnetic fields from target objects add their "echo" to the reflected pulse, making it last a fraction longer than it would without them.

A sampling circuit in the metal detector is set to monitor the length of the reflected pulse. By comparing it to the expected length, the circuit can determine if another magnetic field has caused the reflected pulse to take longer to decay. If the decay of the reflected pulse takes more than a few microseconds longer than normal, there is probably a metal object interfering with it.

The sampling circuit sends the tiny, weak signals that it monitors to a device call an integrator. The integrator reads the signals from the sampling circuit, amplifying and converting them to direct current (DC). The direct current's voltage is connected to an audio circuit, where it is changed into a tone that the metal detector uses to indicate that a target object has been found.

PI-based detectors are not very good at discrimination because the reflected pulse length of various metals are not easily separated. However, they are useful in many situations in which VLF-based metal detectors would have difficulty, such as in areas that have highly conductive material in the soil or general environment. A good example of such a situation is salt-water exploration. Also, PI-based systems can often detect metal much deeper in the ground than other systems.

BFO Technology The most basic way to detect metal uses a technology called beat-frequency oscillator (BFO). In a BFO system, there are two coils of wire. One large coil is in the search head, and a smaller coil is located inside the control box. Each coil is connected to an oscillator that generates thousands of pulses of current per second. The frequency of these pulses is slightly offset between the two coils. As the pulses travel through each coil, the coil generates radio waves. A tiny receiver within the control box picks up the radio waves and creates an audible series of tones (beats) based on the difference between the frequencies.

If the coil in the search head passes over a metal object, the magnetic field caused by the current flowing through the coil creates a magnetic field around the object. The object's magnetic field interferes with the frequency of the radio waves generated by the search-head coil. As the frequency deviates from the frequency of the coil in the control box, the audible beats change in duration and tone.

The simplicity of BFO-based systems allows them to be manufactured and sold for a very low cost. But these detectors do not provide the level of control and accuracy provided by VLF or PI systems.

Metal detectors are great for finding buried objects. But typically, the object must be within a foot or so of the surface for the detector to find it. Most detectors have a normal maximum depth somewhere between 8 and 12 inches (20 and 30 centimeters). The exact depth varies based on a number of factors:

The type of metal detector - The technology used for detection is a major factor in the capability of the detector. Also, there are variations and additional features that differentiate detectors that use the same technology. For example, some VLF detectors use higher frequencies than others, while some provide larger or smaller coils. Plus, the sensor and amplification technology can vary between manufacturers and even between models offered by the same manufacturer.

The type of metal in the object - Some metals, such as iron, create stronger magnetic fields than others.

The size of the object - A dime is much harder to detect at deep levels than a quarter.

The makeup of the soil - Certain minerals are natural conductors and can seriously interfere with the metal detector.

The object's halo - When certain types of metal objects have been in the ground for a long time, they can actually increase the conductivity of the soil around them.

Interference from other objects - This can be items in the ground, such as pipes or cables, or items above ground, like power lines.

Hobbyist metal detecting is a fascinating world with several sub-groups. Here are some of the more popular activities:

Coin shooting - looking for coins after a major event, such as a ball game or concert, or just searching for old coins in general

Relic hunting - searching for items of historical value, such as weapons used in the U.S. Civil War

Treasure hunting - researching and trying to find caches of gold, silver or anything else rumored to have been hidden somewhere

Many metal-detector enthusiasts join local or national clubs that provide tips and tricks for hunting. Some of these clubs even sponsor organized treasure hunts or other outings for their members. Check out LostTreasure.com for more information on clubs.

Add another use.

I used one to detect a staple that the docs accidentally left in after surgery. Eventually, I'll have it removed, unless it works it's way out. ;)

/johnny

We've got a magnet on wheels for that. But it's good for getting the kids out of the house for a couple of hours to put them out there with a metal detector. Wish I had thought about that, back when the girls were younger.

/johnny

I bought one years ago, in my teens. Lots of AA batteries, as I
recall. Had a very loud find in the yard, dug down,
downwards of a foot, and found a piece of aluminum
flashing...

My brother lost an aluminum shaft arrow in a field, and I was able
to find it. Headphones are very helpful, btw.

Did you know the first attempt at a "metal detector", although he
called it something like "resonance oscillator", was by
Alexander Graham Bell in trying to find the bullet in President
Garfield?

It was a failure.

For me, I found contemporary coins, fastener hardware, and the
like. I guess it was a form of recreational exercise, when all
was said and done.

“My brother lost an aluminum shaft arrow in a field, and I was able
to find it.”

High-end carbon fiber target arrows have a metal wire so you can find them with a VLF detector. At $20 each, if they fall to Earth you know not where, a $200 metal detector is a reasonable investment.

I’ve used detectors since the late 1960’s when I bought a cheap Mustang Detector that had no discrimination. Bought my first Whites Detector in 1987, and now have 4.

I used them for ‘Ghost Towning’ here in Michigan. I looked for places where the first settlers built their homes and farms. Usually an old stone cellar or stone foundation is all that remains.

I’ve found a lot of rusty nails and junk, but I’ve found a lot of rare and historical relics and coins too. From silver and copper coins from the early to late 1800’s, a cap and ball boot pistol, an octagon barrel from an early plains rifle in a lake, a mid 1800’s powder measure that still works, to a Civil War soldier’s US belt plate, and a Civil War Artillery coat button, to many early hand forged ax heads and assorted hand forged tools. Research, perseverance, and sweat pays off. A whole lot of walking and hiking to get to those often out of the way places, which is good exercise.

I own a Garrett Ace 150 and really enjoy it. Never found anything of major value, but have found pre-1900 coins, Revolutionary war era musket balls and other relics. While at Myrtle Beach, SC a couple of years ago, we noticed a young couple on the beach looking for something in the sand. I went down to see if I could help. The young mother-to-be had lost her wedding band and was crying like a baby. I grabbed my detector and found the ring within three minutes. Made a couple very happy. My best find to date.

OTOH, I have a nice collection of aluminum pull tabs. LOL

Hey what area of Michigan are you in Main Street? I’m in West Michigan...I’ve been detecting on and off for the last 10 years or so..have a Tesoro Bandito and a Tiger Shark.

I have found several so far. Nails that is.

I also found a strip of aluminum siding about 1 foot deep.

Several nails and screws.

2 zinc pennies.

And some unidentified construction stuff. Aluminum.

2 percent done on the backyard so far.

No treasures yet.

I recently found what looked like a .22 caliber shell and bullet. upon cleaning, I found it was not ammuntion, but a trial size freebie lipstick. It has the engraving “PINK BELLE” on the rim where a bullet would have a caliber. I had to google old 50’s advertisment to figure out what I was looking at.

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