The proposed device differs from the previously published metal detectors such class a energy efficiency, increased sensitivity and simplified signaling. The proposed detector finds in the ground, in the wall of a building magnetic and non-magnetic metal subjects in depth: coin 25 cents - 10...15 cm and larger objects up to 60 see Simplified alarm enables more focus attention to areas of search. The disadvantages of this device include: the slow drift of the frequency of the search oscillator that is characteristic of the detectors of this class.
Block diagram of the detector shown in Fig. 1.
Fig. 1
When exposed metal objects on the search coil PC is an increase in frequency search generator PG. Changing frequency signal PG is enhanced amplifier US. The amplified signal enters the crystal filter CF. The coincidence the GHG frequency with the resonant frequency CF (no metal near PC) signal passes to the amplitude detector AD, converted to a fixed component, that forms in the pulse shaper PHI impetus to the log. "1". Log. "1" acts on the alarm system of the SS, and the audio signal is not produced. When the appearance in the area of the search coil PC metal objects generator PG changes the frequency,resulting in the input of the SS appears log. "0", and alarm begins to work as long as the PC area are metallic objects. All the necessary circuit elements are powered from the regulator voltage CH. Supply current device - up to 8.5 mA.
A schematic diagram is shown in Fig. 2.
Fig. 2
Search generator made according to the scheme capacitive treatacne common-base transistor VT1, which loading are the coil L1 and the input circuit C5R3 signal amplifier, made by the scheme emitter follower transistor VT2. The amplified signal from the resistor R5 enters the crystal filter ZQ1. Signal search generator frequency is equal to the resonant frequency of the quartz filter is fed to the amplitude detector, performed on the diodes VD1 and VD2. Proyektirovanii signal in the form of a permanent component is supplied to the base of transistor VT3 - FI. Through the resistor R7 flows current, creating a voltage drop and generates a log. "1" inlet 1 DD1.1. At the same time on input 2 DD1.1 is given log. "1" with output 4 DD1.2. At this point a single-shot, performed on the elements DD1.1 and DD1.2, is closed and the output 3 DD1.1 there is a log. "0".
The multivibrator is made on the elements DD1.3 and DD1.4.B this moment is not working together with the emitter BQ 1. When approaching the search coil L1 to metal the subject of frequency of GHGs increases, regardless of the "color" of the metal. The PG signal with increased frequency is outside the boundaries of the transmittance of the quartz filter ZQ1. The absence of a signal at the output ZQ1 leads to lock FI, and 1 DD1.1 the single vibrator appears log. "0". The single vibrator DD1.1 and DD1.2 is triggered, and the output 3 DD1.1 appears log. "1", which in turn starts multivibrator DD1.3 and DD1.4. The emitter BQ 1 begins to emit a sound signal frequency. At short-term signal of the quartz filter after (fast PC) the duration of operation of the emitter BQ 1 will depend on the capacitance of the capacitor C10. In the proposed device alarm works instantly and with "memory". For people with impaired hearing can be set the led VD3 connected in the diagram by dotted lines. The current consumption of the device will increase. Voltage stabilizer DA1 simplifies the scheme voltage for the circuit devices.
Details. All type resistors MLT of 0.125 watts. The capacitor C1 type or KVM another type of air-dielectric. .If there are none, you can apply small variable capacitor with solid dielectric from handheld radios with a capacity of up to 50 PF. If there is no such capacitor, you can use a capacitor with a larger capacity, enabling him consistently capacitor capacitance constant required value. The capacitor circuit C2-C4 it is desirable to apply a negative TKE group, for example M47-M. Can to try to mix the groups M and MT. The capacitor C2 can be taken from the diagram of the circuits small-sized radios.
Quartz small from 100 kHz to 1 MHz. Under the corresponding resonator will have to pick the number of turns of the search coil L1. Piezoelectric emitter BQ 1 Chinese production from small phones or watches. You can apply domestic emitter type 3S-1, but he more size and consumes more power. All the electronic part of the device mounted on PCB of one-sided foil fiberglass with a thickness of 1.5 mm. To the end boards at a 90° angle soldered control Board, made of the same material that includes a capacitor C1 and compact switch SA1.
