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METAL DETECTOR

ABSTRACT
In security aspects, metal detector isan essential equipment. But, the metal detector, which is available in the market today, is very costly. Hence this stands as a problem for hobbyists and for small applications. Hence we have taken an endeavor to bring out a metal detector of minimum cost.

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Submitted By
Vineesh A. V.
Vimiya Varkey Guided BY
Viji M. C. M^. K. Gnanasheela
Ms^Sumol N. C.
Ms. Seena George

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CONTENTS
INTRODUCTION
BLOCK DIAGRAM
CIRCUIT DIAGRAM
CIRCUI DESCRIPTION AND WORKING
PCB LAYOUT
COMPONENT LAYOUT
COST ESTIMATION
CONCLUSION
REFERENCES
DATASHEETS
1. INTRODUCTION
In security aspects, metal detector isan essential equipment. But, the metal detector, which is available in the market today, is very costly. Hence this stands as a problem for hobbyists and for small applications. Hence we have taken an endeavor to: bring out a metal detector of minimum cost.
The salient features of our project is that the equipment is
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compact, simple in design and can be used practically anywhere needs. The
metal detector produces an audible alarm signal when a metallic particle comes
near to the sensor. A visible blinking LED is also there to indicate the
presence of metal.
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POWER SUPPLY
4. CIRCUIT DESCRIPTION AND WORKING
The circuit of metal detector is shown in Fig. 1. An astable multi-vibrator is wired around 1C 555. The free running frequency is selected as 1.2 KHz.
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In astable operation, the trigger terminal and the threshold terminal are connected so that a self-trigger is formed, operating as a multi¬vibrator. When the timer output is high, its internal discharging Tr. turns off and the VC1 increases by exponential function with the time constant (RA+RB)*C.
When the VC1. or the threshold voltage, reaches 2Vce/3. the comparator output on the trigger terminal becomes high, resetting the F/F and causing the timer output to become low. This in turn turns on the discharging Tr. and the C1 discharges through the discharging channel formed by RB and the discharging 'TV. When the VCI falls below Vcc/3. the comparator
output on the trigger terminal becomes high and the timer output becomes high asain. The discharging Tr. turns off and the VC 1 rises aaain.
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ASTABLE OPERATION
This frequency is given to the primary of the detecting transformer through a transistor. So due to the pulsating current How. a varying magnetic field will formed in me transformer. A voltage will normally induce in the secondary coil due to the mutual induction. But the transformer has no core (air core) and so the magnitude of this induced emf is very low.
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When a ferrite substance conies near to the winding of the transformer, an effect of core is produced and the induced emf will be much grater than former. This voltage is amplified with the help of a non inverting amplifier build with opamp L.M324.
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NON INVERTING AMPLIFIER
The amplified signal is given to the input of a voltage comparator. The reference voltage is set to a value below the magnitude of induced emf without any core. So. when the ferrite body is near to the transformer, the input of comparator is more then thai of reference pin. Then the output of the comparator goes low.
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This low pulse is given to the triggering input of monostable multi-vibrator build around 555. The time is selected to 2.5 seconds and so the output will high for 2.5 seconds. The transistor BC547 starts conducting and the buzzer beeps to indicate the presence of metal. Also the LED turned off for this time.
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MONOSTAB1 .If. OPERATION
