METAL DETECTOR Introduction:A metal detector is a device device which responds responds to metal metal that may not be readi eadilly app apparen arentt. The sim simplest lest form orm of a metal etal det detect ector consi onsist stss of an oscillator oscillator pr prod oduc ucin ing g an alte altern rnat atin ing g curr curren entt that that pass passes es thro throug ugh h a coil coil producing an alternating magnetic field. field. If a piece of electrically conductive metal is close to the coil, eddy currents will be induced in the metal, and this produces a 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. The first industrial metal detectors were developed in the 1960s and were used extens extensive ively ly for minin mining g and other other indust industria riall appli applicat cation ions. s. Uses Uses includ includee demining (the detection of land of land mines), mines), the detection of weapons such as knives and guns (especially in airport security), security),geophysical geophysical prospecting, prospecting, archaeology and treasure hunting. hunting. Metal detectors are also used to detect foreign bodies in food, and in the construction industry to detect steel reinforcing bars in concrete and pipes and wires buried in walls and floors. The fact is that all of these scenarios are valid. Metal-detector technology is a huge part of our lives, with a range of uses that spans from leisure to work to safety. The metal detectors in airports, airports , office buildings, schools, government agencies and prisons help ensure that no one is bringing a weapon onto the premises. Consumer-oriented Consumer-oriented metal detectors provide millions of people around the world with an opportunity to discover hidden treasures (along with lots of junk).
RESISTORS A resistor is a component of an circuit that resists the flow of electrical current. It has two terminals across which electricity must pass, and it is designed to drop the voltage of the current as it flows from one terminal to the other. Resistors are primarily used to create and maintain known safe currents within electrical components The amount of resistance offered by a resistor is determined by its physical construction. A carbon composition resistor has resistive carbon packed into a ceramic cylinder, while a carbon film resistor consists of a similar ceramic tube, but has conductive carbon film wrapped around the outside. Metal film or metal oxide resistors are made much the same way, but with metal instead of carbon. A wire wound resistor, made with metal wire wrapped around clay, plastic, or fiberglass tubing, offers resistance at higher power levels. Those used for applications that must withstand high temperatures are typically made of materials such as cermets, a ceramic-metal composite, or tantalum, a rare metal, so that they can endure the heat. The unit for measuring resistance is the OHM. (The Greek letter Ω - called Omega). Higher resistance values are represented by "k" (kilo-ohms) and M (Meg ohms).
Fig: Resistor symbols
CAPACITORS A capacitor is a passive electronic component that stores energy in the form of an electrostatic field. In its simplest form, a capacitor consists of two conducting plates separated by an insulating material called the dielectric. The capacitance is directly proportional to the surface areas of the plates, and is inversely proportional to the separation between the plates. Capacitance also depends on the dielectric constant of the substance separating the plates. Electrolytic capacitors are ‘polarized’ which means they have a positive and negative lead and must be positioned in a circuit the right way round They also have a much higher capacitance than non-electrolytic capacitors. Non-electrolytic capacitors usually have a lower capacitance. They are not polarized and can be placed anyway round in a circuit
They are normally used to smooth a current in a circuit.
Fig: Capacitor symbol
COMPONENTS LIST 1
Components name
Specification
No of units
Resistors
56kΩ
2
3.3kΩ
1
22kΩ
1
2.7kΩ
1
2.2kΩ
1
680E
2
15kΩ
1
2
Variable resistor
5K3386Trim
1
3
Capacitors
1PF
2
1KPF
1
220PF
1
270PF
1
12KPF/100V
1
100/25V
1
4
Transistors
BC 547 PH
4
5
Diode
IN 4148
4
6
LED
5MM Red Led
1
7
Coil
Metal Detector
8
Buzzer
VK 27 CT (S)
1
9
Power supply
9V Snapper
1
Coil
1
10
PCB
VK 557 PCB
1
Conclusion After designing, simulating, assembling, soldering and testing the circuit, we came to the conclusion that our circuit of the metal detector is working satisfactorily and has negligible amount of unexpected functioning.
WORKING Metal detectors work on the principle of transmitting a magnetic field and analyzing a return signal from the target and environment. The transmitted magnetic field varies in time, usually at rates of fairly high-pitched audio signals. The magnetic transmitter is in the form of a transmit coil with a varying electric current fl owing through it produced by transmit electronics. The receiver is in the form of a receive coil connected to receive and signal processing electronics. The transmit coil and receive coil are sometimes the same coil. The coils are within a coil housing which is usually simply called “the coil,” and all the electronics are within the electronics housing attached to the coil via an electric cable and commonly called the “control box”. This changing transmitted magnetic field causes electric currents to flow in metal targets. These electric currents are called eddy currents, which in turn generate a weak magnetic field, but their generated magnetic field is different from the transmitted magnetic field in shape and strength. It is the altered shape of this regenerated magnetic field that metal detectors use to detect metal targets. (The different “shape” may be in the form of a time delay.)The regenerated magnetic field from the eddy currents causes an alternating voltage signal at the receive coil. This is amplified by the electronics because relatively deeply buried targets produce signals in the receive coil which can be millions of times weaker than the signal in the transmit coil, and thus need to be amplified to a reasonable level for the electronics to be able to process. In summary: 1. Transmit signal from the electronics causes transmit electrical current in transmit coil. 2. Electrical current in the transmit coil causes a transmitted magnetic field. 3. Transmitted magnetic field causes electrical currents to flow in metal targets (called eddy currents.) 4. Eddy currents generate a magnetic field. This field is altered compared to the transmitted field.