In the original fee is placed in a matchbox in a rectangular aluminum box (screen inverter circuit from radio "Kazakhstan"). Rod made from plumbing tubes made of aluminium and covered on the inside and outside plastic shell with a diameter of 16 mm. the Search coil L1 is made as follows: on the Board or thick plywood plotted a circle with a diameter of 150 mm. At the point of intersection of the chords to hammer metal carnations length of 20 mm at an angle of 45° with the slope away from the center of the circle. On the resulting template to wind the coil L1 wire sew-2, diameter pelsho 0,31...to 0.47 mm. In the original coil wound wire LASO 10x0. 07 - 15 turns. After winding of the coil L1 to the end of the wire do not cut as possible have to demotivate or rewind it during the build. The ends of the coil cleaned and soldered to the connection cable. The number of turns in your embodiment, it is possible to calculate approximately proportionally by the author existing quartz-crystal resonator.
After winding of the coil and securing its ends, the fixing of turns coils by ligation with several strands of thread and attaching them to the hub. This they are fastened around the perimeter of the coil through two cloves, then clove pulled out. The connecting cable of the coil L1 can be shielded. In the author's version used shielded wire, covered top plastic shell with a diameter of 1.2 mm. it is Possible to apply conventional flexible mounting wire, tightly twisted in order to stabilize the vessel.
After you configure the device and adjust the number of turns of the search coil, it put into PVC tubing cut on one sides along the entire length in one plane. Tube length must exceed the length the circumference of the coil is 5 mm, connects the ends of the coil overlap. Cable the connection of the coil output at the point of joining PVC pipe. Further on this the place is a gap between the shielding coating. Try to stay within a size docking tube and output cable, 5... 10 mm. Laid in the receiver coil positioned on a flat surface, cut side up. Underneath you can spread newspaper. Consistently rastafaria cut the tube with a screwdriver, the space in where is the coil, fill the prepared epoxy glue. Places buckling or divergence of the walls of the tube is necessary to seal the threads. Better choose PVC tube, which was stored in round bales of the desired diameter. After section of such a tube is less to break its walls.
After curing epoxy glue (in a day) the coil must be clean of water stains, clean up the threads to make the surface smooth. On a smooth surface the coil is wound shielding layer of copper or brass foil width 8...10 mm thickness of 0.05...0.1 mm. his function is to eliminate capacitive the influence of the earth and other objects on the parameters of the search coil. To start winding shielding layer is necessary from the place of joining of PVC pipes and finish winding from the other end of the coupling PVC tube. The gap between the beginning and the end the shielding layer may be 5 to 20 mm. in Any case can not be connected the beginning and end of the shielding layer, as obtained closed loop. One of the ends of the shielding layer is connected to the output coil and the shielding layer of the connecting cable. The shielding layer of the coil L1 on the inner the perimeter propivaetsya over the entire length in the width soldering 5...10 mm.
In many publications the shielding layer of the search coil is offered to of aluminum foil. When the author tests several designs search coils with aluminum foil revealed the following shortcomings:
- unreliable and short-lived contact of the shielding layer with wire lead cable due to the inability of brazing aluminum at home;
- the contact between the turns of aluminum foil shielding layer is unstable, therefore, the parameters of the search coil change.
Some publications offer the shielding layer of the search coil winding PVC tape. When testing multiple coils covered in this way it was found that when the temperature changes or mechanical stress parameters the search coil was changed. This is due to the fact that tightly wound the shielding layer manually is not possible. Under the action of elasticity of PVC tape at the effects of temperature and other factors, the gaps between the foil shielding layer and the coil are changed, and with them the parameters of the search coil.
