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Simple PI coil for pulsed metal detector

Among the owners of pulsed metal detectors Coil  sensors enjoy deserved fame. It is with their help that it is possible to obtain record depths of detection for small objects the size of a coin. The design features of such sensors make it possible to obtain an increase in the depth of detection of targets by up to 20% compared with a comparable “ordinary” sensor.
At one time, we developed several Coil  sensor designs for our pulse metal detectors. However, they turned out to be difficult for self-repetition, as they required high accuracy in the hard work of special laying winding wire.
Nevertheless, the task of creating a simple and technologically advanced in the manufacture of the Coil  sensor did not leave us, and finally found its solution in a very unexpected way.
The cross section of conductors of a standard twisted pair cable is slightly less than the recommended value, but this is not an obstacle. 2, 3 or 4 turns of the cable easily fit into the standard plastic case of the sensor. Considering that it contains eight insulated conductors, we find that with a corresponding pinout of the ends, we can get a winding of 16 turns, or we can, for example, from 24 or 32.
The first thing to check was the value of the parasitic capacitance of the sensor winding, which contains 32 turns of wire in the form of 4 turns in a special way of the “twisted pair” cable (described below). The first cable on the construction market was chosen; it had the designation UTP   4 PR   24 AWG CAT .5 E , external diameter – about 4.5 mm. Cable designations may vary slightly. Three points are fundamental – the presence of four twisted pairs (and not two), the absence of a screen – the twisted pair must be unshielded (!!!), and the section of conductors – the presence of “24 AWG ” in the designation is necessary.
Creating a winding Coil  sensor occurs in the following sequence. Cut a piece of cable length of 2.5 m. Marker make two notes – at a distance of 10 cm from one end make the first note, and at a distance of 57 cm from the first (or 67 cm from the end) – we make the second note. Then we loop the first loop of the cable, combining the two above-mentioned cable marks, as shown in the photo.

Then we begin to thread the free long end of the cable into the resulting loop, wrapping the first cable round with the second cable. During one revolution of the coil of the future sensor coil, 4–5 wraps must be made, that is, the free end of the cable must be threaded through the ring of the winding created 4–5 times. Below is the first winding second turn of the first turn.

When winding all 4 turns, it is necessary to ensure that the cable fits, strictly repeating the period of twisting of the previous turns. In this case, the final “steering-wheel” of the winding obtained will be compact, dense and neat, as shown in the photo.

The ends of the cable are fixed with tape and unbent inside the winding. The cable ends are shortened to a free length of 6 cm. At a length of 3.5 cm, the outer sheath is removed from the cable ends. You can do this, for example, nail scissors. In any method, the main thing is not to damage the inner conductors and their insulation!

The ends of all wires are trimmed to a length of about 5 mm and served. Then made the wiring ends of the wires. Seven wires from one end of the cable are connected to the seven wires of the other end of the cable. The remaining two wires will be the winding leads. Wire insulation has four colors of designation – orange (О), green (З), brown (К), blue (Г). In each twisted pair of two wires, one – has a solid color of the four specified colors, and the other – any combination of these colors with white. Combination with white color will be denoted as OB, ZB, KB and GB, respectively. The table explains how the cable ends of the cable are connected to create a winding of 32 turns, and the photo shows how it looks in kind.

It is more convenient to insulate the places of soldering of wires by segments of a thin heat-shrinking tube, as shown in the photo. The tube is heated with a mounting hair dryer or just above the flame of a candle or lighter, after which it tightly encloses the soldering spot and is firmly held on it.

The result is the following: which, for convenience, are compressed into a bundle using a heat shrink tube. Finally, the ends of the winding and the ends of the cable are connected by soldering. Wiring is arbitrary. The orange twisted pair wire can be soldered to the blue wire of the cable, and the white-blue twisted pair wire to the brown wire of the cable, and vice versa. The result is the following:

Thus, a low-cost, high-tech and easy-to-repeat basket basket design is proposed. It is not more difficult to manufacture than a traditional sensor with a winding wire, but it has a higher sensitivity. Compared to the classic basket sensor, the sensor with a twisted pair winding loses a little in sensitivity, which is most likely due to the higher Ohmic resistance of the winding.

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