Very good schematic of metal detector.
Metal detector is a relatively simple device, an electronic circuit which provides good sensitivity and stable performance. A distinctive feature of this device is its low operating frequency. Inductor metal detectors operate at a frequency of 3 kHz. This ensures, on the one hand, a weak response to unwanted signals (e.g., signals arising from the presence of wet sand, small pieces of metal, etc.), and on the other hand, good sensitivity when searching for the hidden lines and water pipes of the central heating system, coins and other metallic objects.
For the implementation and configuration of the circuit requires appropriate skills and experience.
Block diagram of the metal detector is shown in Fig. 1.
Detector generator excites vibrations in a transmitter coil at a frequency of about 3 kHz, creating an alternating magnetic field. The receiving coil is perpendicular to the transmitting coil so that passing it through the magnetic field lines create small EMF. At the output of the receiver coil signal is either absent or very small. The metal object entering the field of the coil changes the inductance value and the output appears an electrical signal which is amplified, rectified and filtered. Thus, the system output signal have a DC voltage whose value is slightly increasing when approaching a metal object the coil. This signal is applied to one input of the comparison circuit which compares a reference voltage which is applied to its second input. The reference voltage level is adjusted so that even a slight increase in voltage leads to a change in signal state at the output of the comparison circuit. This in turn actuates electronic switch, resulting in the output stage audible alarm notifying the operator of the presence of a metal object.
Circuit diagram of the detector is shown in Fig. 2.
Transmitter comprising transistors VT1 and related elements excite oscillations in the coil L1. Signals on the coil L2, then amplified and rectified by D1 microcircuit chip D2, enabled by the amplitude detector circuit. The signal from the detector is supplied to capacitor C9, and smoothed by the lowpass filter, which consists of resistors R14, R15 and capacitors C10 and C11. The signal is then fed to the input of the comparison circuit D3, where it is compared with a reference voltage set by the variable resistors RP3 and RP4. RP4 is a variable resistor for fast and coarse adjustment, and RP3 allows precise setting of the reference voltage. Generator collected on a transfer transistor VT2, operates in continuous mode, but the signal generated by them is fed to the base of transistor VT4 only when the transistor is closed VT3, as being in the open state, this transistor bypasses the generator output. When a signal to the input circuit D3 voltage at its output decreases, the transistor VT3 is closed and the signal from the transistor VT2 through the transistor VT4 and Volume RP5 arrives at the output stage, and a speaker.
The circuit uses two sources of power, which eliminates the possibility of any feedback circuit output to its sensitivity. The basic scheme is powered by a battery voltage of 18 V, which is using an IC D4 is reduced to a stable voltage of 12 V. The decrease in battery voltage during operation of the circuit does not change the settings. The output stages are powered from a single power supply voltage of 9 V. The requirements for power consumption is quite low, so to power the device, you can use three batteries. Battery power output stage does not require a special switch, since in the absence of a signal output stage draws no current.
Metal Detector - a rather complex device, so the assembly scheme should be area by area with a thorough inspection of each stage. Mounted on a circuit board on which there are 24 copper strips with 50 holes each with a 2.5 mm pitch. Above all, the strips 64 do section and three drilled mounting holes. Then, on the back side board 20 installed jumper pins for external connections, and two pins for the capacitor C5.
Then mounted capacitors C16, C17 and IC D4. These elements form the power supply voltage of 12 V. The verification of this stage is carried out through a temporary connection of the battery voltage of 18 V. The voltage across the capacitor C16 should be 12 + - 0.5 V. After that the installation of elements of the output stage: the resistors R23-R26 , capacitors C14 and C15 and transistor VT4-VT6. Note that the housing is connected to the transistor VT6 its collector, so the contact body and with adjacent webs invalid. Since the output stage when no signal is no current, it is checked by connecting a temporary speaker of the variable resistor RP5 and 9 V battery
Then set the resistors R20-R22 and the transistor VT2, forming a generator beeps. When two power supplies in the dynamics of listening background sound varies with the position of the volume control. Thereafter, the board mount resistors R16-R19, capacitor C12, and transistor chip VT3 D3. The work of the comparison circuit is verified as follows. By measuring input variable resistor connected D3 RP3 and RP4. This input is formed by two resistors 10 ohm impedance, one of which is connected to the positive supply rail 12 V, and the other - to the zero bus. The second terminals of resistors connected to pin 2 chip D3. Jumper from the pin serves as a temporary connection point. At a rough setting (included both batteries), which is a variable resistor RP4, in particular its position is the failure of the audio signal, while the exact tuning of the variable resistor RP3 must be continuously changing signal near this position. When these conditions are beginning to install the resistors R6-R15, capacitors C6-C11, diodes VD3 and chips D1 and D2.
