Induction Balance Metal Detectors

Tesoro Lobo metal detector

  Tesoro Lobo metal detector is an amusing device. I will not touch on its operating frequency, this is a separate topic. But the gain of the channels of this MD is impressive: for example, the static channel (it also forms the sound signal, or rather its envelope) has a gain of about 45000. White’s Classic and Fiher 1266 have the same channel gain of about 1800. It is clear that you can try to search with this device on great depths. But the fear is just such a big increase – as if there were no numerous misses of targets due to the influence of the soil. I didn’t come across Lobo before, I tried only Amigo , but it doesn’t have a static channel. He made no impression on me. Let’s check the capabilities of Loboon a test instance. It has an anti-glitch regulator – a SENS resistor , which can be used to “crush” excessive sensitivity.

    Here’s what I got.

Tesoro Lobo metal detector

Schematic diagram (for tracing)

    The circuit was reworked for SMD parts, with an accompanying check of the operability of the nodes in the PSpice simulator , on previously tested models. The purpose of the check is to replace the denominations of parts while maintaining the operability of the circuit. The operating modes of the IC were checked against datasheets (maximum supply voltage, input currents, noise characteristics, etc.).

    The scheme implements a more or less static search with dynamic discrimination of metals. You can modify the circuit a little more by introducing an adder for X and Y signals on resistors R15, R17 from synchronous detectors. This will require a balance of the soil displacement so that the ratio of signals used ylo about X:Y= 5:1 . But this is an additional feature of the circuit. In fact, R17 is not installed, a jumper is soldered instead of R15 .

Receiver . Changed the values ​​of resistors and capacitors to common ones, keeping the frequency response and phase response. The receiver is two-stage, the gain is decent. The SENS gain control is located in the receiver assembly, not in the comparator assembly. It seems to me that this is better – there is no excessive amplification (if you “unscrew” the SENS knob ) , it means less distortion when the filters are overloaded. Another plus of the circuit is the presence of a U2A follower after the regulator.

Control signal generator . Immediately struck by the possible overload of the phase shifter on U2B . The simulation confirmed that the maximum input voltage should not exceed 5V. With a transmitter signal amplitude of 8V, which is the maximum possible, signal distortion is very significant. Those. you cannot get the maximum power from the transmitter and you must limit the amplitude of the output signal. But since U2B can have different incarnations (different manufacturers, a different IC in general), the signal level is unknown. It is easier to redo the cascade, which was done. The phase shift is kept at (180-26.5) degrees at 4V output for 8V input. Since the output voltage of the phase shifter changed, we had to add a resistor R18to level the adjustment area.

Synchronous detectors . Since I did not find a BF245 type SMD field without a parasitic diode (and it is unacceptable here), I had to use BFT46 . I don’t think this will affect the performance of the circuit.

Filters for static and dynamic channels . Remained unchanged. For a static channel, it is proposed to replace several switches with one switch “All Metall <-> Disk” and one button with a middle position, as in branded devices. It should be installed on top of the handle. The control looks like this: the lever of the button towards itself – Retune ; from yourself – the balance of the soil. In the middle position – normal search.

Signal analyzer . These are U14 comparators . I simulated the signals and made sure that in the “proprietary” circuits, the 10K resistor and the 4148 diode are connected incorrectly to the output of the comparators. Their task is to limit the level of the audio signal at the output of the static channel when the discrimination circuit is triggered. Therefore, the resistor R39 in my circuit is not needed at all. I left it just in case. But it, rather, interferes with normal operation, since even without the discriminator triggering, the divider R11 / R39 = 100K / 10K is obtained . Why such signal attenuation in an unexpected place? It is easier to throw out the resistor and, if necessary, unscrew the SENS . I think the error just wanders the Internet, and everyone repeats it …

    Here are the signals of this scheme for general development.

Tesoro Lobo metal detector

    In the upper diagram, the signal of the static channel is highlighted in green, which forms the envelope of the audio signal. The YF signal is shown in blue . He is always of the same polarity. The signal of the discrimination channel is shown in red. Its polarity (for a specific metal) depends on the position of the DISK knob . The principle of operation of such a discriminator is discussed in the article Choosing a Balance Point and Discrimination of Metals . In this diagram, the signals correspond to the “allowed” metal, i.e. DF signal has the same polarity as YF . As a result, at the output of the comparators U14 we see that at the moment the sensor passes over the center of the target (approximately 1.12 s), both comparators will turn off and the audio signal will pass to the ULF output. If you look at the circuit, then C13 will not be “set” to the negative supply through the diode D1 . The signal will pass losslessly to U6A , where the background audio level is added to it. Then it will be held at ULF.

Tesoro Lobo metal detector

    And here is the case when the metal will cause a signal in the DF channel of opposite polarity with respect to YF . It can be seen that the comparators U14 will work and through the diode D1 the capacitor C13 will be discharged to almost -8 V. This is the minimum volume. At the moments of zero crossing of signals, short sounds will be heard – at least one, about 1.12 s in the diagram. The rest of the sounds will be with the background volume.

ULF . Everything was as it was, just introduced the volume control on the R63 .

Transmitter . No change.

Power supply unit . No change. The display circuitry corresponds to that proposed by FOMA . I don’t know, maybe it’s better to light the LED when the supply voltage drops below the threshold?

    I nevertheless thought about it and decided to introduce a static search mode with ground signal compensation. This required the introduction of additional elements. In addition, film capacitors are used in all temperature-critical circuits. Since I did not see them with a capacity of less than 1 n , I counted the elements to this capacity.

    It seems to me that such a static search method, which is implemented in the branded Lobo, will not be very good in real conditions. The point is that the ground signal is not subtracted – then there are no false alarms – but is suppressed by the shift of the GB control signal . But this is good for a stationary sensor, or if you maintain strictly the same distance to the ground. If you move the sensor up or down, then an implicit triggering will occur. The sound will be “crushed” by the discriminator, but will be triggered. In addition, the signal can go into negative territory (for example, when the sensor is lowered), and until the soil tracking on U6 is triggered , small targets will be missed. Therefore, it is desirable to introduce compensation for the ground signal, as is done in Whites. Of course, this is no longer quite Lobo, and a simplified W6000 🙂

    This is the trouble with all analog devices with simple filters, not just Lobo . But Lobo has a very large gain in the channels, and close targets have already been selected. So there is no choice …

    Now you can trace the board and make a control copy of the metal detector. .. The board is traced (in jargon – divorced). Here is the result.

Tesoro Lobo metal detector

Location of parts. The vias are marked in green.

Tesoro Lobo metal detector

Tesoro Lobo metal detector

                                              Top                                                      Bottom                                                                   

   Templates for negative photoresist. Via pads 1.5 mm. Drill mask 0.4 mm (black dot in the center of the pad). Mounting holes 3.2 mm, center hole 8 mm. Holes for 1 mm connectors. On the reverse side, the board is filled with “earth”.

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