Metal Detector Circuit Miner 4: A Low-Cost and Easy-to-Make Hobby Project for Beginners

A metal detector is an instrument that detects the nearby presence of metal. Metal detectors are useful for finding metal objects on the surface, underground, and under water. The unit itself, consist of a control box, and an adjustable shaft, which holds a pickup coil, which can vary in shape and size. If the pickup coil comes near a piece of metal, the control box will register its presence by a changing tone, a flashing light, and or by a needle moving on an indicator. Usually the device gives some indication of distance; the closer the metal is, the higher the tone in the earphone or the higher the needle goes. Another common type are stationary "walk through" metal detectors used at access points in prisons, courthouses, airports and psychiatric hospitals to detect concealed metal weapons on a person's body.

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 (inductive sensor) 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.

Metal Detector Circuit Miner 4

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The first industrial metal detectors were developed in the 1960s and were used extensively for mineral prospecting and other industrial applications. Uses include detecting land mines, the detection of weapons such as knives and guns (especially in airport security), geophysical prospecting, archaeology and treasure 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.

In 1841 Professor Heinrich Wilhelm Dove published an invention he called the "differential inductor".[1] It was a 4 coil induction balance, with 2 glass tubes each having 2 well insulated copper wire solenoids wound around them. Charged Leyden jars (high voltage capacitors) were discharged through the 2 primary coils, this current surge induced a voltage in the secondary coils.[2] When the secondary coils were wired in opposition the induced voltages cancelled as confirmed by the Professor holding the ends of the secondary coils. When a piece of metal was placed inside one glass tube the Professor received a shock. This then was the first magnetic induction metal detector, and the first pulse induction metal detector.

In late 1878 and early 1879 Professor (of music) David Edward Hughes published his experiments with the 4 coil induction balance.[3] He used his own recent invention the microphone and a ticking clock to generate regular pulses and a telephone receiver as detector. To measure the strength of the signals he invented a coaxial 3 coil induction balance which he called the "electric sonometer".[4] Hughes did much to popularize the induction balance, quickly leading to practical devices that could identify counterfeit coins. In 1880 Mr. J. Munro, C.E. suggested the use of the 4 coil induction balance for metal prospecting.[5] Hughes's coaxial 3 coil induction balance would also see use in metal detecting.

In July 1881 Alexander Graham Bell initially used a 4 coil induction balance to attempt to locate a bullet lodged in the chest of American President James Garfield.[6] After much experimenting the best bullet detection range he achieved was only 2 inches. He then used his own earlier discovery, the partially overlapping 2 coil induction balance, and the detection range increased to 5 inches. But the attempt was still unsuccessful because the metal coil spring bed Garfield was lying on confused the detector. Bell's 2 coil induction balance would go on to evolve into the popular double D coil.

On Dec 16 1881 Captain Charles Ambrose McEvoy applied for British Patent No. 5518, Apparatus for Searching for Submerged Torpedoes, &c., which was granted Jun 16 1882. His US269439 patent application of Jul 12 1882 was granted Dec 19 1882.[7] It was a 4 coil induction balance for detecting submerged metallic torpedoes and iron ships and the like.[8] Given the development time involved this may have been the earliest known device specifically constructed as a metal detector using magnetic induction.

The modern development of the metal detector began in the 1920s. Gerhard Fischer had developed a system of radio direction-finding, which was to be used for accurate navigation. The system worked extremely well, but Fischer noticed 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 1925 he applied for, and was granted, the first patent for an electronic metal detector. Although Gerhard Fischer was the first person granted a patent for an electronic metal detector, the first to apply was Shirl Herr, a businessman from Crawfordsville, Indiana. His application for a hand-held Hidden-Metal Detector was filed in February 1924, but not patented until July 1928. Herr assisted Italian leader Benito Mussolini in recovering items remaining from the Emperor Caligula's galleys at the bottom of Lake Nemi, Italy in August 1929. Herr's invention was used by Admiral Richard Byrd's Second Antarctic Expedition in 1933, when it was used to locate objects left behind by earlier explorers. It was effective up to a depth of eight feet.[12]However, it was one Lieutenant Józef Stanisław Kosacki, a Polish officer attached to a unit stationed in St Andrews, Fife, Scotland, during the early years of World War II, who refined the design into a practical Polish mine detector.[13]These units were still quite heavy, as they ran on vacuum tubes, and needed separate battery packs.