Including the power supply, first check the availability of the signal at the output circuits D1 (pin 6). It should not exceed one half of the supply (about 6). The voltage on the capacitor C9 should not be different from the voltage output of the circuit, although interference from AC power can cause a slight increase in this strain. Finger-touch input circuits (base capacitor C6) causes an increase in voltage due to the increased level of noise. If regulators are set at the position at which an audio signal is present, finger-touch capacitor C6 leads to the appearance and disappearance of this signal. At this preliminary test the operation of cascades over.
Final check and adjustment carried out after manufacture metal detector coils. After the screening stage of the circuit board mounted on the other elements except the capacitor C5. The variable resistor RP2 is temporarily set to the middle position. Card to the L-shaped aluminum chassis through the plastic washers (to eliminate the possibility of short-circuit) with three screws. The chassis is fixed in the base of the remote control with two bolts that hold the two clips for attaching the housing to the control rod seeker. The side of the chassis for fixing the power source housing. When assembling the control panel, make sure that the conclusions of the switch on the back of the variable resistor RP5 not relate to components of the motherboard. After drilling a rectangular hole is attached speaker.
And connecting rod parts forming the holder head crawler made of plastic pipes with a diameter of 19 mm. Seeker head itself is a plate with a diameter of 25 cm, made of durable plastic. The inner part of her carefully trimmed with sandpaper to ensure a good bonding with epoxy.
The main characteristics of the metal is largely dependent on the used coils, so their manufacture requires special attention. Coils having the same shape and size, is wound on the D-shaped contour, which is formed from shtypey attached to a suitable piece of the board (Figure 4).
Each coil consists of 180 turns of enamelled copper wire with a 0.27 mm from the outlet of 90nd coil. Before removing the coil from the pins, they are tied up in a few places. Then, each coil is wound strong thread that the coils are flat against each other. This manufacturing transmitting coil ends. Prinmnaya a coil must be provided with a screen. Shielding coil is provided as follows. First it is wound wire and then wrapped with an aluminum foil, which is wrapped over the wire. This dual coil ensures good contact with the aluminum foil. In the winding wire and the foil must be provided a small gap or play, as shown in Fig. 8, prevents the formation of a closed loop around the circumference of the coil.
Thus produced coils are secured with plastic clips on the edges of the plates and connected to the control unit with four-core shielded cable. Two central tap the screen and the receiver coil are connected to the bus via a zero-shielding wire. If you turn on the radio and is located close to the coil, you can hear the high-pitched whistle (at a frequency detector) due to blank range sound in radio. This points to the serviceability of the generator metal detector. In this case, no matter which band the radio set, so checking may be used instead of any cassette. Fitting coils operating position is determined either by the output detector, which should be minimized or from the measuring instrument (voltmeter) is connected directly to a capacitor C9.
The second option for adjusting coils much easier. The voltage on the capacitor should be approximately 6 V. After that the outer parts of the coils are glued with epoxy resin, and the interior, passing through the center, stay loose, allowing you to make the final adjustments.
Final adjustment is set loose parts coils in a position in which the objects of ferrous metal, such as coins, causing a rapid increase in the output signal, and the other objects - slight decrease it. If the desired result is not achieved, it is necessary to swap the ends of one of the coils. It should be remembered that the ultimate tuning or adjustment of the coils should be conducted in the absence of metal objects. After installation and durable fastening coil coated with an epoxy resin, and then superimposed on them all fiberglass and sealed with epoxy resin.
After the head by a searcher in the scheme of integrated capacitor C5, a variable resistor RP1 is set to the middle position, and a variable resistor RP2 is set to minimum output. Thus on one side of mid-position variable resistor RP1 provides recognition steel objects, and on the other side - non-ferrous metal objects. It should be borne in mind that with each change of the nominal value of the resistance of the variable resistor RP1 need for a re-adjustment of the device.
In practice, the metal detector is a light, well-balanced, sensitive. During the first few minutes after power imbalance may be zero, but after a while it disappears or becomes insignificant.
-------------------------BOM-------------------------------------------------------------
R1, R6, R7, R8 100 K
R2, R3, R22, R23 100 R
R4, R5 6,8 K
R9, R11, R21, R25 K
R10 220 K
R14 15 K
R15, R19 68 K
R16 8,2 K
R17 18 K
R18 3,9 M
R12, R13 47 K
R24 4,7 K
R20 33 K
R26 1,8 K
RP1, RP4 10 K
RP2 10 K
RP3 100 K
RP5 10 K (with the switch)
C1 100 M, 16 V
C2, C5, C14 0,01 M
C3, C4 0,22 M
C6, C13 0,1 M
C7, C8, C12 1 M
C9 47 M, 16 V
C10 2,2 M, 35 V
C11 0,47 M, 35 V
C15, C16 220 M, 16 V
C17 470 M, 25 V
VT1, VT5 BC214L
VT2 TIS43
VT3, VT4 BC184L
VT6 BFY51
VD1, VD2, VD3 1N914
D1, D2, D3 CA3140
F4 mA78L12AWC
