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Improvised coil for pulsed metal detector
I decided to collect my first pulsed metal detector Clone PI-W and, lo and, lo and come to the manufacture of search mono-coil. And since I am currently experiencing some financial difficulties, I had a difficult task - to make the reel itself from the cheapest materials.

Looking ahead, I will say at once that I have coped with the task. In the end, I got this sensor:

By the way, the resulting coil-ring is perfect not only for Clone, but also for almost any other pulse (Koschi, Tracker, Pirate).

Next, I’ll tell you how to make a search coil for a metal detector with your own hands, spending less than 500 rubles.

I will tell in great detail, as the devil is often in the details. Moreover, that short stories of making coils in the inethepond pond pond (kindo, take this, here cut off, wrap, glue and ready!) and you begin to do it and it turns out that the most important mentioned casually, and some of what is forgotten to say. And it turns out that everything is more complicated than it seemed at the beginning.
It’s not going to happen here. Ready? Let’s go!

Idea

The easiest way for self-manufacturing seemed to me such a design: take a disk of sheet material thick

4-6 mm. The diameter of this disk is determined by the diameter of the future winding (in my case it should be 21 cm).

Then to this pancake on both sides stick two discs of a little more diameter, so that it turned out as if a spool for winding the wire. That is, such a strongly enlarged in diameter, but flattened by the height of the coil.

For clarity I will try to portray it in the drawing:

I hope the basic idea is clear. Just three discs glued together all over the square.

Choosing material

As a material I planned to take the plexiglass. It is perfectly processed and glued to dichloroethane. But, unfortunately, I could not find it for free.

All collective farm materials such as plywood, cardboard, bucket covers, etc. I immediately threw away as unsuitable. I wanted something durable, durable and preferably waterproof.

And then my eyes turned to fiberglass.

It is no secret that glass cloth (or glass, glass canvassing) do anything you want. Even motor boats and bumpers for cars. The fabric is impregnated with epoxy resin, give it the desired shape and leave until fully hardened. It turns out a strong, water-resistant, easily readdressed material. And that’s exactly what we need.

So we need to make three pancakes and ears to fasten the barbell.

Making individual parts

Pancakes No.1 and No.2

The calculations showed that to produce a sheet 5.5 mm thick you need to take 18 layers of fiberglass. To reduce the flow of epoxy, fiberglass is better cut in advance with circles of the required diameter.

100 ml of epoxy resin was enough for a 21 cm diameter disk.

Each layer should be carefully smeared, and then put the whole pile under the press. The more pressure there is, the better - the extra resin will be squeezed out, the mass of the final product will be a little less, and the strength is a little more. I loaded about a hundred kilos on top and left it until the morning. The next day it turned out to be a pancake:

This is the most massive part of the future coil. It weighs - be healthy!

Then I will tell you how this part will significantly reduce the mass of the finished sensor.

In the same way, a disk with a diameter of 23 cm and a thickness of 1.5 mm was made. Its weight is 89.

Pancake No.3

The third disc did not have to be glued. At my disposal was a sheet of glass textiles of a suitable size and thickness. It was a circuit board from some ancient device:.
What is a metal detector and why it is needed
Unfortunately, the board was with metallic holes, so I had to spend some time drilling them.

I decided it would be the top drive, so I made a hole in it to enter the cable.

Ears for barbells

The residues of the textolite were just enough on the ears to attach the sensor hull to the rod. Sawed two pieces on each ear (to be solid!)

In the ears it is necessary to immediately drill holes under a plastic bolt, as then it will be very inconvenient to do it.

By the way, it’s a fastener bolt for a toilet chair.

So, all the components of our coil are ready. It remains all this glued together in one big sandwich. And don’t forget to get the cable inside.

Build into one

First, the top disc of the holey glass-textolite was glued together with a medium pancake of 18 layers of fiberglass. It took just a few milliliters of epoxy - it was enough to smear both glued surfaces all over the area.

Ear editing

With the help of a jigsaw sawed grooves. In one place, of course, slightly overdone:

To make the eel well lay, made a small slant on the edges of the saws:

Now it was necessary to decide which option is better? Ears can be put in different ways.

Industrial-made coils are more often made on the right version, I prefer the left. I often make left-wing decisions.

In theory, the right way is better balanced, because the fastener is closer to the center of gravity. But it is far from the fact that after the relief of the coil, its center of gravity will not shift in one direction or another.