5. Receive coil detects the magnetic field generated by eddy currents as a very small voltage.
6. Signal from receive coil is amplified by receive electronics, then processed to extract signal from the target, rather than signals from other environment magnetic sources such as earth’s magnetic field. As with most introductions, the above brief description is over-simplified. The signal induced in the receive coil, by the magnetic field of the eddy current, can be thought of as made up of two simultaneous components, not just an altered component: • One component is the same shape as the transmit signal. This is called the reactive signal (“X”). Because it is the same shape as the transmit field, the signal, by definition, responds immediately to whatever the transmit signal is doing. • When this X component is subtracted from the eddy current induced signal in the receive coil, the shape of the remaining Signal depends only upon the history of the transmitted field, and not the instantaneous value. This signal is called the resistive or loss component (“R”).Both the target X and R signals vary depending on the distance of the target from the coil; the further away, the weaker the transmitted magnetic field at the object, and the weaker the received signal from the eddy currents; thus the weaker the receive coil R and X signals which, as stated, may be very weak for deep targets. The received signal is usually processed by the electronics to produce at least 2 signals: the strength of one signal is proportional to the R signal strength or magnitude, but is no longer an alternating signal. Similarly, the other signal is also not an alternating signal, but rather a signal simply related to X signal strength or magnitude only. Unfortunately, both the terms “X signal” and “R signal” may refer to both these two different meanings: the one meaning referring to the alternating receive signal at the transmit frequency, and the other meaning to the strength of the received signals or magnitude (how big they are). So the term “X signal” may refer to the alternating X signal waveform at the transmit frequency, or just the X signal strength or magnitude, which of course changes as the coil is moved about over different areas of ground. The same applies to the R signal.
This dual meaning of the same term is common in electronics. For example, when referring to a received medium-wave signal, it is not always clear if an engineer is referring to the signal at the medium-wave frequency, or its varying magnitude; namely, the information transmitted regardless of the transmit frequency. In metal detectors, the terms “X” and “R” signal, usually refer to their magnitudes, not the alternating signals. These X and R signals (magnitudes) are further processed to give an output signal which may be reported to an operator in a number of different ways, the two most common being: 1. A ground balanced audio signal, whose loudness is usually proportional to the received signal strength from the eddy currents in metal targets. 2. A discriminated signal which only makes an audio “beep” when a target with selected properties is detected. These properties may be varied by a metal detector operator varying the controls of the metal detector. Most discriminating metal detectors also have a visual display which indicates properties of a detected metal target.
INDEX SL NO
CONTENTS
1
INTRODUCTION
2
COMPONENTS LIST
3
RESISTORS
4
CAPACITORS
5
VARIABLE RESISTORS
6
COIL
7
CIRCUIT DIAGRAM
8
WORKING
9
CONCLUSION
10
BIBLOGRAPHY
VARIABLE RESISTORS Variable resistors consist of a resistance track with connections at both ends and a wiper which moves along the track as you turn the spindle. The track may be made from carbon, cermets (ceramic and metal mixture) or a coil of wire (for low resistances). The track is usually rotary but straight track versions, usually called sliders, are also available. Variable resistors may be used as a rheostat with two connections (the wiper and just one end of the track) or as a potentiometer with all three connections in use. Miniature versions called presets are made for setting up circuits which will not require further adjustment. Variable resistors are often called potentiometers in books and catalogues. They are specified by their maximum resistance, linear or logarithmic track, and their physical size. The standard spindle diameter is 6mm
SYMBOL:
COIL An electromagnetic coil (or simply a "coil") is formed when a conductor (usually an insulated solid copper wire) is wound around a core or form to create an inductor or electromagnet. When electricity is passed through a coil, it generates heat. One loop of wire is usually referred to as a turn, and a coil consists of one or more turns. For use in an electronic circuit, electrical connection terminals called taps are often connected to a coil. Coils are often coated with varnish or wrapped with insulating tape to provide additional insulation and secure them in place. A completed coil assembly with taps is often called a winding . A transformer is an electromagnetic device that has a primary winding and a secondary winding that transfers energy from one electrical circuit to another by inductive coupling without moving parts. The term tickler coil usually refers to a feedback coil, which is often the third coil placed in relation to a primary coil and secondary coil. A coil tap is a wiring feature found on some electrical transformers, inductors and coil pickups, all of which are sets of wire coils. The coil tap(s) are points in a wire coil where a conductive patch has been exposed (usually on a loop of wire that extends out of the main coil body). As self induction is larger for larger coil diameter the current in a thick wire tries to flow on the inside. The ideal use of copper is achieved by foils. Sometimes this means that a spiral is a better alternative. Multilayer coils have the problem of interlayer capacitance, so when multiple layers are needed the shape needs to be radically changed to a short coil with many layers so that the voltage between consecutive layers is smaller
BIBLOGRAPHY
www.google.com www.wikipedia.com
CIRCUIT DIAGRAM