In this mode, the timer generates a fixed pulse whenever the trigger voltage falls below Vcc/3. When the trigger pulse voltage applied to the #2 pin falls below Vcc/3 while the timer output is low; the timer's internal flip-flop turns the discharging Tr. off and causes the timer output to become high by charging the external capacitor CI and setting the flip-flop output at the same time.
The voltage across the external capacitor CL VC 1 increases exponentially with the time constant t=:RA*C and reaches 2Vcc/3 at td=l. 1 RA*C. Hence, capacitor C1 is charged through resistor RA. The greater the time constant RAC. the longer it takes for the VC1 to reach 2Vcc/3. In other words, the time constant RAC controls the output pulse width. When the applied voltage to the capacitor CI reaches 2Vcc/3, the comparator on the trigger terminal resets the flip-flop, turning the dischi irtiiim 1 r. on. At tins time, CI begins to discharge and the timer output converts to low. In this way. the timer operating in monostable repeats the above process.
The output of monostable is given to base of transistor BC547 and a buzzer is connected to the colector. So when the monostable Iriggeres, the transistor will conduct and hence the buzzer will be on and it produce a tone to indicate the presence of metal.
After the time over, the device will be read)' for next detection.
5. WINDING DETAILS OF DETECTING
TRANSFORMER
The detecting trams former has a 1:1 winding (the primary and secondary has the same number of turns). The primary is wound over a 1cm diameter plastic former with 35 SWG enamaled copper wire. The primary winding has 210 turns and is wound very closely. An insulating coating is applied over the primary for isolation. Then the secondary is wounded over the primary with 40 SWG copper wire and has also 210 turns. The whole assemly is covered with a well insulator and apply warnish to allow the winding to set. The leads are taken from the primary and the secondary.
6. IMLB LAYOUT
7. COMPONENT LAYOUT
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8. COST ESTIMATION
COMPONENTS QTY RATE COST
LM324 1 14.00 14.00
LM555 2 10.00 20.00
BD139 1 3.00 3.00
BUZZER 1 40.00 40.00
DETECTING TRANSFORMER 1 300.00 300.00
3mm LED 2 . LOO 2.00
1/4W RESISTOR 10 0.25 2.50
47K PRESET 5.00 10.00
BC547 2 2.50 5.00
12-0-12/1A TRANSFORMER 1 100.00 100.00
1N4007 2 1.00 2.00
4700MFD/30V CAPACITOR 1 25.00 25.00
10MFD/25V CAPACITOR 4 2.50 10.00
0.1 MFD DISC CAPACITOR 5 1.00 5.00
14PIN IC BASE 1 2.00 2.00
8P1N IC BASE 2 1.00 2.00
LM7805 1 10.00 10.00
PCB 1 150.00 150.00 ¢
WIRE lOMir 5.00 50.00
HANDLE 75.00 75.00
SOLDERING IRON (25W) 1 200.00 200.00
SOLDER & FLUX 1 50.00 50.00
EXTRA 350.00
TOTAL 1427.50
9. CONCLUSION
This project has been developed considering the need for "'intelligent" low cost and longer lasting lamp. With this purpose in mind, a study has been conducted to understand about the electronic components available in the market. The equipment is compact, simple in design and can be used practically anywhere needs.
10. REFERENCE
OP-AMPS and Linear Integrated Circuits by Ramakant A . Gayakwad.
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SEMICONDUCTOR"
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LM555/NE555/SA555
Single Timer
Features
¢ High Current Drive Capability (200mA)
¢ Adjustable Duty Cycle
¢ Temperature Stability of 0.005WC
¢ Timing From p.Sec to Hours
¢ Turn off Time Less Than 2u.Sec
Applications
¢ Precision Timing
¢ Pulse Generation
¢ Time Delay Generation
¢ Sequential Timing
Description
The LM555/NE555/SA555 is a highly stable controller capable of producing accurate timing pulses. With monostable operation, the time delay is controlled by one external resistor and one capacitor. With astable operation, the frequency and duty cycle are accurately controlled with two external resistors and one capacitor.
Internal Block Diagram
©2002 Fairchild Semiconductor Corporation
Rev. 1.0.2
Application Information
Table 1 below is the basic operating table of 555 timer:
Table 1. Basic Operating Table
Threshold Voltage (VthMPIN 6) Trigger Voltage (VtrXPIN 2) Reset(PIN 4) Output(PIN 3) Discharging Tr. (PIN 7)
Don't care Don't care Low Low ON
Vth > 2Vcc / 3 Vth > 2Vcc / 3 High ' Low ON