Many manufacturers of these new devices brought their own ideas to the market. White's Electronics of Oregon began in the 1950s by building a machine called the Oremaster Geiger Counter. Another leader in detector technology was Charles Garrett, who pioneered the BFO (Beat Frequency Oscillator) machine. With the invention and development of the transistor in the 1950s and 1960s, metal detector manufacturers and designers made smaller, lighter machines with improved circuitry, running on small battery packs. Companies sprang up all over the United States and Britain to supply the growing demand. Beat Frequency Induction requires movement of the detector coil; akin to how swinging a conductor near a magnet induces an electric current; except the pulse is electric EMF and not magnetic EMF[further explanation needed].

Modern top models are fully computerized, using integrated circuit technology to allow the user to set sensitivity, discrimination, track speed, threshold volume, notch filters, etc., and hold these parameters in memory for future use. Compared to just a decade ago, detectors are lighter, deeper-seeking, use less battery power, and discriminate better.

State of the art metal detectors have further incorporated extensive wireless technologies for the earphones, connect to Wi-Fi networks and Bluetooth devices. Some also utilize built in GPS locator technology to keep track of searching location and the location of items found. Some connect to smartphone applications to further extend functionality.

The biggest technical change in detectors was the development of a tunable induction system. This system involved two coils that are electro-magnetically tuned. One coil act as an RF transmitter the other as a receiver; in some cases these can be tuned to between 3 and 100 kHz. When metal is in their vicinity, a signal is detected owing to eddy currents induced in the metal. What allowed detectors to discriminate between metals was the fact that every metal has a different phase response when exposed to alternating current; longer waves (low frequency) penetrate ground deeper, and select for high conductivity targets like silver, and copper; than shorter waves (higher frequency) which, while less ground penetrating, select for low conductivity targets like iron. Unfortunately, high frequency is also sensitive to ground mineralisation interference. This selectivity or discrimination allowed detectors to be developed that could selectively detect desirable metals, while ignoring undesirable ones.

Coil designers also tried out innovative designs. The original induction balance coil system consisted of two identical coils placed on top of one another. Compass Electronics produced a new design: two coils in a D shape, mounted back-to-back to form a circle. This system was widely used in the 1970s, and both concentric and double D type (or widescan as they became known) had their fans. Another development was the invention of detectors which could cancel out the effect of mineralization in the ground. This gave greater depth, but was a non-discriminate mode. It worked best at lower frequencies than those used before, and frequencies of 3 to 20 kHz were found to produce the best results. Many detectors in the 1970s had a switch which enabled the user to switch between the discriminate mode and the non-discriminate mode. Later developments switched electronically between both modes. The development of the induction balance detector would ultimately result in the motion detector, which constantly checked and balanced the background mineralization.

At the same time, developers were looking at using a different technique in metal detection called pulse induction.[15] Unlike the beat frequency oscillator or the induction balance machines which both used a uniform alternating current at a low frequency, the pulse induction (PI) machine simply magnetized the ground with a relatively powerful, momentary current through a search coil. In the absence of metal, the field decayed at a uniform rate, and the time it took to fall to zero volts could be accurately measured. However, if metal was present when the machine fired, a small eddy current would be induced in the metal, and the time for sensed current decay would be increased. These time differences were minute, but the improvement in electronics made it possible to measure them accurately and identify the presence of metal at a reasonable distance. These new machines had one major advantage: they were mostly impervious to the effects of mineralization, and rings and other jewelry could now be located even under highly mineralized black sand. The addition of computer control and digital signal processing have further improved pulse induction sensors. 2ff7e9595c