When developing this detector the task was to create a small highly economical device with good repeatability and high performance, using common and inexpensive details.
Analysis of the most common schemes showed that they all eat from source with a voltage not lower than 9 (i.e. "Crown"), but it is expensive and uneconomical. So, assembled on the chip CLE metal detector works from one battery is not more than 6-8 hours.
Search coil most devices or outlets, or have several windings. Sensitivity of the simple metal detectors are low, and more complex require the use of quartz resonators or other scarce items. In the end, A. Melnikov scheme has been developed (Fig. 2.44 a) of the metal detector the beatings as the easiest to setup and use.
Fig. 2.44. Metal detector with low-voltage power supply and a schematic diagram; b - schematic diagram, assembled entirely on a silicon type transistors CT; - example surface mounting
Schematic diagram
Functionally, the scheme includes:
- two generators (coil oscillating circuit of one of them and is search);
- balanced mixer;
- the audio frequency amplifier loaded on headphones.
For such low-voltage devices could not be more perfect barrier generators that operate consistently from the supply voltage 0.8 V or more (for silicon transistors).
Another advantage is that their output is a constant voltage (with respect to the collector connected to the housing unit) is 0.65 In and stable (emitter-base junction of transistor plays a role stabistor). This effect is used to stabilize the operating point balanced mixer.
Sound amplifier anotherstory. For such an amplifier is desirable to use a transistor by a factor of current transfer not less than 200.
Generators operating at different frequencies:
- search approximately at a frequency of 100 kHz;
- support - at a frequency of 200 or 300 kHz.
Thus, the balanced mixer allocates the beating between the frequency of the reference generator and 2-nd or 3-rd harmonic search. This solution can dramatically decrease the phenomenon of "capture" frequency search generator that simple circuits can not install the beat frequency below 200 Hz, and increases sensitivity - change the frequency of the search oscillator at 10 Hz changes the frequency of the beats of the 20 (or 30) Hz.
Of course, it is possible to further increase the frequency of the reference oscillator, but in this case the level of the beats becoming very small, which means that:
- decreases the sound volume;
- instability increases, which complicates the work.
It should be noted that temperature stability of the scheme low, but practically it does not affect the results:
- first, in the resonant circuits are subject to the same contour capacitors, so their frequency varies equally in the same direction, and as a result, the beat frequency does not change;
- secondly, some simple constructive measures have helped to improve the thermal stability of the detector.
Namely such requirements are relevant:
- the search coil must be of rigid construction;
- must be applied properly escaping;
- fee and the housing must be rigidly fixed on the rod.
The wire from the coil to the circuit should be shielded, it is advisable to use thin cable television. The rod itself should be of dry wood or fiberglass. The Board in the case, it is desirable to pour paraffin. This will not only protect against moisture but from quick temperature changes.
The manufacture of the search coil
The search coil is made of "twisted pair" cable, which conduct local network. The cable must be shielded, 5-th category, it is desirable to the outer strip (it has thicker insulation and the coil turns are tougher).
Four turns of the cable should be laid by a ring with an outside diameter of about 25 cm, and:
- lay the first two external loop over each other;
- then wrap in four places with duct tape;
- then wrap two turns inside.
All this then needs to be cut in the middle of the wrap and duct tape. For such a winding is better to use cloth tape. Both ends of the cut strip the insulation about half an inch, the ends of wires tinned. Shielding foil should be cut off.
The wire that goes together with foil, with one side breaking off from the other to connect with one of the wires of the cable. This wire will output the start winding. You should note that the screen in no case should not form a closed loop!
Further the conclusions of cable you need to connect in series, in accordance to err it is almost impossible, because all eight wires of different colors. In the end you should get a coil 32 coils with decent resistance and rigidity.
The fabrication of another embodiment of the coil
Another version of the reel spool winding wire thickness not less than 0.3 mm. You can drive a few nails into a Board at a distance of 40 cm and a winding of wire (34 turns) on them, then gently remove the coil wrap and duct tape. Then the coil should be screened. It is best to wrap it in foil, taken from old electrolytic capacitor.
We must bear in mind that electrolytic capacitors inside is an alkaline electrolyte, so unwinding the foil from the condenser preferably under a stream of water to the electrolyte corrodes the fingers. The foil must be wound so that it formed a closed loop, between the beginning and the end windings must be a gap of about 1 cm.
Try to solder the wire to the foil is useless - it is aluminum and not ludisia, therefore, on top of foil needed to wrap a few turns of bare tinned wire - this will be the output of the screen. It is already possible to connect one of the ends of the coil. Further this end to connect with braid shielded wire going from the coil to the Board, and the Board, with the common wire.
The second end of the coil need to connect with Central core wire and on the Board the base of the first transistor of the search generator. On top of the foil to reel again the duct tape.
The application features different coils
The size of the coil depend on the performance characteristics of the detector. Coil diameter 35 cm surely catches the truck from caterpillar tractor at a depth of 80 cm, but does not detect the coins, rings, nails, and other trifles. This option is perfect for finding black metal, when it is of interest a massive piece of iron (scrap).
Search for rings, coins on the beach needed a coil diameter of 15 cm Small coil diameter of 15 cm and consists of 6 turns of cable or 50 turns of wire. Depth of detection of coins approximately 15 cm Coil diameter of about 25 cm - a compromise solution, it has 40-45 turns.
Element base
Details of the metal detector is the most affordable. Resistors and capacitors are almost any type transistors in the generators can be applied KT315 (better with the letters B, G, E, some instances with the letters A and b refused to work - the small current transfer ratio). Excellent work KT3102, KT368.
A transistor balanced mixer have to be germanium. Any transistor radio 70-80s release will provide them in abundance. Suitable P with any letter, P, P, P401, GT, GT, GT. In the selectors TVs SCM-24 includes transistors GTA.
Since the operating frequency of the circuit is not very high, it is suitable even P, L28, MPB, MP42B that were used in the playback amplifiers reel to reel recorders. The amplifier 34 is desirable to use a transistor with the largest coefficient current transfer from available.
The reference oscillator coil is wound on a standard armature circuits from the inverter Chinese stereos and receivers. Some coils have a built-in capacitor, want to delete. Coil gently unwound, and if there are more than 85 turns it gently wrap the same wire. If it turns less, 85 of coils is wound any of the wrapping wire. The wire should be thin enough, otherwise the desired number of coils will fit. In the extreme case can be wound with 75 turns.
Capacity contour of the capacitors is not necessary to observe accurately, preferably only apply in both generators, the capacitors of the same value and type for the best thermal stability. The capacitance of 4700 pF can be from 3300 to 5100 pF instead 2200 pF, you can use 1500 or 1800 pF.
Mounting the unit
Circuit Board was not developed, was wiser to abandon print installation and to assemble the device on a thin (0.5 mm) piece of PCB, coupling between a detail of their own conclusions. An example of such an installation, which occupies the size less than half a matchbox, in Fig. 2.44 V.
Used transistors KT3102 and GT in metal housings.
Diagram alternative metal
In the manufacture of several detectors suddenly there was a problem with search old germanium transistors. And, in case at hand hams they will not appear, scheme has been developed, assembled entirely on silicon transistors type KT315B. Despite some reduction in sensitivity, the scheme shows good performance. The scheme presented in Fig. 2.44 b.
Author: A. Melnikov; Publication: www.mao-sim.nm.ru