The left method of fastening purely visually looks more pleasant (IMHO), besides in this case the total length of the metal detector in the folded form will be a couple of centimeters less. For someone who plans to carry the device in a backpack, this can be important.

In general, I made my choice and started to paste. Abundantly smeared with bauxite, securely fixed in the right position and left to freeze:

After freezing, all sticking out on the back side shinged off with a sandpaper:

Entering the cable

Then, with the help of a round top, he prepared the grooves for the conductors, started the connecting cable through the hole and pasted it dead:

To prevent severe inflections, the cable at the entry point had to be reinforced somehow. For this purpose, I’ve got a little thing i’ve come from, here’s a rubber ying:

It was left to glue the third pancake (bottom).

Finish the frame

It took a few milliliters of bauxite and a couple of hours of time to glue the third pancake together. Here’s the result: So I got a hard and sturdy frame, fully prepared for winding the wire.

Winding sealing

A copper enamelled wire with a diameter of 0.71 mm was used as a winding wire. After winding 27 turns, the sensor weighed another 65 grams:

Now the reece had to be legalized somehow. As a smear used a mixture of epoxy resin and finely chopped fiberglass (learned about this super recipe from this article).

In short, I scolded a little glasscloth:

and coolly mixed it with a bauxite with the addition of ballpoint paste. It turned out to be a viscous substance, similar to wet hair. This composition can smear any cracks without problems:

Pieces of fiberglass give the putty the necessary viscosity, and after hardening provide increased strength of adhesive seam.

To seal the mixture properly, and the resin soaked the threads of the wire, wrapped all this tape into a stretch:

The isolent should necessarily be green or, at worst, blue.

After everything was well frozen, I was wondering how strong the design turned out. It turned out that the coil can withstand my weight (about 80 kg).

In fact, we do not need such a super-strength coil, its weight is much more important. Too much weight of the sensor is sure to make itself felt by shoulder pain, especially if you plan to conduct a long search.

Facilitates

To reduce the weight of the coil, it was decided to cut some parts of the structure:

This manipulation allowed to lose 168 grams of excess weight. At the same time, the strength of the sensor has not decreased, as can be seen thanks to this video:

Now in hindsight I understand how it was possible to make the coil a little easier. To do this, it was necessary to make large holes in the average pancake (before gluing everything together). Something like this:

The voids inside the structure would have almost no impact on the strength, but would reduce the total mass of another gram by 20-30. Now, of course, it’s too late to rush, but I’ll take into account the future.

Another way to facilitate the design of the sensor is to reduce the width of the outer ring (where the threads are stacked) by 6-7 millimeters. Of course, it can be done now, but there is no such need yet.

Finishing coloring

I found excellent paint for glass and fiberglass products - epoxy resin with the addition of dye of the right color. Since the entire design of my sensor is made on the basis of bauxite, the paint based on resin will have excellent adhesion, and will lie like a native.

As a black dye, ilyd enamel PF-115 was used, adding it to the desired cover.

As practice has shown, a layer of such paint is held very firmly, and it looks as if the product is dipped in liquid plastic:

In this case, the color can be any depending on the enamel used.

The final mass of the search coil together with the cable after painting - 407 g

The cable weighs separately

80 grams.

Check

After our homemade coil for the metal detector was fully ready, it was necessary to check it for the absence of an internal cliff. The easiest way to test is by tester to measure winding resistance, which normally should be very low (maximum 2.5 Oma).

In my case, the resistance of the coil together with two meters of the connecting cable was in the area of 0.9 Om.

Unfortunately, in such a simple way it will not be possible to detect the inter-revolution closure, so you have to rely on your accuracy when winding. Closure, if there is, immediately manifests itself after the start of the circuit - the metal detector will consume an increased current and have extremely low sensitivity.

Conclusion

So, I think that the task was successfully accomplished: I managed to make a very strong, water-resistant and not too heavy coil of the most throwaway materials. Spending list:

A sheet of glass textolite 27 x 25 cm is free;
Fiber sheet, 2 x 0.7 m - free;
Epoxy resin, 200 g - 120 rubles;
Enamel PF-115, black, 0.4 kg - 72 rubles;
Pet-2 0.71 mm winding wire, 100 g - 250 rubles;
Connecting cable PVS 2×1.5 (2 meters) - 46 rubles;
Cable input is free.
Now I have the task of making the exact same bar. But that’s another story.