Deep metal detectors based on Tracker PI

We have received numerous questions about using very large (more than 0.5×0.5m) coils together with Tracker PI. At first, we were quite skeptical about these ideas. Especially since Tracker PI, equipped with standard firmware, worked poorly with such coils. However, as subsequent events showed, we were wrong not to pay due attention to this topic.

The main positive qualities that large coils provide are an increase in the depth of detection of large objects and a decrease in sensitivity to small objects. For searching for certain types of objects, these properties are indispensable.

This year (2004), representatives of the search expedition “Dolina” approached us with a request to help equip the detachment with a deep pulse metal detector. In our cynical times, these guys are engaged in a noble cause – the search and reburial of the remains of Soviet soldiers at the sites of the Great Patriotic War battles in the Novgorod region. We decided to help the detachment to the best of our ability.

After a series of experiments, it was found that Tracker PI can work with large coils. However, the firmware had to be significantly reworked for this. Firstly, it was necessary to increase the probing frequency to 400 Hz, secondly, the new program has a reduced time constant for automatic zero adjustment, and thirdly, the time intervals have been slightly changed. As a result, it was possible to develop the V4.3 program, which can work both in the Tracker PI-1 (NM8042) circuit and in the Tracker PI-2 circuit. To do this, you need to add elements C14, R31, R32 from the PI-2 circuit to the PI-1 circuit or simply connect pin 2 of the microcontroller to the +5V point (pin 20 of the microcontroller). In the latter case, battery discharge monitoring will not be performed.
For the PI-2 circuit, you only need to replace the nominal value of resistor R30 with 1 Ohm. For both circuits, it is advisable to replace resistor R7 with a multi-turn one.
Firmware V4.3 is designed to work with a sensor whose inductance is about 550 μH. The device with such firmware consumes about 150 mA.

Full setup of the circuit with large coils indoors is practically impossible due to the fact that such a coil cannot be removed at a sufficient distance from metal objects and network wires, which are stuffed in modern buildings. Therefore, the setup is carried out in two stages. In the first stage, the board is configured with the coil disconnected. Resistor R7 must be adjusted until approximately the following oscillogram is obtained at pin 7 of the D1 microcircuit:

The second stage of adjustment is performed outdoors with the coil connected. Resistor R7 is adjusted within small limits until the maximum detection range is obtained. After each adjustment of resistor R7, the reset button must be pressed.

We tested the operation of the device with three sensors measuring 0.6×0.6 m, 0.9×0.9 m and 1.5×1.5 m. All sensors were wound with multi-core electrical installation wire in PVC insulation with a conductor cross-section area of ​​0.75 sq. mm. The use of such wire allows making flexible detachable sensors. The first sensor contained 16 turns, the second 13 turns and the third 10 turns. The sensors were tested with the same device. In this case, the device was initially adjusted for the first coil and worked normally with the remaining coils without additional adjustment. The data on the detection depth of some typical objects (by air) are summarized in the following table. These data were obtained in urban conditions. In field conditions, far from industrial interference, the depths are 5-20% greater.

This table shows that different sensors are beneficial to use in different cases. For example, a 0.6×0.6m sensor has relatively small dimensions, it is more convenient to work with it in the forest. This sensor gives the maximum depth for a coin box. However, this sensor is sensitive to small metal objects. Therefore, working in metal-cluttered areas can be problematic.
But a 1.5×1.5m sensor is absolutely insensitive to small objects, it gives the maximum depth limit. But at the same time, such a sensor has a rather impressive design in terms of dimensions, which requires certain skills to work with it.

Speaking of sensor designs. Below are photos of some possible design solutions. This is how a 0.6×0.6m sensor can be made. Material – plastic pipes. Traditional mounting to the rod.

This is what a 1.5×1.5m sensor might look like when traditionally attached to a rod. The spacers are made of plastic pipes. The flexible sensor is 10 turns of wire wrapped in two layers of electrical tape. The entire structure is disassemblable. Working with such a sensor requires significant physical effort, but it allows you to examine hard-to-reach places – for example, flooded funnels.

And this is a “machine” design of a metal detector with a 0.9×0.9m sensor. The main construction material is also plastic pipes. Two six-volt batteries are used to balance the design.

Searching with such a device turns into a walk. However, it is difficult to survey rough terrain with such a design.

After disassembly, such a device also takes up little space.

And finally, a little information from the search expedition. During the last Watch of Memory, the remains of several soldiers were found with the help of our deep metal detector. The search was conducted with 0.9×0.9m and 1.5×1.5m sensors. Most often, the helmet gave a response.

Another potential target for a deep metal detector. By the way, even in such a deplorable state, the weapon is handed over to the police after the shift.

And this is what this difficult work looks like in reality:

And such finds will help to restore the names:
“Death” medallions.

Chest and other military insignia.

A spoon with the fighter’s last name and other information scratched into it.