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LIST OF FIGURES
FIG.NO. PAGE NO.
Fig:(1) Block diagram of a transmitter-receiver metal……………………………………………2
Fig:- (2) Circuit diagram of metal detector……………………………………………………….4
Fig:-(3) Concept of magnetic flux……………………………………………………………...…5
Fig:-(4) As transmitter current from the antenna generates the electromagnetic field …………...7
Fig:- (5) When any metal com…………………………………………………………………….8
Fig:- (6) This diagram of “perfect coupling” illustrates the general shape es within the detection
pattern of a search coil,…………………………………………………………………………….8
Fig:- (7) This illustration shows the location and approximate proportional size of the fringe
detection area………………………………………………………………………………………9
Fig:- (8) block diagram of regulated power supply……………………………………………….10
Fig:- (9) transformer input and output ……………………………………………………………11
Fi g:- (10.1) Bridge rectifier…………………………………………………………………,,,….11
Fig:-(10.2) Output: full-wave varying DC………………………………………………………..11
Fig: - (11) Transformer + Rectifier input and output………………………………………..........11
Fig:- (12) Transformer + Rectifier + Smoothing input and output ……………………………....11
Fig:- (13) Transformer + Rectifier + Smoothing + Regulator input and output………………….12
Fig :-(14) resistance and it’s circuit symbol………………………………………………………12
Fig:- (15) Capacitor and its circuit symbol…………………………………………………….....12
Fig:- (16) Transistor and it’s circuit symbol……………………………………………………...13
Fig:-(17)Diode and it’s circuit symbol………………………………………………..…………..13
Fig:- (18) Red (3mm)LED and it’s circuit symbol………………………………………….……13
Fig:- (19) 8 pin 555 timer…………………………………………………………………...…….14
Fig:-(20)Buzzer…………………………………………………………………………….……..14
Fig:-( 21) Soldering on strip board…………………………………………………………….....15
Fig:-( 22) Strip board layout………………………………………………………………...……15

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INTRODUCTION
Metal detectors are devices that use electromagnetic fields to detect and
signal the presence of metallic or ferromagnetic objects. Metal detectors vary
in their effective operating ranges and the amounts and types of metals
necessary to generate a signal. They may be fixed, as in the familiar airport
walk-through detectors, or hand-held and portable. Uses include sport (finding
coins, jewellery and artefacts), prospecting, industrial and security. Metal
detectors have been used to identify metal objects placed into or upon patients
either therapeutically, through injury or ingestion, or purely diagnostically. This
article reviews the history of metal detection in the practice of medicine and
provides an overview of the utility of metal detectors in current diagnostic
practice. Non-diagnostic, medically related uses include scanning of hospital
patients and visitors for weapons and of entrances to magnetic resonance
imaging (MRI) centres to prevent flying metal objects and subsequent
injury.[1]
. The simplest form of a metal detector consists of an oscillator
producing an alternating current that passes through a coil producing an
alternating magnetic field. If a piece of electrically conductive metal is close to
the coil, eddy currents will be induced in the metal, and this produces an
alternating magnetic field of its own. If another coil is used to measure the
magnetic field (acting as a magnetometer), the change in the magnetic field
due to the metallic object can be detected. A metal detector is not an
instrument that detects energy emissions from radioactive materials. A metal
detector simply detects its presence and reports this. Metal detectors are
fascination machines.
History and development, toward the end of the 19th century, many
scientists and engineers used their growing knowledge of electrical theory in an
attempt to devise a machine which would pinpoint metal. The use of such a
device to find ore-bearing rocks would give a huge advantage to any miner
who employed it. The German physicist Heinrich Wilhelm Dove invented. The
modern development of the metal detector began in the 1920s. Gerhard Fisher
had developed a system of radio direction-finding, which was to be used for
accurate navigation. The system worked extremely well, but Fisher noticed
that there were anomalies in areas where the terrain contained ore-bearing
rocks.
Fig:(1) Block diagram of a transmitter-receiver metal

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COMPONENTS
• Resistors: 1kΩ,47kΩ,12kΩ,1.2kΩ,10kΩ,4.7kΩ.
• capacitors: 1000µF ,1 µF,.01 µF,50 µF
• 1N4001 diode (4)
• 1N4148 diode (1)
• red LED (3mm best)
• 555 timer IC
• Transformer 230/12 v/A.C.
• Buzzer
• Variable resistance,1k,10k
• strip board 51 rows × 21 hole