To overcome the above disadvantages of shielded coil was placed in cut PVC pipe and filled with epoxy glue. The finished coil is attached to textolite plate shape of a Crescent with thick threads, missed through holes drilled in the plate, in the field adjoining coils. Places fit the coil to the textolite plate and fixing bandages from threads promazyvaetsya epoxy glue. The plate coil is attached to the curved barbell at the end in the form of "sticks" in the middle by a clamp made of sheet brass, steel, aluminum width of 30 mm and a thickness of 0.5... 1 mm Clamp around the perimeter contracted with two M3 bolts. Straighten legs of the yoke are attached to textolite plate coil with 2, 4 M3 bolts. Connecting cable coil is passed into the rod through the hole and connected to the electronic block. Battery "Krona" is located under the e-block and secured with using a rectangular yoke. Metal detector with battery "Krona" weighs 300 g.
Establishment. Connect the device to a power source voltage through 9 In milliammeter. The meter should show a current of 8mA. The emitter should Bq 1 to emit a low frequency signal. By adjusting the resistor R9 achieve maximum volume. For disable the alarm you want to disable output 1 DD1.1 from the circuit or resistor R7. Instead of the capacitor C2 connect the variable capacitor 0...500 pF. Better to build to use a dual capacitor 2x500 pF air a dielectric. "Unfinished" search coil connect to the schema using the connecting cable of a certain length. Connect the oscilloscope to the emitter of VT2. On the screen you should see an RF component with a level of about 3 V. connect digital frequency meter to the emitter of VT2 and determine the frequency of the search oscillator. Variable capacitor C1 is set to the middle position.
Through adjustment of the capacitor set the frequency of the search oscillator equal to the frequency of a quartz resonator ZQ1. If the frequency of the search oscillator above & tune up condenser impossible to downgrade, connect in parallel commissioning of the second section of the capacitor of the capacitor. If this operation is not helped to reduce the rate of GHG to the resonance KF, it is necessary to demotte few turns PC. If, on the contrary, the frequency of low GHG and adjustment capacitor is not able to raise it, you need to rewind from the PC a few turns. After comparison of frequencies of GHGs and KF connect the oscilloscope to the output of the CF at the point of connection VD1 and VD2. The slider of the resistor R5 set the top position. With proper ZQ1 and the PG adjustment on the oscilloscope screen will be a picture of the HF component. While connected to the resistor R7 to the emitter VT3 should appear in the log. "1", i.e. a voltage of 2.4...of 5.7 V. When SS connected to the emitter should be silent. The number of turns of the PC must be selected such that the capacitance of the capacitor C2 was approximately 50 pF.
In the further development of the PC, i.e. applying the shielding layer, fill epoxy the mount to the boom, coil inductance is reduced. For this before the full completion of fabrication of the PC, add another 2-4 turns. After the completion of the manufacture of PC you need to re-make re-adjustment and to determine the value of the capacitor C2 with the capacitance meter. In the absence of the above devices for the generation of GHGs can be identified by a permanent component on R5 by disconnecting and connecting the capacitor C3. The presence of coincidence frequency PG-KF can be determined by the DC R7 and the work of SS. To determine the capacitance of the capacitor C2 can empirically by the rotor position adjustment of the condenser.
When final adjustment is necessary to set GHG on resonance frequency of the KF with by using the capacitor C1 until the cessation of the sound emitter. In this capacity capacitor C2 must be such that the resonance frequency occurred at an average position the capacitor C1. The slider of the resistor R5 turn down to the end, this should trigger an alarm. Turn the slider R5 returning to the disappearance of the signal of alarm and even a couple of degrees. For final settings after complete Assembly in the housing of the device must drill a hole for the adjustment of the resistor R5.
You must remember that the maximum sensitivity of the detector will be at a frequency of GHGs on the edge of the upper bandwidth CF. When you are prompted metal objects in the field of PC, the frequency is changed up into units tens of Hertz, depending on the values of search objects and distance between them PC. When you configure the PG to the lower bandwidth of KF, the impact of metal items on the PC will lead to the restructuring of PG average bandwidth KF that will not cause actuation of the SS. Based on the above it is the better device to have the frequency drift "up",which automatically increases the sensitivity to alarm than drift down, which reduces the sensitivity to for a long time. Therefore, in the circuit PG is better to use capacitors with negative TKE or together with negative and positive TKE.
Literature
Author: B. N. Dubinin, Novoyavorivsk, Lviv region.