The Treasure hunting
The detection of metal objects within the ground is based on two principles of Physics:
- First, a metal object that comes close to a coil causes a change in inductance of the searching coil. If there are more than one coils in the same space, then the coil between the coils is changed. This change may be incremental or diminishing. This phenomenon is due to the change in the so-called relative magnetic permeability (μ) of the space near the metal. The three types of ferromagnetic, paramagnetic and diamagnetic are divided by the relative magnetic permeability that surrounds these materials. Although it is very difficult to separate the diamagnetics from the paramagnetic materials that may be buried in the subsoil, it is certainly possible to separate the ferromagnetic materials from the other materials.
- Secondly, on any metal near a coil which flows through an alternating current (cyclic streams), the so-called eddy currents, are induced. These cause heat loss in the metal, thereby increasing the resistivity in the coil, increasing its losses and decreasing its quality (Q). The intensity of eddy currents depends on the size, shape and type of the metal, as well as its distance from the coil and the frequency of the leakage current. Generally, a large metal sheet, without holes, develops stronger eddy currents than a smaller or shredded metal object.
Finally, in addition to the depth of the object on the ground, the type and humidity of the soil or the soil in general is important. From the foregoing, it is clear that precise metal separation based on only one recognition parameter is practically impossible.
Basic metal searching circuit types
The practical detector metal circuits are divided into three basic categories:
BFO (Beat Frequency Oscillator). In this system there are two oscillators, one of which is constant and the other one has a coil as the coil for resonance. The frequency difference between two oscillators, as it is shown by a non-linear mixer of the two signals, varies during metal detection, remaining in the low-waist region of the audible spectrum. The user understands the existence of a metal by changing the frequency of the loudness, heard from the headphones, or by a loudspeaker.
TR/IB (Transmission/Reception/Induction Balance). Here is a transmitter and receiver, coupled with loose inductive coupling. The occurrence of metal in the subsoil alters the value of the inductive coupling. The result is signaled by the level change of the oscillator output signal.
PI (Pulse Induction). In this system, impulses are pulled into the buried metal, which is reflected and distorted back. The type of deformation indicates whether or not the nature of the detected metal that is present.
Each of the above categories of detectors has its disadvantages and advantages. The ideal metal detector should therefore combine all three detector categories. This unfortunately would result in a very complex system.
The Simple Method
The detection method used by the circuit is very similar to that of the BFO, except that it has no sound, but signal turns into a current and drives a microameter.
You will agree that it is hard to find a simpler detector circuit. Only one integrated circuit, 4030, a moving coil microammeter, some resistors, few capacitors and two coils are used. The total cost of parts should be less than $15. The circuit uses two Colpitts-type LC oscillators formed by the IC1a and IC1c gates. C7 makes loose capacitive coupling between the two oscillators.
The oscillator IC1a has a fixed frequency of approximately 300kHz. The oscillator IC1c operates at exactly the same frequency when there is no metal in the range because it is synchronized through C7 by the constant IC1a oscillator at a phase angle of about 90 degrees. When a metal sufficiently changes the inductance of L2, the oscillator IC1c changes frequency. The IC1d gateway performs the exclusive EX (EXOR) logic on the output signals of the two oscillators, providing at its output the double frequency (approximately 600KHz without the detected metal) at a period ratio of about 50%.
The period ratio and consequently the current through the M1 instrument changes when a metallic object is detected. The IC1b gate leads M1 to act as an inverter while P1 adjusts the sensitivity of the instrument.
The circuit is powered by a 9V battery that lasts for several hours. D1 prevents reversed supply of the circuit.
Constructing the metal detector
The PCB of the detector is shown in the figure below. Apart from L2, there are no other critical components in the circuit, so construction should not present particular difficulties. Simply attach each component to the position indicated by the parts view. Turn the cursor P1 to the left to the lowest sensitivity position. The detection coil has a core of air and a diameter of about 7.5cm, with 40 turns of enameled copper wire or 20cm diameter and 25 turns of enameled copper wire. The coil is wrapped around the circumference of a round piece of wood or a piece of plastic drainage pipe or gutter. Sensitivity range increases by increasing the diameter of the coil. L1 is purchased ready and if it has a threaded ferrite core, C8 can be omitted. If in practice the circuit is not very sensitive, you can raise the R2 value slightly.
Setting the finished circuit is very simple. Power up the circuit and set C8 so that the needle of the instrument moves from right to left, without of course metal near L2. The correct setting of C8 is where the needle stays firm at the beginning of the scale (without tapping left). You may want to make the C8 at the exterior so you can adjust the sensitivity of the detector while looking for metal.
A disadvantage resulting from the simplicity of the circuit is the thermal instability of the detection oscillator. If this proves to be problematic, you should put L2, C4, C5 and C6 in close proximity to co-operate thermally. Therefore, if the coil is relatively far from the rest of the circuit, these capacitors should be placed near the coil instead of on the board. The stability of the coil can be improved by closing it in an epoxy resin casing or by covering it with adhesive tape. From an ergonomic point of view, the stabilized coil should be secured to the end of a stick. The rest of the circuit is placed in a small plastic box fixed to the handle of the stick.
The prototype has a plastic drain pipe of 32mm diameter for stick. The box with the circuitry was mounted on the "T". The detection head has a 200mm plastic plug cap. The connection of the coil carrier with the stick was made with a 135 degree angle. At the top of the stick a corresponding plug of 32mm was plugged. Coil winding can be done on the outer surface of the carrier plug and then secure the coil to its place with an insulating tape (PVC adhesive). The photo of the detection head is quite informative. The cost of the entire construction is obviously small. All non-electronic materials can be found in plumbing shops.
Ready for searching treasures
Having now finished with the construction of the detector, you will be eager to go looking for treasures. But you have to be familiar with the use of the detector before you get the mountains and gullies looking.
In practice, diammagnetic and paramagnetic materials will have very little effect on the detector readings. Ferromagnetic materials, however, cause intense evidence in the instrument, increasing the inductance of L2. However, because you are usually looking for coins, that is, small metal objects and because the 300kHz frequency is relatively high, you will have heavy losses from eddy currents that lower the frequency of the oscillator. Since here the main detection factor is the presence of eddy currents, the instrument needle diverges in the same direction for all kinds of metals. If you are looking at the needle and its moving, it means something is buried on the ground. So start digging and good luck. :-)
List of Components
R1, R2 = 22KΩ
P1 = 10KO, trimer
C1, C3, C4, C6 = 680pF
C2 = 220pF
C5 = 270pF
C7 = 3p3
C8 = 100pF, Trimmer
C9 = 10μF / 63V
L1 = 330μH
L2 = hand coil, see text
D1 = BAT85
IC1 = CD4030
M1 = 100μA moving coil instrument
yjhxtjxjhxrft Jun 26, 2020
Miguel Jun 23, 2018