The Pirate and other similar impulses are good for everyone – high repeatability, simplicity of the circuit, good parameters. But something is missing, for example, discrim – the ability to distinguish between non-ferrous metals and iron. Therefore, 15 years later, wanting to give a metal detector to his 8-year-old son (for the sake of joining this most interesting business), he again assembled the famous Volksturm-S .
I think this is one of the simplest MDs, capable of discriminating the found metal, and the most pleasant memories of its assembly and operation have been preserved. The circuit, with its apparent complexity, is so easy to start that it was tuned without any problems all the time without any oscilloscopes and other amateur radio exotics. I just measured the voltage at a pair of points and the total current consumption with a multimeter. Naturally, there will be some difficulties with the manufacture and resonance of the DD search coil, but almost dissertations have already been written about this.
So, back to the diagram. It has been slightly changed – a two-color indication of metal (iron-flower) has been introduced and ULF has been improved. In the old version, it was with an error, because of which it was not set to an exact zero and the overall sensitivity fell, but here the problem has been fixed.
There is also a small addition to the circuit to improve selectivity, but I have not personally tested it – this can be done at any time in the form of a small plug-in scarf.
Next is the printed circuit board. It has also been improved in terms of placing color and sound indication nodes on it, just do not forget to transfer the emitter Q2 and the 100 Ohm resistor from Q3 to ground (they are on the minus on the board).
The speaker here is small, low-impedance. If the current consumption with it is large, increase the resistance value of 10 ohms to 40-60 ohms. But so that the sound is not completely quiet (although the volume is nothing special, so as not to attract attention). Also pick up current limiting resistors on the LEDs, on a bright day they can be seen much worse than in the room when setting up.
Setting up a new Volksturm-S metal detector
First of all, check the power supply floor on the LM358 voltage divider. If everything is fine, supply power to the ULF with a generator and a target signal amplifier. By turning the threshold knob, you will see how one or the other LEDs light up in its extreme positions. And the tone of the sound changes. And in the middle position (it is caught quite accurately), everything falls silent. So this is the norm here.
Now the generator with coils. We put any clock quartz and listen with satisfaction to the 8 kHz squeak on the resistor R23 or the emitters of transistors Q3, Q4. They wound, tuned, mixed, filled the coils in the case (for it I took a round plastic stand from the CRT monitor).
Everything was fully connected and checked on the table how it works here. And the MD works like this: the reaction goes to the MOVING metal, so even putting it on a reinforced concrete floor, you won’t get any special problems. The coil will not “see” that armature, only metal things that you wave in front of a motionless coil.
We wave aluminum – there is a glow of the red LED and the sound of one key, we wave iron – the blue LED flashes and the sound is different, with a howl, as it were. In general, metal can be distinguished without problems.
Assembling the metal detector in the case
Well, the last stage is the installation of a configured device in a suitable case. For him, I took an aluminum box (just the shielding will be for the board) from an external old HDD. It is a little big in size, but will be as a reserve for possible improvements in the future.
There are 2 LEDs on the front, a speaker hole, and a flair control with a threshold that is combined with a power switch.
Speaking of food. The Volksturm-S draws about 30 mA in silence, so with 3 18650 batteries installed, you can literally walk with it for days. How convenient it is compared to branded ones, for example, Garrett 250, where 8 AAs landed in a couple of hours!
The batteries are installed in a common holder removed from the case. On the rear panel of the metal detector there are 2 RCA sockets for coils.
Moreover, an interesting effect is observed: if the connection is reversed, discrimination disappears. Probably, in order to avoid this, it is worth using some kind of single double connector – you yourself will come up with something there.
The rod is an ordinary plastic water pipe 500 mm, the body of the metal detector board is attached to it with a clamp and a cable from the search coil passes inside.
That’s all. Compared to other similar devices of medium complexity, Volksturm-S starts with one turn (with a resistor), which is why I once again assembled it, and not any others. And if you have already outgrown simple circuits – make Volkshurm, which, although inferior to Quasars and Fortunes in terms of sensitivity, is much easier to assemble and configure on your own!
Video of MD work