Vcc / 3 < Vth < 2 Vcc / 3 Vcc / 3 < Vth < 2 Vcc / 3 High - -
Vth < Vcc / 3 Vth < Vcc / 3 High High OFF
When the low signal input is applied to the reset terminal, the timer output remains low regardless of the threshold voltage or-the trigger voltage. Only w hen the high signal is applied to the reset terminal, timer's output changes according to threshold voltage and trigger voltage.
When the threshold voltage exceeds 2/3 of the supply voltage while the timer output is high, the timer's internal discharge TV. turns on. lowering the threshold voltage to below |/3 of the supply voltage. During this time, the timer output is.maintained low. Later, if a low signal is applied to the trigger voltage so that it becomes I/3 of the supply voltage, the timer's internal discharge Tr. turns off. increasing the threshold voltage and driving the timer output again at high.
Figure 1. (Vlonoalable Circuit
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Figure 3. Waveforms of Monostable Operation
1. Monostable Operation
Figure I illustrates a monostable circuit. In this mode, the timer generates a fixed pulse whenever the nigger voltage falls below Vcc/3. When the trigger pulse voltage applied to the #2 pin falls below Vcc/3 while the timer output is low. the timer's internal flip-flop turns the discharging Tr. off and causes the timer output to become high by charging the external capacitor CI and setting the flip-flop output at the same time.
The voltage across the external capacitor C1. Vci increases exponentially with the time constant 1=Ra*C and reaches 2Vce/3
at ld=l. I Ra*C. Hence, capacitor CI is charged through resistor Ra. The greater the time constant RaC. the longer it lakes
for the Vci to reach 2Vcc/3. In other words, the lime constant RaC controls the output pulse w idth.
When the applied voltage lo the capacitor C1 reaches 2Vcc/3. the comparator on the trigger terminal resets the Hip-flop.
turning the discharging Tr. on. Al this time. CI begins lo discharge and the timer output converts to low.
In this way. the timer operating in monostable repeals the above process, figure 2 shows the time constant relationship based
on Ra and C. figure 3 shows the general waveforms during monostable operation.
It must be noted that, for normal operation, the trigger pulse voltage needs to maintain a minimum of Vcc/3 before the timer output turns low. That is. although the output remains unallected even if a different nigger pulse is applied while the output is high, it ma\ he affected and the waveform not operate properly if the trigger pulse voltage al the end of the output pulse remains al below Vcc/3. I'igure -I shows such timer oulpul abnormality.
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Figure 4. Waveforms of Monostable Operation (abnormal) 2. Astable Operation
wjt LM124
kT#@ LM224 - LM324
LOW POWER QUAD OPERATIONAL AMPLIFIERS
¦ WIDE GAIN BANDWIDTH : 1.3MHz
¦ INPUT COMMON-MODE VOLTAGE RANGE INCLUDES GROUND
¦ LARGE VOLTAGE GAIN : 100dB
9 VERY LOW SUPPLY CURRENT/AMPLI : 375|iA
¦ LOW INPUT BIAS CURRENT : 20nA
¦ LOW INPUT OFFSET VOLTAGE : 5mV max. (for more accurate applications, use the equiv¬alent parts LM124A-LM224A-LM324A which feature 3mV max.)
¦ LOW INPUT OFFSET CURRENT : 2nA
¦ WIDE POWER SUPPLY RANGE : SINGLE SUPPLY : +3V TO +30V
¦ DUAL SUPPLIES : ±1.5V TO ±15V
DESCRIPTION
These circuits consist of four independent, high gain, internally frequency compensated operation¬al amplifiers. They operate from a single power supply over a wide range of voltages. Operation from split power supplies is also possible and the low power supply current drain is independent of the magnitude of the power supply voltage.
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Output 1 1 C
Inverting Inpul 1 2 []
Non-inverting Input 1 3
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Non-inverting Input 2 5 £ \t
Inverting Input 2 6 L~
Output 2 7 £
PIN CONNECTIONS (top view)
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ABSOLUTE MAXIMUM RATINGS
Symbol Parameter LM124 LM224 LM324 Unit
Vcc Supply voltage ±16 or 32 V
Vi Input Voltage -0.3 to +32 V
Vid Differential Input Voltage 1' + 32 V
Plot Power Dissipation N Suffix
D Suffix 500 500 400 500 400 mW mW