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CIRCUIT DIAGRAM
Fig:- (2) Circuit diagram of metal detector

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WORKING OF A METAL DETECTOR
The basic principle on which a metal detector works is that when electric
current passes through a coil, it produces a magnetic field around it. A metal
detector consists of an oscillator which produces alternating current. When this
alternating current passes through the transmit coil present in the metal
detector, a magnetic field is produced around it. Whenever an electrically
conductive metallic object comes in contact with the coil, it produces another
magnetic field around it. The metal detector contains another coil in its loop
called receiver coil, which detects the changes in the magnetic field caused due
to presence of the metallic object. The modern metal detectors are based on
any one of the three technologies, which are VLF (very low frequency), PI
(pulse induction) and BFO (beat-frequency oscillation).
The operation of metal detectors is based upon the principles of
electromagnetic induction. Metal detectors contain one or more inductor coils
that are used to interact with metallic elements on the ground. The single-coil
detector illustrated below is a simplified version of one used in a real metal
detector.
Fig: - (3) Concept of magnetic flux
A pulsing current is applied to the coil, which then induces a magnetic field
shown in blue. When the magnetic field of the coil moves across metal, such as
the coin in this illustration, the field induces electric currents (called eddy
currents) in the coin. The eddy currents induce their own magnetic field, shown
in red, which generates an opposite current in the coil, which induces a signal
indicating the presence metal.
(I):- Very Low Frequency (VLF) Technology
The most commonly used technology in metal detector is the VLF. Metal
detectors contain two sets of coils, namely, transmitter coil and receiver coil.
Electricity is passed through the transmitter coil to create a magnetic field. This
constantly pushes the electricity into the ground and pulls it back up. The
magnetic field so generated interacts with any metallic or conductive object
that comes in its way. The receiver coil passes the electric current whenever

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the metal detector passes over a conductive object. This amplifies and sends
the frequency of the current to the control box. Metal detectors, using VLF
technology, detect metals and determine the difference between different
types of metals and the depth at which they are located.
(II):- Pulse Induction (PI) Technology
PI technology uses a single coil, which plays the role of both the transmitter
and receiver. In some cases, it can make use of two or three coils also. Short
bursts or pulses of current are passed through the coil to generate a short
magnetic field. The end of each pulse results in the magnetic field reversing its
polarity suddenly and then collapsing. This, thus, creates electrical spikes
lasting for a short period. As soon as the spikes and magnetic field collapse,
another current, known as reflected pulse, runs through the coil which again
lasts for an extremely short period.
When a metal detector detects a metallic or conductive object, the reflective
pulse lasts for a longer duration. The reason behind this is that the pulse sent
by the metal detector produces an opposite magnetic field, causing the
reflective pulse to last longer. The metal detector monitors the spikes and
reflected pulses and sends the signals to the device called integrator. This
integrator reads, amplifies and converts the signals to direct current. The audio
circuit connected produces a tone indicating the presence of a metal or metallic
object.
(III):-Beat Frequency Oscillator (BFO) Technology
BFO uses two coils of wire just like VLF technology. One coil is connected in
the control box of the device, while the other is situated in the search end. The
coil in the control box is smaller than the one in the search end. Both are
connected to the oscillators that send thousands of electric pulses in a single
record. When pulses pass through each coil of wire, radio waves are created
that are collected by a receiver located in the control box.
On the frequency of the radio waves, the receiver creates audible tones.
When the metal detector passes over a metal or metallic object, the electric
current passing through the coil of the search end creates a magnetic field,
which in turn creates another magnetic field around the metallic objects. The
magnetic field interferes with the radio waves and causes a change in the
tones produced by the receiver. Hence, the metal detector beeps up.
How Metal Detectors Work:
A Simple Explanation We don't need to
understand all the science of how a metal
detector finds various metals. You can find
coins, rings, jewelry, gold, relics, artifacts, small
buried caches and even deep treasures without
knowing scientifically how a metal detector
works. Look at this simple illustration:
Illustration 'A' shows a typical metal detector
user. He has followed the instructions supplied
by the manufacturer and has his metal detector turned on. After testing his

