The capabilities of the metal detector
This detector is able to detect:
- large metal objects (iron bucket, the cover from the hatch, water pipe) at a depth of one meter;
- small objects (coins or screws) up to a depth of 15-20 cm.
The principle of operation
The device is constructed on the basis of the most common parts that are available in stocks of any radio Amateur. The metal detector made by well-known and widely used in such devices the principle of beating between the two frequencies high-frequency generators. The frequency of one of them (reference) constant, and the frequency of the second (search) is changed under the action of external metal items that change the inductance of its coil when injected into the area of its action.
Schematic diagram of the detector of the available elements
Schematic diagram of the detector shown in Fig. 2.37. Reference oscillator performed on the transistor VT1. The frequency of oscillation is determined by the parameters contour L1C3 and is about 1 MHz.
Fig. 2.37. Schematic diagram
Search generator made on the transistor VT2, he also generates a signal approximately the same frequency. The difference is that in the contour of the support the generator uses a small coil with a ferrite core. Therefore, the inductance of the external metal objects practically have a significant action.
The search coil circuit of the generator is wound on a larger frame in a frame. It has no core. As a result of its inductance varies with its approaching a metal object, which in this case begins to run the functions of the moving core.
The signals from both generators come to the diode mixer diode VD1. In a result, the capacitor C12 is obtained subtracting the product of the frequencies of the generators. The closer the values of these frequencies, the lower audio tone on this capacitor, but more than the difference between the generator frequency, the higher the pitch of the sound, which receives the signal (the product of the diode mixer).
The signal goes through a low frequency amplifier transistors VT3-VT6. By means of the variable capacitor C7 search generator can be set a way that in the absence of nearby metal objects the tone of the sound in dynamics was the lowest.
Then when approaching the coil L2 to the metal oscillator frequency starts to VT2 to change. The frequency difference between the oscillators is increased, and hence, the tone in the dynamics will rise. When finding the exact metal sound go into a shrill squeak.
The coils of
The coil L1 should be wound on a ferrite rod with a diameter of 8 mm, for example, from magnetic antenna of the radio. Stem length is reduced to 30 mm. Previously on the rod need to wear the frame - casing cemented from Whatman, which moves it to some friction. Coil L1 must contain 110 turns of wire sew with a diameter of 0.2-0.3 mm Outlet must be done from the 16th round of counting from the collector VT1.
Coil L2 retrieval. It must be wound on the frame constituting a frame dimensions 120 x 220 mm, made of plexiglass, plastic or wood. The need to keep winding wire sew diameter 0,4 x 0,6 mm. Coil should contain 45 turns taped the 10th counted from the collector of VT2. The coil must be connected to the main block three conductor shielded wire. The coil should be located at a distance of about 1 meter from the main unit is mounted on an aluminum tube or wood rail).
Design
The instrument itself (the main block containing the generator on VT1 and USC with speaker and a battery) can be mounted in the housing of the radio. From the same receiver it is advisable to use:
- speaker;
- variable capacitor;
- the spool pin L1.
Design may be different, all depends on the capabilities and desires.
Components and options replacement elements
Capacitor C7 can be with a minimum capacity of not more than 10 pF, and the maximum not less than 150 pF. Transistors KT315 can be replaced KT3102 or CT, CT. Transistors MP you can replace MP-MP, and the transistor MP39 MP39 at-MP42.
Diodes D9 - any letter, or D2, D18, GD. The speaker - any impedance from 4 Ohms to 100 Ohms, for example, from the speaker radio or head phones.
The battery is at 9V, you can use the "Crown" or the right battery. The power supply from AC source of 220 V is not desirable, because it occurs background of alternating current and decreases the sensitivity of the device as a whole.
Setting of the metal detector
Tuning is the tuning coil L1 so that when the average the position of the rotor of the capacitor C7 and in the absence of external metal items in dynamics was heard the sound of the low tone.
Later on, while the adjustment before beginning the search will be performed capacitor C7.
In the absence of oscillations from the generator to VT1 need to pick up the value of C4 and / or to adjust the mode of operation of the cascade the selection of the nominal value of R2. If not excited generator to VT2, it is necessary to adjust C8 and to adjust the mode of operation of the transistor the selection of the nominal value of R6.
Features of use
The device has a high sensitivity, and it requires certain skills. So you need to practice.
When working it is important to note that when approaching the ferrous metals (iron, steel, cast iron) frequency generator to VT2 decreases, and when approaching color - increases.
Author: Pavlov S.