Output Short-circuit Duration z) Infinite
'in Input Current 3' 50 50 50 mA
^oper Opearting Free-air Temperature Range -55 to +125 -40 to +105 0 to +70 °C
Tstg Storage Temperature Range -65 to +150 X
dnuii-LiiLuu^ iFuui me uuipui iu v^*^ lchi LdUbe excessive neduny u vpc > ov. i ne maximum uuipui currer of the magnitude of Vcc. Destructive dissipation can result from simultaneous short-circuit on all amplifiers.
Either or both input voltages must not exceed the magnitude of Vcc* or Vcc".
Short-circuits from the output lo VCC can cause excessive healing if VgC > 15V. The maximum output current is approximately 40mA independent
This input current only exists when the voltage at any of the input leads is driven negative. It is due to the collector-base junction of the input PNP transistor becoming forward biased and thereby acting as input diodes clamps. In addition to this diode action, there is also NPN parasitic action on the IC chip, this transistor action can cause the output voltages of the Op-amps to go lo the Vcc voltage level (or to ground for a large overdrive) for the time duration than an inpul is driven negative.
This is not destructive and normal output will set up again for input voltage higher than -0.3V.
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TYPICAL SINGLE - SUPPLY APPLICATIONS
HIGH INPUT Z ADJUSTABLE GAIN DC INSTRUMENTATION AMPLIFIER
Rl 100k
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iLM124 R3 100k R4 100k
R2 2k I T o R5 100k ¦ 1/4 <V. .LM124
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MOTOROLA
SEMICONDUCTOR TECHNICAL DATA
Order this document by BC546/D
Amplifier Transistors
NPN Silicon
EMITTER
BC546, B
BC547, A, B, C
BC548, A, B, C
CASE 29-04, STYLE 17 TO-92 (TO-226AA)
REV 1
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© Motorola. Inc. 1996
SEMICONDUCTOR TU
BD135/137/139
Medium Power Linear and Switching Applications
¢ Complement to BD136, BD138 and BD140 respectively
1 TO-126
1. Emitter 2.Collector 3.Base
NPN Epitaxial Silicon Transistor
Absolute Maximum Ratings TC=25°C unless otherwise noted
Symbol Parameter Value Units
VCBO Collector-Base Voltage : BD135 45 V
BD137 60 V
BD139 80 V
VfJEO Collector-Emitter Voltage BD135 45 V
BD137 60 V
BD139 80 V
VEBO Emitter-Base Voltage 5 V
lc Collector Current (DC) 1.5 A
'CP Collector Current (Pulse) 3.0 A
lB Base Current 0.5 A
Pc Collector Dissipation (TC=25°C) 12.5 W
Pc Collector Dissipation (Ta=25°C) 1.25 w
Tj Junction Temperature 150 c
TSTG Storage Temperature - 55 - 150 ¦ =c
Electrical Characteristics Tc=25°C unless otherwise noted
Symbol Parameter Test Condition Min. Typ. Max. Units
VCE0(sus) Collector-Emitter Sustaining Voltage : BD135 : BD137 : BD139 lc = 30mA, lB = 0 45 60 80 V V V
'CBO Collector Cut-off Current VCB = 30V, lE = 0 0.1 uA
'EBO Emitter Cut-off Current VEB = 5V. Ic = 0 10 pA
hFE1 nFE2 hFE3 DC Current Gain : ALL DEVICE : ALL DEVICE : BD135
: BD137, BD139 VCE = 2V, lc = 5mA VCE = 2V, lc = 0.5A VCE = 2V, lc = 150mA 25
25 40 40 250 160
VCE(sat) Collector-Emitter Saturation Voltage lc = 500mA, lB = 50mA 0.5 V
VBE(on) Base-Emitter ON Voltage VCE = 2V, lc = 0.5A 1 V
hFE Classification
SEMICONDUCTOR*
fairchildsemi.com
KA78XX/KA78XXA
3-Terminal 1A Positive Voltage Regulator
Features
Output Current up to IA
Output Voltages of 5. 6. 8. 9. 10. 12. 15. 18. 24 V Thermal Overload Protection Short Circuit Protection
Output Transistor Sale Operating Area Protection
Description
The KA78XX/KA78XXA series of tin 'ce-tcrminul positive regulator are available in the TO-220/D-PAK package and with several fixed output voltages, making them useful in a wide range of applications. Each type employs internal current limiting, thermal shut down and safe operating area protection, making it essentially indestructible. If adequate heal sinking is provided. the\ can deliver over IA output cunenl. Although designed primarily as fixed voltage regulators, these devices can be used with external components lo obtain adjustable \oltages and currents.
INPUT
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PASS
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©2001 Fairchild Semiconductor Corporation
GND
Rev. 1.0.0

