The feature of the proposed detector is an interesting circuit solution analyzer and indicator. Thus as an indicator used pointer instrument.
It should be noted that the metal detector has a relatively high sensitivity. In addition in the direction of deflection the indicator can be defined and the type of metal (color or black), from which made detected object.
Schematic diagram
Metal detector (Fig. 2.7) consists of two generators, circuits indicating and the voltage stabilizer.
Fig. 2.7. Schematic diagram of the detector with a needle indicator
Transistors T1 and T2 is assembled measuring oscillator, the oscillation frequency which depends on the parameters of the circuit formed by the coil L2, and a parallel-connected capacitors C1 and C2. Reference oscillator assembled on the transistors T3 and T4 in a similar way. The oscillation frequency of this oscillator is determined by the parameters of the circuit, performed on the elements L3, C4, and C5.
The scheme, which analyzes the emergence of the deviation (deviation) of the frequency signal measurement of the generator is compared with the signal frequency of the reference oscillator, contains the measuring circuit, which consists of a dial gauge with PA1 a zero mark in the middle of the scale, capacitor C6 and diodes D1-D4. In the same the circuit evaluates the sign of the frequency deviation. The oscillations of the reference oscillator is served in the measuring circuit through the coil L4, and the signal from the measuring generator through the coil L1. The entire circuit is balanced so that when the coincidence of the frequencies of both oscillators an indicator arrow PA1 will be be on the zero division of the instrument scale.
With the appearance in the area of the search coil L2 of a metal object the resonant frequency of the circuit L2C1C2 will change. This will change the working the frequency of the measuring oscillator and, consequently, to the deflection indicator PA1. The deflection of the needle and serves as a source of information about the discovery metal object. The angle of deflection of the indicator depends on PA1 the dimensions of the object and the distance from it to the measuring coil.
When measuring near the coil L2 will be the subject of black metal, operating frequency of the measuring oscillator, transistors T1 and T2. will decrease, and the meter needle will deflect to one side. If the subject of made of non-ferrous metal (for example brass), frequency generator UNODC, the meter needle will deflect in the opposite direction.
The direction of deflection depends on the polarity of connection of the indicator PA1, on the scale after calibration can be applied to the appropriate labels.
Fig. 2.8. The design of the frame coils L1 and L2
Coil L1 contains 20 turns, and L2 is 60 turns of wire PEV-2 in diameter 0,31 mm, wound round. Coil protected electrostatic screen, which a semi-circular strip of brass wound on the surface frame. The gap between the beginning and the end of winding of the tape shall be not less than 10 mm.
Coils of particularly necessary to ensure that no short circuit happened the ends of the tape, as in this case, a closed loop is formed.
Coil L3 has 160 turns, and L4 - 50 turns of wire diameter pelsho 0,12 mm, is wound in bulk on a frame with a diameter of 7.5 mm. is mounted Inside the frame turn core with a diameter of 2.5 mm and a length of 12 mm, made of ferrite NN. Frame with coils L3 and L4 is placed in an electrostatic screen the hole opposite the trimming of the core. The screen must be earthed.
Supply voltage of 12 V is fed to the measuring and reference generators from source V1 via a voltage regulator, assembled in the Zener diode D5 and the transistor T5.
Details and design
For the manufacture of the considered detector, you can use any the breadboard. So used to detail without facing any limitations associated with dimensions. Installation can be either hinged, they print.
The resistors may be, for example, type MLT-0,5, capacitors C1, C2, C4, C5, C7 type km or KLS. As the capacitances C3 and C6, you can use any metalloboranes capacitors, for example the type of MBM or BMT. Capacitors C8, C9 you can replace any electrolytic, such as the type K50-6, transistors CTG - other transistors in this series or the series transistors KT315 with the current transfer ratio of not less than 60, the transistor MPA - any of the series MP39 - MP42 or KT361, diodes DB - other diodes in this series. As an indicator PA1 recommended pointer instrument type M24 with a current total deflection of 100 μa and a zero in the middle of the scale.
Coils L1 and L2 are placed on the frame (Fig. 2.8), made of fiberglass or other insulating material. Board with the controls it contains and the power source are placed in any a suitable plastic or wooden case. On the housing cover set the indicator PA1, the switch S1, and also connector X1 for connection coils L1 and L2. These elements are connected to the dress flexible stranded wire.
Frame with coils L1 and L2 is placed at the end of any convenient handles. In this case the findings of the coils are connected with the mating part of connector X1 flexible stranded shielded wire. As the power source B1 can be used, for example, three batteries 3336L connected in series, or a rechargeable battery.
Establishing
Before setting up the appliance must be positioned so that the search coil L2 was at a distance of at least 1.5 m away from any metal objects.
To the coil L1 must be connected to the oscilloscope and, selecting the values of the capacitances capacitors C1, C2, set the frequency of the measuring generator transistors T1 and T2, equal to 100 Hz. The waveform is adjusted by selection of resistors R1-R3. Similarly adjusted reference oscillator, this oscilloscope is connected to the coil L4, and the shape of the oscillations is adjusted the selection of the resistances of resistors R4-R6. Before you begin adjusting the trimpot the core of the coil L4 should be set to the middle position.
Next, you need to install on the coils L1 and L4 is equal to the amplitude of oscillations which should be in the range of 0.8-1 V. If necessary, the amplitude alarms you can change the selection of the number of turns of the coils L1 and L4. After that, rotating trimmer core of the coil L3, you should set the arrow indicator PA1 to the zero mark of the scale.
The order of work
A feature of this detector is that when conducting a search works to do any additional settings and adjustments required.
When approaching the measuring coil L2 of the subject of black metal working the frequency of the measuring oscillator is reduced. In this case, the indicator arrow PA1 trending in any direction. If the object is made of colored metal, for example brass, the oscillation frequency of the measuring oscillator increases. When the indicator needle is deflected in the opposite side. Thus it is possible to determine not only the presence of metallic of the subject in range of the search coil, but also to assess the kind of metal, color or black, it is made.
Using the metal detector metal objects type canning cans can be detected up to a depth of 20-30 cm
Author: M. V. Adamenko