Comprehensive Literature Review on Metal Detector Technology

This report presents a comprehensive literature review on metal detectors, beginning with a historical overview of their development, from early induction balance devices to modern computerized systems. It explores various metal detecting technologies, including VFLEX, Very Low Frequency (VLF), Beat Frequency Oscillator (BFO), Pulse Induction (PI), and others, detailing their principles and applications. The report also covers different types of metal detectors, such as body scanning detectors, walk-through units, and hobby detectors, highlighting their specific uses in security, treasure hunting, and archaeological applications. Furthermore, it discusses the uses of metal detectors, the potential effects of exposure to them, and the electronic components involved, including oscillators (specifically Colpitts oscillator), diodes, and peak detectors. This detailed review offers insights into the evolution, functionality, and diverse applications of metal detector technology.

CHAPTER TWO
LITERATURE REVIEW
2.1 BRIEF HISTORY OF METAL
Metal detector have been around for some times. Towards the twilight of the 10th century,
German physicist Heinrich William Dove designed a metal detector based on the “induction
balance principle” that could find ore bearing rocks strictly for the economy value. But, the early
devices were crooked, used lot of battery power and had limited operational capacity.
If anyone is to be considered the originator of metal detectors, it will be English geologist and
mining engineer R. W. Fox. It was Fox who first discovered that electricity will flow through
metallic ores as well as solid metal objects. He created a simple metal locator which consisted of
a battery, several metal rods and a suitable length of wire.
His first method of detection was as follows: one metal rod would be driven into the earth
where the suspected vein of ore was located; it was connected to one terminal of the battery. The
other battery terminal was connected to a floating wire. Other metal rods were driven into the
ground at several different points and consecutively touched with the floating wire. Where a
spark occurred, it was an indication that metal was present. Circa 1870, this device was improved
to two rods insulated from each other in a common probe and connected through battery to a bell
and plunged into the earth. When contact was made by metallic ore, nugget or metal pipe, the
bell rang, thus indicating the presence of a conductive object.
In 1874 Parisian inventor Gustav Trouve invented a handheld device for locating and
extracting metal objects such as bullets from human patient. Motivated by Trouve, Alexander
Graham Bell invented a similar device to attempt to locate a bullet stocked in the chest of
American president James Garfield. In 1881, Alexander Graham Bell invented the first metal

detector. As president James Garfield lay dying of an assassin’s bullet, Bell hurriedly devised a
crude metal detector in an unsuccessful attempt to locate the fatal slug. Bells metal detector was
an electromagnetic device he called the induction balance.
In 1925, Gerhard Fischer invented a portable metal detector. Fischer’s version was first
sold in 1931 and Fischer was behind the first large scale production of metal detectors.
According to the expert at A&S company: “In the late 1920s, Dr. Gerhard Fisher the founder of
Fisher Research Laboratory was appointed as a research engineer with the federal telegraph Co.
and the western Air Express to develop airborne direction-finding equipment. He won some of
the first patent issued in the field of airborne direction finding by means radio. In the course of
his work, he came across some strange mistakes and once he solved these problems he had the
foresight to apply the solution to a completely unrelated field, that of metal and mineral
detection.
Modern improvement of metal detector systems began in the 1930’s. Dr. Gerhard Fisher
established that if a radio beam could be distorted by a metal then a metal detecting device could
be fabricated using a search coil resonating at radio frequency. In 1937, he applied for and was
granted patent for a metal detector. During the second world war these detectors were used as
“mine detectors” to detect mines hidden underground, the survival of each soldier depended on
this. They were heavy, operated on vacuum tubes and required separate battery packs. After the
war, there were lots of mine detectors on the market, bought up by relic and treasure hunters who
used them for pleasure and profit. The hobby of metal detecting had taken off.
Another leader in detector technology was Charles Garrett, who initiated the beat
frequency oscillator machine (BFO). With the invention and improvement of the transistor in the
1950s and the 1960s, metal detector manufacturers and engineers require smaller lighter

machines with improved circuitry, running on small battery packs. Companies sprang up all over
USA and Britain with the supply and growing demand. Modern top versions are fully
computerized using integrated circuit technology to enable the user to set sensitivity,
discrimination, track speed, threshold volume, notch filters etc. and hold these factors in memory
for future use. Compared to just a decade ago, detectors are lighter, deeper seeking, use less
battery power, and discriminates better.
A lot of portable metal detectors are used by archeologists and treasure hunters to locate
metallic items, such as jewelry, coins, bullets and various artifacts hidden shallowly
underground.
2.2 METAL DETECTING TECHNOLOGIES
The various technologies used in metal detecting are:
1. VFLEX
2. Broad Band Spectrum (BBS)
3. Full Band Spectrum (FBS)
4. Multi Period Sensing (MPS)
5. Dual Voltage Technology (DVT)
6. Smart Electronic Timing Alignment (SETA)
7. Very Low Frequency or Inductive Balance (VLF)
8. Zero Voltage Transmission Technology (ZVT)
9. Beat Frequency Oscillator (BFO)
10. Pulse Induction (PI) among others.