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detector on some surface targets (coins) to make sure it is working, he now
starts searching for buried coins and treasures.
Notice the "red" signal pattern being
transmitted from the search coil into the ground.
(Note: we have enlarged the illustration of the
signal pattern for easier understanding). As long as
the signal entering the ground does NOT come in
contact with metal, there will be no audio signal, no
flashing light, no vibration, nothing will happen.
Illustration 'B' shows what happens when the
detector user's metal detector search pattern comes
in contact with metal objects, in this case both
shallow and deep coins. When the search pattern
touches metal it interrupts the transmitted signal
and this interruption or disturbance of the search
pattern will cause the metal detector to alert the detector user (you) with an
audio signal, usually a distinct loud sound. In some cases flashing or blinking
lights will accompany the audio signal.
Types of detecting:-
(1)Eddy Currents Secondary Electromagnetic Field Generation
Whenever metal comes within the detection pattern, electromagnetic field
lines penetrate the metal’s surface. Tiny circulating currents called “eddy
currents” are caused to flow on the metal surface as illustrated in the figure on
the facing page. The power or motivating force that causes eddy currents to
flow comes from the electromagnetic field itself. Resulting power loss by this
field (the power used up in generating the eddy currents) is sensed by the
detector’s circuits. Also, eddy currents generate a secondary electromagnetic
field that, in some cases, flows out into the surrounding medium. The portion
of the secondary field that intersects the receiver winding, causes a detection
signal to occur in that winding. Thus, the detector alerts the operator that
metal has been detected.
Fig:-(4) As transmitter current from the antenna generates the electromagnetic field, detection
patter (dotted lines) is the area within which Metal detection occurs. Mirror-image pattern atop
coil is not used.

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(2)Electromagnetic Field Distortion
The detection of non-conductive iron (ferrous) minerals takes place in a
different manner. When iron mineral comes near and within the detection
pattern, the electromagnetic field lines are redistributed, as shown in the figure
on the following page. This redistribution upsets the “balance” of the
transmitter and receiver windings in the search coil, resulting in power being
induced into the receiver winding. When this induced power is sensed by the
detector circuits, the detector alerts its operator to the presence of the iron
mineral. Iron mineral detection is a major problem for both manufacturers and
users of metal detectors. Of course, the detector of iron mineral is welcomed
by a gold hunter who is looking for black magnetic sand which can often signal
the presence of placer metal. On the other hand, the treasure hunter, who is
looking for coins, jewelry, relics, gold nuggets, etc., usually finds iron mineral
detection a nuisance.
Fig:- (5) When any metal comes within the detection pattern of a search coil, eddy currents
flow over its surface, resulting in a loss of power in the electromagnetic field, which the
detector’s circuits can sense.
(3)Salt Water Detection
Salt water (wetted salt) has a disturbing effect upon the electromagnetic
field because salt water is electrically conductive. In effect, salt ocean water
“looks like” metal to some detectors! Fortunately manufacturers are able to
design detectors capable of “ignoring” salt water.
Fig:- (6) This diagram of “perfect coupling” illustrates the general shape of a detection
pattern that occurs when the electromagnetic field from a search coil penetrates earth or any
other nearby object.

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(4)Fringe Area Detection
Fringe area detection is a phenomenon of detection, the understanding of
which will result in your being able to discover metal targets to the maximum
depth capability of any instrument. The detection pattern for a coin may
extend, say, one foot below the search coil. The detection pattern for a small
jar of coins may extend, perhaps, two feet below the search coil as illustrated
in the drawing on the facing page. Within the area of the detection pattern, an
unmistakable detector signal is produced.
Fig:- (7) This illustration shows the location and approximate proportional size of the fringe
detection area in which faint target signals from around the outer edges of a normal detection
pattern can be heard.
Types of Metal
The sensitivity of a metal detector is not the same for all types of metal. For
simplicity, we tend to categorize all metals into three types:
• Ferrous:
Any metal that can easily be attracted to a magnet (Steel, iron, etc.).
Typically the easiest metal to detect and usually the most common
contaminant.
• Non-Ferrous:
Highly conductive non-magnetic metals (copper, aluminum, brass, etc.)
When inspecting dry products these metals produce almost the same signal
size as ferrous due to the fact that they are good conductors. When inspecting
wet products, de-rate the sphere size by at least 50%.
• Non-Magnetic Stainless Steel:
High quality 300 series stainless steels (Type 304, 316). These are always
the most difficult metals to detect due to their poor electrical conductive
qualities and by definition are have low magnetic permeability. When
inspecting dry products a stainless sphere will have to be 50% larger than a
ferrous sphere to produce the same signal size. When inspecting wet products
a stainless sphere would have to be 200 to 300 % larger than a ferrous sphere
to produce the same signal size.