metal detector material

hiii..watsay i wana see the project

Hi,
we only have the report of this topic. No implementation of the metal detector is available. Please search the net for further details of this like circuit diagrams, components required etc...

i hav attached projct proposal and i need report according to this till tomorrrow 3 pm...plz help me..send it to me on havoc _dx[at]yahoo.com plzzzz

bfo metal detector
BFO ( beat frequency oscillator ) metal detectors use two oscillators, each of which produces a radio frequency.The one called the the search loop oscillators uses a coil of wire. The second oscillator called the reference oscillator uses a much smaller coil of wire, and is usually inside the control box . By adjusting the oscillators so their frequencies are very nearly the same, the difference between them is made audible as a beat note, this beat note changes slightly when the search loop is moved over or near to a piece of metal. This is the principle of operation of this circuit. Usually, the search oscillator is kept fixed say at 100khz and to arrange for the reference oscillator to be adjustable 100khz plus or minus 250hz. a beat note of 250hz to 0 to 250hz is provided by this setting. This type of detector is most sensitive when the beat note is close to zero, about 5hz .

The circuit basically consists of two balanced oscillator. One acts as the detector element, the other provides the reference signal.The reference oscillator can be constructed using various circuit topology: inductor-capacitor (LC), resistor-capacitor (RC), or even a crystal (quartz) oscillator. the detector oscillator always use inductor-capacitor topology as the change in inductance as the coil nears a metal object is used as the working principle of the circuit. the detector oscillator is tuned to have same frequency as the reference oscillator in the absense of the metal object near it.

The circuit diagram and the details on construction of the circuit are given in this page:
http://schematicwiringsimple-electronic-applications/metal-detector-circuit-schematic-using-beat-frequency-oscillator-bfo/

please post me metal detector full report

Metal detectors
Theory and practice

Metal detectors are used in a wide range of applications from landmine detection to safety in airports, office buildings or schools. They can also be useful around the house to help locate lost coins, jewelry, keys and gas lines.
Metal detectors helped archaeologists in the discovey of precious artifacts and coins that were once the everyday items of use by our ancestors. Until recently, this privilege was reserved for the lucky few who could afford the expensive instrument. But with the advances in electronics and technology the price of these machines dropped to an affordable level.
Today, inexpensive, high quality consumer-oriented metal detectors provide millions of hobbyists around the world with an opportunity to discover hidden treasures, providing relaxation, excitement, the thrill of discovery and why not - profit.
As you start your search for the perfect metal detector, you will quickly discover that there are a multitude of detectors from which to choose from. There are machines using different technologies such as BFO (Beat Frequency Oscillator), Off-Resonance, IB (Induction Balance), VLF (Very Low Frequency), VLF/TR, TR (Transmit-Receive), PI (Pulse Induction), or RF (Radio-Frequency or Two-box detectors). Innovations in the field of metal detecting are a still going on -- there are new patents and original designs born every day.
We will cover only the three main detector types you will likely to encounter in your quest for the perfect metal detector for treasure hunting and exploration:
VLF or Very Low Frequency
PI or Pulse Induction
BFO or Beat-frequency oscillator

For more details, please visit
http://hobby-hourelectronics/metal_detectors.php

[attachment=7253]

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INTRODUCTION
Metal detectors use electromagnetic induction to detect metal.
THE FIRST DETECTORS
Towards 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 induction balance system, which was incorporated into metal detectors a hundred years later.
Early machines were crude, used a lot of battery power, and worked only to a very limited degree.
Alexander Graham Bell used such a device to attempt to locate a bullet lodged in the chest of American President James Garfield in 1881; the attempt was unsuccessful because the metal bed Garfield was lying on confused the detector.
MODERN DEVELOPMENTS
The modern development of the metal detector began in the 1930s. 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. He reasoned that if a radio beam could be distorted by metal, then it should be possible to design a machine which would detect metal using a search coil resonating at a radio frequency.
In 1937 he applied for, and was granted, the first patent for a metal detector. However, it was one Lieutenant Josef Stanislaw Kosacki, a Polish officer attached to a unit stationed in St Andrews, Fife, Scotland during the early years of World War II, that refined the design into a practical detector. They were heavy, ran on vacuum tubes, and needed separate battery packs. The design invented by Stanislaw was used extensively during the clearance of the German mine fields.
As it was a wartime research operation to create and refine the design of the detector, the knowledge that Stanislaw created the first practical metal detector was kept secret for over 50 years.
After the war, there were plenty of surplus mine detectors on the market; they were bought up by relic hunters who used them for fun and profit. This helped to form metal detecting into a hobby. Metal detector can be used for robotics.


ANATOMY OF 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.
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".

Garrett GTI 1500 metal detector
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.

METAL DETECTOR TECHNOLOGIES

o Very low frequency (VLF)
o Pulse induction (PI)
o Beat-frequency oscillation (BFO)


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 ground.

This Land Ranger metal detector from Bounty Hunter uses VLF.
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.



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.
Resistance - An object that does not conduct electricity easily (is resistive) is quick to react to changes in the current.
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.
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 thousa.nd.

This Garrett metal detector uses pulse induction.
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.