2.2.1 VFLEX TECHNOLOGY
VFLEX uses state of the art digital electronics and signal processing to improve
conventional single frequency (VLF) metal detector technology. VFLEX provides increased
detecting execution with perfect sine wave transmission, an in-coil signal booster and coil
selectable transmit frequencies. Conventional single frequency metal detection technology by
incorporating two microcontrollers, one inside the control box and the other inside the coil. Each
time the detector starts up, the microcontrollers allow communication via a digital data link. The
coil microcontroller communicates the coil configuration, size and exact frequency, so the
control box can produce a perfectly matched transmit signal. This significantly reduces alteration
and increases target ID accuracy. VFLEX technology produce and transmits a perfect high
quality sine wave using the same technology that is found in high quality digital audio players
and is produced without alterations. The elimination of distortion maximizes power transmitted
from the coil, thereby increasing detection depth and sensitivity. This also results in improved
target ID accuracy and greater immunity to both ground and environmental noise.
2.2.2 VERY LOW FREQUENCY TECHNOLOGY
Very low frequency technology is a conventional single frequency sine wave
transmission technology. This is an outdated type of technology used in most basic metal
detectors. Single frequency sine wave detectors generate an electromagnetic field, which is
transmitted into the ground in a continuous wave. While the basic way that the signal is sent out
and received back has not altered, the received signal processing has improved greatly. This has
proven to be a reliable and easy to use technology for finding metal.

2.2.3 BFO (BEAT FREQUENCY OSCILLATOR) THEORY
The basic BFO uses two radio frequency (RF) oscillators which are tuned to almost the
same frequency. The frequency of each oscillator is often determined by an L-C resonant circuit,
that is, an inductor and a capacitor. One is called the search oscillator and uses the search coil as
its inductor; the other is called the reference oscillator and uses an internal inductor. The outputs
of the two oscillators are fed into a mixer which produces a signal that contains the sum and
difference frequency components of the two input signals. The frequencies of the two oscillators
are normally chosen such that a change in the search oscillator frequency will produce a mixer
output signal that is in the audio frequency range.
2.3 TYPES OF METAL DETECTOR
1. Body Scanning Detector
2. Walk Through Units
3. Hobby detector.
2.3.1 BODY SCANNING DETECTOR
They are also known as scanners or handheld detectors. They must be small enough for a
single operator to run the unit over the person being searched while keeping a high degree of
sensitivity. They are basically battery powered, and due to their small size, have to be used very
close to the area being searched, because its area of capture is very small.

2.3.2 WALK THROUGH UNITS
As the name implies, as person walks through this type of metal detector, they are
scanned for weapons, bombs, and other related dangerous weapons. Due to the unit’s size, its
signal strength is greater and its efficiency total, no metal can be concealed from this unit and
they can even detect metal implant in a person’s body.
The walk through and body scanning detectors are basically security metal detectors, adopted
because of the threats of the worldwide terrorism in order to enhance public security in airports,
courthouses, legislative houses, post offices, prisons and so on.
2.3.3 HOBBY DETECTORS
These are also known as consumer “detectors”; they are purchased by consumers for
varying reasons ranging from location of hidden treasure, to searching for precious metals buried
underground.
2.4 USES OF METAL DETECTOR
1. Security of buildings
2. Search for lost
3. Detection of relics
4. Food processing
5. Geological detection
6. Security of lives and properties.

2.5 EFFECT OF REPETITIVE EXPOSURE TO METAL DETECTORS
Security metal detectors discharges a non-ionizing radiation that bounces its beams off of
object to get their outline. This is the radiation that people are bothered about but it is not any
reason to be concerned about because the amount of radiation emitted from them is relatively
harmless, even for people who regularly fly or regularly go through metal detectors. Security
metal detectors are also safe for pregnant woman and their luggage/ baby supplies. The only
time a pregnant woman can be affected by the radiation emitted from a security metal
detector is through long term exposure. So, if someone walks around metal detector a lot,
care should be taken to keep as much distance as possible, monitor radiation levels, and to
follow normal safety instructions.
2.6 OSCILLATOR
An electronic oscillator is an electronic circuit that generates a periodic, oscillating
electronic signal, often a sine wave or a square wave. Oscillators converts direct current
(DC)from a power supply to an alternating current (AC) signal. They are generally used in
many electronic devices. The oscillator’s output varies its amplitude with time, this output
may be sinusoidal, square, pubes, triangular or saw tooth.
An oscillator provides a source of repetitive AC signals across its output terminals
without needing any input (except a DC supply). The signal generated by the oscillator is
usually of constant amplitude. The wave shape and amplitude are controlled by the design of
the oscillator circuit and choice of components value. The frequency of the output wave may
be fixed or variable depending on the oscillator design. The oscillator used in this project is
the oscillator, since the search coil is meant to be an inductor.