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POWER SUPPLY
Types of Power Supply
There are many types of power supply. Most are designed to convert high
voltage AC mains electricity to a suitable low voltage supply for electronic
circuits and other devices. A power supply can by broken down into a series of
blocks, each of which performs a particular function. For example a 12V
regulated supply:
Fig :-(8) block diagram of regulated power supply
Each of the blocks is described in more detail below:
• Transformer - steps down high voltage AC mains to low voltage AC.
• Rectifier - converts AC to DC, but the DC output is varying.
• Smoothing - smoothes the DC from varying greatly to a small ripple.
• Regulator - eliminates ripple by setting DC output to a fixed voltage.
Transformer only
Fig:- (9) transformer input and output
The low voltage AC output is suitable for lamps, heaters and special AC
motors. It is not suitable for electronic circuits unless they include a rectifier
and a smoothing
Bridge rectifier
A bridge rectifier can be made using four individual diodes, but it is also
available in special packages containing the four diodes required. It is called a
full-wave rectifier because it uses the entire AC wave (both positive and
negative sections). 1.4V is used up in the bridge rectifier because each diode
uses 0.7V when conducting and there are always two diodes conducting, as
shown in the diagram below. Bridge rectifiers are rated by the maximum
current they can pass and the maximum reverse voltage they can withstand
(this must be at least three times the supply RMS voltage so the rectifier can

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withstand the peak voltages). Please see the Diodes page for more details,
including pictures of bridge rectifiers.
Fig:- (10.1) Bridge rectifier
Alternate pairs of diodes conduct, changing over
the connections so the alternating directions of
AC are converted to the one direction of DC.
Fig:-(10.2) Output: full-wave varying DC
Transformer + Rectifier
Fig: - (11) Transformer + Rectifier input and output
The varying DC output is suitable for lamps, heaters and standard motors. It
is not suitable for electronic circuits unless they include a smoothing capacitor.
Transformer + Rectifier + Smoothing
Fig:- (12) Transformer + Rectifier + Smoothing input and output

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The smooth DC output has a small ripple. It is suitable for most electronic
circuits.
Transformer + Rectifier + Smoothing + Regulator
Fig:- (13) Transformer + Rectifier + Smoothing + Regulator input and output
The regulated DC output is very smooth with no ripple. It is suitable for all
electronic circuits.
Resistors Function:-
Resistors restrict the flow of electric current, for example a resistor is
placed in series with a light-emitting diode (LED) to limit the current passing
through the LED.
Example: Circuit symbol:
Fig :-(14) resistance and it’s circuit symbol
Capacitors Function:-
Capacitors store electric charge. They are used with resistors in
timing circuits because it takes time for a capacitor to fill with charge. They are
used to smooth varying DC supplies by acting as a reservoir of charge. They
are also used in filter circuits because capacitors easily pass AC (changing)
signals but they block DC (constant) signals.
Example: Circuit symbol:
Fig:- (15) Capacitor and its circuit symbol

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Transistor:-
Transistor transfers a signal from a low resistance to high resistance.
‘Trans’ means the signal transfer of the device. ‘Istor’ classified it as a solid
element in the same general family of resistor. A transistor is a semiconductor
device used to amplify and switch electronic signals and power. Transistor is a
“current” operated device which has a very large amount of current (Ic) which
flows without restraint through the device between the collector and emitter
terminals. But this is only possible if a small amount of biasing current (Ib) is
present in the base terminal of the transistor making the base to act as a
current control input. The symbol hfe or sometimes referred to as Beta (β) is
actually the ratio of these two currents (Ic/Ib) and is described as the DC
Current Gain of the device.
Fig:- (16) Transistor and it’s circuit symbol
Diodes Function:-
Diodes allow electricity to flow in only one direction. The arrow of the
circuit symbol shows the direction in which the current can flow. Diodes are the
electrical version of a valve and early diodes were actually called valves.
IN4001 diode used in the project.
Example: Circuit symbol:
Fig:-(17)Diode and it’s circuit symbol
Light Emitting Diodes (LED’s):-
LED is a solid state light source .it is a low power device and fast on off
switching.LEDs emits light when an electric current passes through them.
Example: Circuit symbol:
Fig:- (18) Red (3mm)LED and it’s circuit symbol

