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Improved second magnetic field indicator
In this post we will present an improved version of the second magnetic (scalar) field indicator. Our first experience in this direction was presented by here . The new circuit and design made it possible to expand the frequency range of the indicator: from 1 kHz to 30 MHz - and this is what was experimentally verified, but in fact the frequency range can be wider. Also, the supply voltage range became wider: 5-27 V. In addition, the stability of the circuit parameters has increased and its configuration has been simplified. All this became possible due to the use of a high-frequency operational amplifier and the exact values ??of the L1 sensor, from which we will begin our description.
The sensor is a P14/8 armored ferrite core, which means that its outer diameter is 14 mm and its height is 8 mm. In the table, it is underlined with a red line. The board presented below in the section 'Industrial version' is designed for this standard size.
This sensor is assembled as follows. 50 turns of wire with a diameter of 0.1-0.15 mm are wound onto the frame from the armor core. This will be our L1 coil. It is placed in a core consisting of two halves, between which a gap of 0.3-0.5 mm must be made. This gap can be made using thin cardboard paper, cutting out a circle with a diameter of 15 mm, in which you also need to cut a 12 mm hole. Another option for the gap is to cut in half the insulating washer M3, and lay these two halves between the core and the frame, then the gap is automatically formed due to the height difference. This option is presented below in the photo. Putting both halves on the frame, it is necessary to fix this structure using the M2 hole stand and two screws with a thread diameter of 2 mm. Such a rack fits well into the center of the armor core and does not distort the field; its official name is TFF-M2x8 metal rack.
You can do even easier without using a brass stand and washers: just glue the whole structure together. It will not be very technologically advanced and not maintainable, but this option will require fewer parts.
The most surprising thing about this indicator is that a ferrite ring with a wire evenly wound around it can be used as the L1 sensor. Although the sensitivity of the sensor will decrease several times, but the very fact of obtaining data for indication from completely closed magnetic lines of such a ring is of theoretical and practical interest. The diameter and height of the ring should be as small as possible. Rings with a diameter of 8 mm and with a number of turns - 15-20 work well.
The circuitry of this indicator is designed for a wide range of supply voltages: from 5 V to 27 V. In addition, the indicator can be powered by a USB connector or three 'AAA' batteries.
If the indicator is to be powered from a power supply unit, then it must be supplied from the XS1 connector. In this case, the power supply unit can be designed for any voltage in the range from 9 to 27 volts. If the indicator is powered from the USB connector, then the circuit must be powered from the XS2 connector. It also needs to be powered by three AAA batteries connected in series if you use this option. In the last two cases, the DA3 and C6 elements can be omitted from the board.
List of circuit elements:
  • L1 - armored ferrite core standard size P14/8;
  • DA1 - dual operational amplifier MCP6022 . Replacement with other op amps is not allowed;
  • DA2 - signal level meter KA2284 (AN6884, LB1403);
  • DA3 - 5V voltage regulator - L78L05;
  • VD1..VD5 - LEDs blue or red;
  • C1..C2 - polypropylene capacitors with a tolerance of 10%;
  • C3..C6 - ceramic capacitors with a tolerance of 10%;
  • R1..R10 - resistors for 0.065-0.125 W with a tolerance of 5%;
  • SW1 - any latching switch, for example PB22E06 ;
  • XS1 - power connector DC-005 5.5/2.1 .
To tune the circuit, set the two trimmers to the middle position (this is how they are by default). Bring the scalar coil some distance. If this sensitivity suits you, then the setup is complete. If the sensitivity is insufficient, then first slightly reduce the resistance R4, then R9, achieving the desired sensitivity of the device. But it should be remembered that in the absence of a field, the LEDs should not light up.
Industrial version: PCB (open)
The industrial version provides a set of documentation for the industrial production of the board: GERBER file for the production of a printed circuit board, BOM file of the specification of components and a schematic diagram showing the values of the elements. All this allows you to immediately order a PCB, for example, here, and then quickly assemble it.
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