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Theory of metal detector by Amit sahu

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.

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Theory of metal detector by Amit sahu

  1. 1. 1 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
  2. 2. 2 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
  3. 3. 3 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
  4. 4. 4 CIRCUIT DIAGRAM Fig:- (2) Circuit diagram of metal detector
  5. 5. 5 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
  6. 6. 6 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
  7. 7. 7 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.
  8. 8. 8 (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.
  9. 9. 9 (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.
  10. 10. 10 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
  11. 11. 11 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
  12. 12. 12 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
  13. 13. 13 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
  14. 14. 14 555 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
  15. 15. 15 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
  16. 16. 16 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.
  17. 17. 17 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
  18. 18. 18 Bibliography Electronic Club Guide www.kpsec.freeuk.com www.wikipedia.org

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