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555 Timer Circuit:-
The 555 timer IC is an integrated circuit (chip) used in a variety of timer,
pulse generation, and oscillator applications. The 555 can be used to provide
time delays, as an oscillator, and as a flip-flop element. Derivatives provide up
to four timing circuits in one package.
Fig:- (19) 8 pin 555 timer
The 8-pin 555 timer must be one of the most useful ICs ever made and it
is used in many projects. With just a few external components it can be used
to build many circuits, not all of them involve timing.
A popular version is the NE555 and this is suitable in most cases where a
'555 timer' is specified. The Low power versions of the 555 are made, such as
the ICM7555, but these should only be used when specified (to increase
battery life) because their maximum output current of about 20mA (with a 9V
supply) is too low for many standard 555 circuits. The ICM7555 has the same
pin arrangement as a standard 555.
The circuit symbol for a 555 (and 556) is a box with the pins arranged to
suit the circuit diagram: for example 555 pin 8 at the top for the +Vs supply,
555 pin 3 outputs on the right. Usually just the pin numbers are used and they
are not labeled with their function.
The 555 and 556 can be used with a supply voltage (Vs) in the range 4.5 to
15V (18V absolute maximum).
Alarm:-
The most common alarm is a
flashing beacon, activated by the
metal detector output relay. A siren,
horn, or bell may also be used, with
or without the beacon. Other
commonly used alarm devices are
buzzers; flag drop markers, paint
spray markers, and flashing.
Fig:-(20)Buzzer

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Strip board:-
Strip board has parallel strips of copper track on one side. The tracks are
0.1" (2.54mm) apart and there are holes every 0.1" (2.54mm).
Strip board is used to make up permanent, soldered circuits. It is ideal for
small circuits with one or two ICs (chips) but with the large number of holes it
is very easy to connect a component in the wrong place. For large, complex
circuits it is usually best to use a printed circuit board (PCB) if you can buy or
make one. Strip board requires no special preparation other than cutting to
size. It can be cut with a junior hacksaw, or simply snap it along the lines of
holes by putting it over the edge of a bench or table and pushing hard, but
take care because this needs a fairly large force and the edges will be rough.
You may need to use a large pair of pliers to nibble away any jagged parts.
Avoid handling strip board that you are not planning to use immediately
because sweat from your hands will corrode the copper tracks and this will
make soldering difficult. If the copper looks dull, or you can clearly see finger
marks, clean the tracks with fine emery paper, a PCB rubber or a dry kitchen
scrub before you start soldering.
Fig:-( 21) Soldering on strip board
Strip board layout:-
Fig:-( 22) Strip board layout

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Salient Features:-
• Auto setting.
• Lowest False Alarm Rates.
• Low Power Consumption.
• Easy to Operate.
• High Reliability, long life performance.
• High Sensitivity and High Accuracy.
• Detects all metals including non magnetic Stainless Steel.
• Totally indigenous. Spare parts and maintenance readily available.
• Audio / Visual alarm on Detection.
• High Sensitivity for maximum penetration.
• Detects all metals.
•
Applications:-
• Detects all metals.
• Identifies metallic objects by speaker sound and needle movement.
• Detection of weapons such as knives and guns, especially at airports,
malls, geophysical prospecting, archaeology and treasure hunting.
• Airport and Building Security.
• Archaeological exploration.
• Geological research.

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Rate list of components
SNo. Part No. Part Description Qty. Price
1. 230/12V Transformer (12V-1A) 2 80.0
2. NE555 Timer 1 25.0
3. 14X8 CM Strip board 1 15.0
4. Q1,Q2 (BC548B) Transistor 2 20.0
5. D1- (1N4148)
D2-D5 (1N4007)
Diode 1
4
15.0
6. D6 Red LED 1 3.0
7. R1-12KΩ
R2-10Ω
R3-1.2KΩ
R4-47KΩ
R5-4.7KΩ
R6-82KΩ
Resistances 1
1
1
1
1
1
12.0
8. C1-50µF
C2-0.01µF
C3-0.01µF
C4-1µF
C5-1000µF
Capacitor 1
1
1
1
1
15.0
9. VR1-1KΩ
VR2-10KΩ
Variable Resistance 1
1
6.0
10. 12V Buzzer 1 20.0
11. Connecting wires 5.0
12. General Purpose Vero Board 1 20.0
13 Mains connecting lead 1 10.0
14 Connecting nuts 4 4.0
Total 250

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Bibliography
Electronic Club Guide
www.kpsec.freeuk.com
www.wikipedia.org