The chain of elements R1 and HL1 serves solely for visual indication of the mode of switching on the device and is not mandatory. Resistance R1 is selected depending on the type of LED and the resistances of other circuit elements.
This unit (demagnetizer), as the name suggests, is used to demagnetize various metal objects, the parasitic "magnetism" of which negatively affects their functionality. Such objects can be, for example, magnetic heads of tape recorders or tools - screwdrivers, tweezers, scissors.
The principle of operation consists in the action on the object of an alternating magnetic field of sufficient intensity with a gradually decreasing amplitude.
The easiest way to get such a field is using a small mains transformer, the core of which has been converted into a "non-closed" one. To do this, the core should be disassembled and then reassembled, leaving only the “package” of W-shaped plates and removing all “straight” plates. How it will look can be seen in Figure 1.
In addition, when disassembling the transformer, you should remove (wind up) its secondary winding, leaving only the primary, designed for 220/230 volts. In some cases, this winding can be rewound if it is damaged or wound with wire. too small a cross-section (for low-power transformers this can be a wire with a cross section of the order of 0.1 mm. sq.).
The fact is that during operation, such a transformer will heat up quite significantly and an increase in the cross section of its winding can somewhat facilitate work in this mode. The number of turns depends on the size and power of the transformer and for small-sized specimens it can reach two or more thousand turns of a thin wire.
If it is difficult to wind a new winding, or it is not possible, there is an option with the use of a ballast (damping) resistor, which will help "dissipate" excess power. In this case, of course, the power of the magnetic field itself created by the transformer will also decrease somewhat. But with a reasonable selection of resistance, this option can still be justified.
The resistance of the damping resistor depends on the transformer, its size (power) and the resistance of the primary winding. You can select it empirically, starting from a value of 1 kΩ and gradually lowering it, to achieve an optimal ratio of the degree and speed of heating the winding and the strength of the generated magnetic field. The power of this resistor must match the power of the transformer itself.
In the above circuit, as you can see, three MLT-2 resistors of 1.2 kΩ are connected in parallel. Their total power dissipation is 6 watts. During operation, these resistors will heat up quite significantly, but within acceptable limits. In this case, the power of the magnetic field created by the transformer will be quite sufficient, and the heating of its primary winding will significantly decrease.
In any case, this device should not be kept turned on for more than 5-10 seconds, therefore, it is better to use the S1 power button without latching in order to exclude the possibility of turning it on for a long time.
This time is quite enough to demagnetize any object and exclude possible overheating of the device.
If you need to demagnetize several objects, or repeat the demagnetization process, you should take a break of 1-2 minutes before turning it on again. The demagnetization process itself is not complicated. Bring the device to the object for demagnetization at a minimum distance, press the power button and, keeping it pressed, slowly and smoothly remove the device at a distance of 40 - 50 cm (not less).