best reviews metal detectors for gold
Interesting information about treasure hunters
How a metal detector works
Electronic metal detector with your own hands

A metal detector is a relatively simple device whose electronic circuitry provides good sensitivity and stability. A distinctive feature of such a device is its low operating frequency. Metal detector induction coils work at 3 kHz. This provides, on the one hand, a weak response to unwanted signals (e.g. signals arising from wet sand, small pieces of metal, etc.), and on the other hand, good sensitivity when searching for hidden water pipes and trails central heating, coins and other metal objects.

Implementing and tweaking the scheme requires the appropriate skill and experience..
The metal detector generator excites the vibrations in the transmitting coil at a frequency of about 3 kHz, creating a variable magnetic field in it. The receiving coil is positioned perpendicular to the transmitting coil in such a way that the magnetic power lines passing through it will create a small EDS. At the exit of the receiving coil the signal is either absent or very small. The metal object, getting into the coil field, changes the value of inductiveness, and at the exit there is an electrical signal, which is then amplified, straightened and filtered. Thus, at the exit of the system we have a signal of constant voltage, the value of which increases slightly when the coil approaches the metal object. This signal is sent to one of the entrances of the comparison scheme, where it is compared to the supporting voltage that is applied to its second entrance. The level of support voltage is adjusted in such a way that even a small increase in signal voltage leads to a change in the state at the output of the comparison scheme. This in turn triggers an electronic switch, resulting in an amplifier cascade at the weekend, alerting the operator to the presence of a metal object.

The concept of the metal detector is presented on rice. 2.

The transmitter, consisting of a VT1 transistor and related elements, excites vibrations in the L1 coil. Signals coming to the L2 coil are then amplified by the D1 chip and straightened by the D2 chip, which is switched on by an amplitude detector. The signal from the detector is delivered to the C9 capacitor and smoothed by a low-frequency filter, which consists of R14, R15 and C10 and C11 capacitors. The signal is then sent to the d3 comparison circuit, which is compared to the voltage that is set by the RP3 and RP4 variable resistors. The RP4 variable resistor is quick and rough, and the RP3 provides accurate voltage adjustment. The generator assembled on a transistor with a single VT2 transition works in continuous mode, but the signal it produces is delivered to the VT4 transistor base only when the VT3 transistor closes, because, while in the open state, this transistor shunts the output of the generator. When the D3 chip enters, the voltage at its exit decreases, the VT3 transistor closes and the VT2 transistor is released through the VT4 transistor and the RP5 volume control arrives at the output cascade and loudspeaker.

The diagram uses two power sources, which eliminates the possibility of any feedback coming out of the scheme to its sensitive input. The main circuit is powered by a 18B voltage battery, which with the D4 chip drops to a stable voltage of 12 B. Weekends are powered by a separate 9B power source. Power requirements are quite low, so you can use three batteries to power the device. The power battery of the output cascade does not require a special switch, as in the absence of a signal the output cascade does not consume current.

The metal detector is a complex device, so the diagram should be assembled cascading with a thorough check of each cascade. The scheme is mounted on a board with 24 copper strips of 50 holes each with a 2.5 mm increment. First of all, 64 incisions are made in the strips and three installation holes are drilled. Then on the back of the board install 20 jumpers, pins for external connections, as well as two pins for the capacitor C5.

Then install c16, C17 and D4. These elements form a power source with a voltage of 12 B. This cascade is checked by temporarily connecting the battery with a voltage of 18 B. R23-R26, C14 and C15 capacitors and VT4-VT6 transistors. Keep in mind that the VT6 transistor body is connected to its collector, so contact of the case with neighboring elements and jumpers is unacceptable. Since the output cascade does not consume current in the absence of a signal, it is checked by a temporary connection of the loudspeaker, the RP5 variable resistor and the 9B voltage battery