2.61 COLPITTS OSCILLATOR
The Colpitt’s Oscillator uses two capacitors placed across a common inductor L and the
center of the capacitors is tapped. The tank circuit, in the Oscillator, is made up of C1 , C2 ∧L
.
2.7 DIODES
A diode is a two-terminal electronics component that conducts primarily in one direction; it
has low resistance to the current in one direction, and high resistance in the other. The most
common function of a diode is to allow an electric current to pass in one direction (called the
diode’s forward direction), while blocking current in the reverse direction. Thus, the diode can
be viewed as an electronic version of a checked valve. This unidirectional behavior is called
rectification, and is used to convert alternating current to direct current, including extraction of
modulation from radio signals in radio receivers-these diodes are forms of rectifiers.
Figure 2.1: Diode symbol
The anode represents the P- type region while the cathode represents the N-type region.
When voltage is applied across the diode’s terminals, it gives three possibilities: No
bias(VD=0V). Forward bias (VD>0V) and reversed biased (VD<0V). VD is the voltage
across the diode. The forward bias condition is realized by applying a positive terminal to the

P-region and negative terminal to the N-region. The reversed bias condition is obtained by
applying a negative terminal to the P region and the positive terminal to the N-region, this
prevents any current flowing because the P-N junction offers high resistance; though a very
small current known as reversed saturation current exists under reverse bias conditions. The
bias conditions are shown below:
(A)
(B)
Fig 2.2 PN junction diode A) forward biased B) reversed biased
2.8 PEAK TO PEAK DETECTOR
A peak detector is a series connection of a diode and a capacitor outputting a DC voltage
equal to the peak value of the applied AC signal. In the peak detector circuitry, the highest point

of the input waveform charges the capacitor, which holds that value while the diode is back-
biased.
2.9 LIGHT EMITTING DIODES
A light emitting diode (LED) is a semiconductor device that emits a visible light when an
electric current passes through it. The light is not very bright, but in most LEDs it is
monochromatic, occurring at a single wavelength. The output from a LED can vary from red
(at a wavelength of approximately 700 nanometers) to blue-violet (about 400 nanometers).
Some LEDS emits infrared (IR) energy (830 nanometers or longer); such a device is known
as an infrared-emitting diode.
A LED or IRED comprises of two elements of processed material called P-type
semiconductors and N-Type semiconductors. These two elements are placed in direct
contact, forming a region called the PN junction. In this respect, the LED or IRED looks like
most other diode types, but there are important differences. The LED or IRED has a
transparent package, allowing visible or IR energy to pass through. Also the LED or IRED
has a large PN junction area whose shape is fitted to the application.
2.10 OP-AMP AND COMPARATORS
2.10.1 OP-AMP
An operational amplifier (often OP-Amp or op-amp) is a DC coupled high gain electronic
voltage amplifier with a differential input and, mostly, a single-ended output. OP-AMP are
among the most widely used electronic devices today being used in a great array of
consumer, industrial, and scientific devices. Many standard IC OP-AMPs cost only a few
cents in a normal production volume; however, some integrated or hybrid operational

amplifiers with special performance specifications may cost over US$100 in small quantities.
OP-AMPs may be packaged as components or used as element of more complex integrated
circuit. A basic op-amp unit is shown below:
FIG 2.5 OP-AMP unit
2.10.2 COMPARATORS
A comparator circuit consist two voltage signals and determines which one is greater.
The result of this comparison is shown by the output voltage. If the op-amp’s output is
saturated in the positive direction, the non-inverting input (+) is a greater, or more positive,
voltage than inverting input (-), all voltages measured with respect to ground. If the op-amp’s
voltage is close to the negative supply voltage, it implies that the inverting input (-) has a
greater voltage applied to it than the non-inverting input (+).

FIG 2.6 Basic comparator unit.
2.11 VOLTAGE REGULATOR
A voltage regulator is an electricity regulation device designed to automatically convert
voltage into a lower, usually direct current (DC), constant voltage. The term signifies a
voltage regulator integrated circuit which is usually found in computers and other electronic
devices that are plugged directly into an alternating current wall outlet but require only a
small DC voltage. The term can also refer to voltage regulation or power module devices,
such as cellphone or laptop chargers. Some regulators do not increase or decrease a devices
voltage, but simply ensure constant output value.
2.12 OTHER PASSIVE COMPONENTS
Passive components are components that cannot develop power or require an external
applied power source to operate. They include resistors, capacitors, inductors.
In this project resistors were used as potential dividers to control the flow of current and
capacitors were employed for blocking unwanted DC voltages.