Metal detector Thunder-4
- Power supply: 6-7.5V
- Frequency: 8192Hz
- Current consumption: ~120mA (excluding sound)
- Current in the circuit: ~140mA
- Coil diameter: 160mm
- Sensitivity: In the air: ~35cm (5-kopeck coin USSR) / In the ground: ~25cm Solid metal: ~1m
- Discrimination: 2-tone audio, black metal – low tone, colored metal – high tone Presence of pinpoint mode (pinpoint) – sliding tone (VCO or VCO)
- Ground balance, sound indication of the balance process in PP mode (similar to Groza-3)
The system as a whole allows (with the right coil) to “cut” through the grass and not hear the ground, does not react to small impacts. The board dimensions and control elements are similar to Groza-3. It works with any coil system: 2D, Ring, Big Foot. Three active regulators “GAIN,” “SENS,” “GB,” P/P button, and a jack for headphones. Built-in speaker, external power from 6 to 7.5V. Power input protection against polarity reversal and voltage exceeding (>8V) with self-resetting fuse.
The system is quite resistant to household interference, etc.
The phase control scheme with sync detectors is similar to Thunder-3. The idea itself, applied in that device, is simple and effective in terms of combating ground and stone influences. The phase method of combating ground has proven to be quite effective but revealed some drawbacks, which have been addressed in this model.
Sync detectors: Similar to Thunder-3. Sensor imbalance during operation, influenced by all destabilizing factors, provides a significant mismatch signal. There will never be zero balance there, even from the influence of the ground itself. With the appropriate amplification of the first stage, this “error” can reach unacceptable levels for the preamp’s saturation and system failure, especially at ambient temperatures. To combat this undesirable phenomenon, a corresponding circuit solution, described for Groza-3, has been applied. The use of a special sensor signal level regulator (GAIN) allows matching the actual signal level for the normal operation of the circuit and overcoming operational difficulties. In other words, there is no failure at any level of imbalance signal. Sensitivity losses are not significant in this case. A 60% reduction in the signal level reduces sensitivity by 4-5cm.
The channel amplifier circuit has been modified to improve transient processes and settling time, enhancing resolution in search operations, especially in cluttered areas. The output section of the channels has been redesigned, incorporating a “soft” comparator, and ground compensation occurs through direct summation of the two channels Y1Y2. Frequency modulation of the audio signal at 16Hz has been introduced, making the tone less flat and monotonous. The ground balancing technology is the same as in Thunder-3.
The discrimination tone formation is similar to Thunder-3.
The Pinpoint mode is similar; thanks to this solution, it becomes possible to “hear” the ground and its balance, qualitatively assess it, and use the mode for its intended purpose of target pinpointing. The ground balancing process itself is similar to Thunder-3.
The use of a linear audio amplifier for the output signal allows connecting any load from 8 to 150 ohms, including various speakers and headphones.
A power transformer with very high efficiency is applied, providing a power supply in the range of 12-15V. This adds dynamics to overload situations. Power control is similar to Thunder-3.
The main reason that prompted the construction of the device using this scheme is that the phase-cut method used in Thunder-3 (similar to A50) is effective in terms of heavily “bad” soil. However, such soil is encountered quite rarely. In most cases, soils are more or less calm and homogeneous. The advantages of this method have already been discussed. What is the drawback of such a solution? It lies in the limitation of sensitivity to “weak” targets in the soil. Precisely in the soil, it should be emphasized…
We have 2 channels configured mirror-symmetrically relative to the 0 ground. Their reaction to the ground is directly opposite. In the absence of metal, the levels of ground signals always pull their vectors in opposite phase, thus preventing the matching scheme, i.e., the sound comparator, from activating. It will only activate when the vectors are directed in one, i.e., positive direction, coincide. And this will only happen in the presence of metal. That’s understandable.
Ground signals (vectors) are always in motion. And this motion has both + and -, i.e., different phases, and also has amplitude. Here lies all the problems, precisely in the amplitude. When a certain level is reached, the signal from the target, always having a positive vector (polarity in our case), becomes smaller than the level of the ground signal. Moreover, the target signal can both add up to and subtract from it. What’s more, the target signal must ensure the equality of the phases of both channels.
In short, there comes a moment when the level of the ground signal pulls its vector over the vector of the target. As a result, we lose sensitivity to the target. What comes out in the end: by creating a large pumping current in the TX circuit, we have a good signal from the target, a good signal-to-noise ratio. But this is in the air. In the ground, we get a considerable signal from both the ground and the target as a sum. And the more pumping current, the larger the coil area, the stronger the influence of the ground signal, its amplitude. This also affects discrimination.
Since the ground signal is present in channel X. And you can’t subtract it from there. I.e., no measures are taken… Hence the conclusion. Another method must be applied. The method of direct subtraction of mirror channels. Having different vectors in phase (mirror-like) but equal in amplitude, it is possible to obtain accurate subtraction of the ground signal. Equality of these signals is ensured by precise tuning of the phase of the SD channels during ground balance, initially. The outputs of the channels are summed. The subtracted signal is practically equal to 0. Metal, any metal, entering the field of view, disrupts the balance of subtraction and, as a result, activates the threshold comparator. This allows registering a fairly weak signal. With optimal tuning of the time constant of the channels, its selection, decent discrimination and sensitivity are achieved.
This is what happened in Groza-4. The detection depth on the sand is practically the same as “in the air”. The Soviet five-kopeck coin is well discriminated in average soil up to 22-25 cm, despite the fact that the ground is not subtracted in channel X. The compensation scheme manages to maintain the polarity of channel X for colored metal.
Special points X Y are provided for connecting the hodograph. Although the use of target hodograph output systems, calculating VDI targets, is a separate issue. This problem will be addressed in the Groza-5 device.