R20-R22 and VT2 transistor are then installed to form a beep generator. When you connect two power sources, the speaker listens to the sound background, changing as the volume control handle changes. The R16-R19, c12 capacitor, VT3 transistor and D3 chip are then installed on the board. The comparison scheme is checked as follows. The D3 meter ingeshers are connected by RP3 and RP4 variable resistors. This input is formed by two 10-kOm resistance resistors, one of which connects to a positive power-up of 12 B and the other to a zero-scar. The second conclusions of the resistors connect to the conclusion of two D3 chips. The jumper from this output serves as a time point of connection. When the rough adjustment (both batteries are included), which is carried out by the RP4 variable resistor, a beep is broken in a certain position, while the RP3 variable resistor is finely adjusted change the signal near this position. In compliance with these conditions, R6-R15 resistors, C6-C11 capacitors, VD3 diode and D1 and D2 chips will be installed.
How to choose the right metal detector
Turning on the power source, first check the presence of the signal at the output of the D1 chip (conclusion 6). It should not exceed half the value of the power source (approximately 6 B). The voltage on the C9 capacitor should not be different from the output voltage of this chip, although guidance from the AC network may cause a slight increase in this voltage. Touching the chip’s entry finger (the base of the C6 capacitor) causes an increase in voltage due to the increased noise level. If the settings are in a position where there is no beep, the C6 finger tap causes the signal to appear and disappear. This is the end of the preliminary health check of the cascades.

The final check and setting of the metal detector is carried out after the manufacture of coils of inductivity. After pre-checking the circuit cascades, the rest of the elements are installed on the board, with the exception of the C5 capacitor. The RP2 variable resistor is temporarily placed in an average position. The board is attached to the L-different aluminum chassis through plastic washers (to eliminate the possibility of short circuit) with three screws. The chassis is fixed in the body of the control with two bolts holding two clamps, designed to attach the body of the remote to the seeker’s bar. The side of the chassis ensures that the power sources in the case are fixed. When assembling a remote, make sure that the switch’s conclusions on the back of the RP5 variable resistor do not apply to the board elements. After drilling a rectangular hole, the speaker is glued.

The bar and the connective parts that form the finder’s head holder are made of plastic tubes with a diameter of 19 mm. The head of the seeker is a plate with a diameter of 25 cm, made of durable plastic. The inside of it is carefully cleaned with sandpaper, which provides a good bonding with epoxy resin.

The main characteristics of the metal detector largely depend on the coils used, so their manufacture requires special attention. The coils, which have the same shape and size, are reeled on a D-shaped contour, which is formed from a pin, fixed on a suitable piece of board (Figure 4).

Each coil consists of 180 turns of enamelled copper wire 0.27 mm with a diversion from the 90th turn. Before removing the coils from the pins, they are bandaged in several places. Then each coil is wrapped with a strong thread, so that the revolutions fit tightly together. This is where the transmission coil ends. The own coil should be equipped with a screen. This is how the coil is screened. First it is wrapped with wire, and then wrapped with a layer of aluminum foil, which is wrapped again with wire. This double remp guarantees good contact with aluminum foil. In the windings of the wire and in the foil should be a small gap or gap, preventing the formation of a closed twist on the circumference of the coil.

The coils, made in this way, are secured with clamps at the edges of a plastic plate and connected to the control unit with a four-dwelling shielded cable. Two central drains and a screen of the receiving coil are connected to the zero-sash through the shielding wires. If you turn on the device and radio near the coil, you can hear a high-tonal whistle (at the metal detector frequency) caused by a signal in the radio. This indicates that the metal detector generator is healthy. In this case, it does not matter on what range the radio is configured, so you can use any cassette tape recorder to check instead. The position of the coils is determined either by the output signal of the metal detector, which should be minimal, or by the readings of the measuring device (voltmeter) connected directly to the C9 capacitor.

The second option for the roll-out is much easier. The voltage on the capacitor should be approximately 6 B. After that, the outer parts of the coils are glued with epoxy resin, and the internal parts passing through the center remain unattached, allowing for the final adjustment.

The final setting is to place unattached coil parts in a position where objects from colored metal, such as coins, cause a rapid increase in the output signal and the rest of the items to decrease slightly. If the desired result is not achieved, you need to swap the ends of one of the coils. It should be remembered that the final adjustment or fit of coils should be carried out in the absence of metal objects. After installation and strong fastening of the coil is covered with a layer of epoxy resin, then they are superimposed with fiberglass and all this is sealed with epoxy resin.

The final setting is to place unattached coil parts in a position where objects from colored metal, such as coins, cause a rapid increase in the output signal and the rest of the items to decrease slightly. If the desired result is not achieved, you need to swap the ends of one of the coils. It should be remembered that the final adjustment or fit of coils should be carried out in the absence of metal objects. After installation and strong fastening of the coil is covered with a layer of epoxy resin, then they are superimposed with fiberglass and all this is sealed with epoxy resin.

In practice, a metal detector is a light, well-balanced, sensitive device. Within the first few minutes of turning on the device, there may be a zero-level imbalance, but after a while it disappears or becomes insignificant.