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
Photo courtesy Bounty Hunter Metal detectors can provide hours of
enjoyment when you use them to search for buried
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 edition of HowStuffWorks,
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.
Anatomy of a Metal Detector A typical metal
detector is light-weight and consists of just a few parts:
Stabilizer (optional) - used to keep the unit
steady as you sweep it back and forth
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"
Photo courtesy Garrett
Electronics Garrett GTI 1500
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
Metal detectors use one of three technologies:
Very low frequency (VLF)
Pulse induction (PI)
Beat-frequency oscillation (BFO)
In the following sections, we will look at each of these
technologies in detail to see how they work.
VLF Technology 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:
Transmitter coil - This is the outer coil loop.
Within it is a coil of wire. 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
Photo courtesy Bounty Hunter This LandRanger metal detector from Bounty
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 animation above demonstrates VLF
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.
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
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
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.
PI Technology 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.
Photo courtesy Garrett
Electronics This Garrett
metal detector uses pulse
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 animation above demonstrates PI
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
that generates thousands of pulses of current per second. The
frequency of these pulses is slightly offset between the two
As the pulses travel through each coil, the coil generates
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 animation above demonstrates BFO
The simplicity of BFO-based systems allows them to be
manufactured and sold for a very low cost. You can even make
one at home following the instructions on this
page. But these detectors do not provide the level of
control and accuracy provided by VLF or PI systems.
Buried Treasure 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
The type of metal in the object - Some metals, such as
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
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.
Photo courtesy Bounty Hunter The control unit for a Bounty Hunter Tracker
Hobbyist metal detecting is a fascinating world with
several sub-groups. Here are some of the more popular
Coin shooting - looking for coins after a major
event, such as a ball game or concert, or just searching for
old coins in general
Prospecting - searching for valuable metals, such
as gold nuggets
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
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.
Detective Work In addition to recreational
use, metal detectors serve a wide range of utilitarian
functions. Mounted detectors usually use some variation of PI
technology, while many of the basic handheld scanners are
Photo courtesy Garrett
Electronics A Garrett
Some nonrecreational applications for metal detectors are:
Building security - screen people entering a
particular building, such as a school, office or prison
Event security - screen people entering a
sporting event, concert or other large gathering of people
Item recovery - help someone search for a lost
item, such as a piece of jewelry
Archaeological exploration - find metallic items
of historical significance
Geological research - detect the metallic
composition of soil or rock formations
Photo courtesy Garrett
Electronics A Garrett Super
Scanner handheld metal
Manufacturers of metal detectors are constantly tuning the
process to make their products more accurate, more sensitive
and more versatile. On the next page, you will find links to
the manufacturers, as well as clubs and more information on
metal detecting as a hobby.
For more information on metal detectors and related topics,
check out the links on the next page.