[attachment=9886]
Description
This is the circuit diagram of a low cost metal detector using a single transistor circuit and an old pocket radio..This is nothing but a Colpitts oscillator working in the medium band frequency and a radio tuned to the same frequency.First the radio and the circuit are placed close.Then the radio is tuned so that there is no sound from radio.In this condition the radio and the circuit will be in same frequency and same frequencies beat off to produce no sound.This is the set up.When the metal detector circuit is placed near to a metal object the inductance of its coil changes , and so do the frequency of oscillations.Now the two frequency will be different , there will be no canceling and radio produces a hissing sound.The metal is detected.
Note
To make L1 make 60 turns of 36SWG enameled Copper wire on a 1 cm PVC tube.
Powering the circuit using a adapter rather than a battery induces noise. It is always good to power radio projects from battery.

Presented By-
P.Sindhuri
B.T.Vishnu Vihar
M.Vani Divyatha
M.Tejeswar

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[attachment=9944]
ABSTRACT
Nowadays, there are a lot of criminal incidents such as terrorist attacks in hotels & malls. This device helps in detecting metallic objects residing in body. The device is called metal detector. This project is developed to detect lost metals like iron, nails, keys and coins .The first step is to design the control circuit to detect different electrical pulse wave which has a different frequency. It is based on electromagnetic field produced by coil entwined at metal substance.
OBJECTIVES OF THE PROJECT
To study how to develop a system of metal detector
To design a device which can use to detect any changing frequency & pulses and output comes as alarm by speaker
To detect metal substance by using a simple circuit
To know and familiarize the electronic components in terms of function, operation and the characteristic.
WHAT IS METAL DETECTOR ?
A metal detector is simple electronic circuit
used to detect & locate metallic objects
above and below the surfaces
HISTORY
The German physicist Heinrich Wilhelm Dove invented the induction balance system, which was incorporated into metal detectors a hundred years later.
The modern development of the metal detector began in the 1930s, Gerhard Fisher design a machine which would detect metal using a search coil resonating at a radio frequency, he was granted the first patent for a metal detector.
The first industrial metal detectors were developed in the 1960s and were used
extensively for mining and other industrial applications.
INTRODUCTION
METAL DETECTOR STYLES
Metal detectors come in several physical styles "beachcomber," hand-held, and mounted styles can all be effective for their intended use.
Hand-held detectors are smaller and less obtrusive than beachcomber detectors. They are also usually "powerful and sensitive enough to detect all concealed metal weapons, including the smallest knives and guns".
COMPONENTS USED
Transistors -BC547
Diodes -IN4148
Resistors
Capacitors
LED-5mm red
Inductor coil
Buzzer -27mm C.Tone
Battery -9v
Metal detector comprises of three modules
CIRCUIT WORKING
HEART OF THE CIRCUIT-COLPITTS OSCILLATOR
Positive feedback
Voltage divider
Frequency is determined by the inductor and the two capacitors
Metal detector comprises of three modules
a. Oscillating circuit
b. Regulator circuit
c. Indicating circuit
a. Oscillating circuit consists of
1.a first transistor;
2.a second transistor;
3.adjustable resistor coupled to the emitter of the first transistor; and
4.inductor coupled to the base of the second transistor to control the conductive state.
b. Regulating circuit consists of
1.Charging circuit
2.Discharging circuit
3.Regulating transistor
c. Indicating circuit consists of
1.Diode
2.LED
3.Buzzer
PROJECT APPLICATIONS
Coin shooting
Prospecting
Relic hunting
Treasure hunting
Beach combing
REAL-TIME APPLICATIONS
Airport security
Building security
Event security
Item recovery
Archaeological exploration
Geological research
ADVANTAGES
In Schools: The metal detectors in schools are used to reduce any possible risk of violence by right and prompt signals such as beeps and alarms when the detectors find person carrying destructive weapons like knives, guns, bomb etc.
IN INDUSTRIES: Industrial metal detectors are used in the pharmaceutical, food, beverage, textile, garment, plastics, chemicals, lumber, and packaging industries.
DISADVANTAGES
False Reading
Constitutional Rights
Medical Interference
FUTURE SCOPE
MODIFICATIONS IN METAL DETECTOR HELPS TO DISTINGUISH DIFFERENT METALS LIKE FERROUS AND NON FERROUS METALS (GOLD,SILVER ETC..,) AND ALSO HOW MUCH DISTANCE IS THAT METAL FROM COIL
CONCLUSION
Generally, this project become as a role that can give a contribution for all people with one system which is effective and accomplished to handle stolen, robbery and used for finding something that were lost. This is because, this project was build using simple electronic circuit and not too big also having a high performance to protect people asset from stolen and helps to find lost objects.

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