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Magnetic field meter circuit

2018-02-27 09:35  
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These days when electronic circuits can be found almost anywhere, there is, some people fear*, a different kind of environmental pollution. They call it electrosmog (the word does not appear in any dictionary – not even technical ones), but in this article we will call it stray magnetic fields (SMFs). Some ‘experts’ think that SMFs may affect the physical well-being of people. If you believe that these experts are right, the magnetic-field meter described will help you find sources of SMFs and determine their strength. These findings may help you reduce the fieldstrength.

The input amplifier, based on IC1a, ensures that the signal from the induction coil, L1, is amplified x 101. The coil is terminated into a high impedance, so that its output is buffered by the op amp. The integrator consists of IC1B, another of the four op amps contained in IC1.The (active) rectifier, based on IC1c, is, in fact, a differential amplifier that lessens the average voltage by the output potential of the integrator. Since the op amp is powered asymmetrically, the output is a half-wave rectified alternating voltage. This voltage is averaged by R16-C6 or, in case a DVM is used as the meter, by R18-R20- C7. The form factor (2.22) is corrected by the rectifier. The level matching is purposely carried out by the rectifier since this op amp has a much larger swing than IC1a or IC1b.

Magnetic field meter circuit design

The principle of the present meter is shown in the block diagram in Figure 1. The induction coil used to detect the magnetic field is represented by an alternating- voltage source, V1, whose average output is 1 μV. The output of the source is amplified x 101 by op amp X1. The op amp is linked to integrator X2 which provides frequency-dependent amplification. For direct-voltage signals this is 1000, for high-frequency signals it is 0. The cross-over frequency is chosen so that the amplification is uniform over the range in which magnetic induction is to be measured (40 Hz – 10 kHz). Feedback network R4-R6 automatically ensures that the circuit has a stable d.c. operating point at all times. This makes it possible for relatively inexpensive op amps to be used. Also, the internal attenuator ensures that the maximum d.c. amplification is x 101 (1 R6/R5). The value of R6/R5 also determines the lower limit of the frequency range.

Magnetic-field meter circuit diagram

The circuit diagram of the meter is shown in Figure 2. It consists of an input amplifier, integrator, automatic offset correction network, rectifier with d.c. suppression, display and associated drive, power supply, and a socket for connection to a digital voltmeter (DVM).

Op amps IC1a and IC1b carry a pure sinusoidal signal that alternates symmetrically around a direct voltage of 3 V, whereas that of IC1c alternates around 0 V. This means that this op amp can handle an amplification of x 2.2 much better than the earlier two. The drop across C6 is used by the display driver, IC2, to represent the strength of the magnetic field. The driver has its own reference-voltage source. This 1.25 V source is also used to derive an auxiliary voltage for op amps IC1a and IC1b. The potential at node A is [ ( R14 R15 ) / R15 ] x 1.25 = 3 V.

The minimum voltage at which IC2 provide full drive is 1.2 V. Since the IC is driven by an averaged potential, the signal level required for full drive is 1.2 x Pi = 3.77 Vpp. Because the signal amplification takes place in the rectifier, that is, the op amp with the largest drive range, a drop in battery voltage does not immediately affect the accuracy of the meter. The display driver controls ten LEDs. The diagram clearly shows which LED lights at a given fieldstrength. When D10 lights, the measured fieldstrength >=2.3 μV, rather greater than the upper limit specified in MPRII (250 nT). If the meter is linked to a DVM, this must be set to its 200 mV direct- voltage range. The measurement range is then 50 nT – 2 μT. Measuring levels below 50 nT is not possible owing to the noise floor.

Magnetic-field meter construction

The circuit is best built on the printed circuit board shown in Figure 3, which reduces the necessary work to a minimum.

Mangneto field meter PCB

The induction coil is a DIY job. The core on to which it is wound is made from two strips cut off the PCB. These strips are formed into a ‘sandwich’ separated by four 1.3 mm dia. solder pins in the indicated positions. Note that the track side of both strips must face the motherboard. The broad strip has two solder pads to which the coil terminals are soldered. The third pad merely serves to increase rigidity. Close-wind 121 turns of 0.2 mm enamelled copper wire on to the core. If this is done carefully, the winding will consist of exactly five layers. Place the coil against the motherboard in such a way that the three copper pads at the edge of the motherboard coincide with the corresponding pads on the coil. Solder the coil to the motherboard.

The remainder of the wiring is straightforward. First lay the wire bridge at the centre of the board. After the two solder pins have been soldered into place, solder the resistors and capacitors on to the board. Mind the polarity of the electrolytic capacitor. The ICs may be soldered directly to the board, but sockets may be used as well. The last components to be placed are the LEDs. These diodes are in three groups, each of a different colour. The green of D1-D3 indicates a safe level; the yellow of D4-D6 a dubious level; and the red of D7-D10 a risky level. After switch S1 and the 9-V battery have been connected, the circuit is ready for use. It is best to build it into a suitable case to make a compact measuring instrument. The meter need not be calibrated since the measurement error is negligible, provided the correct components have been used and the induction coil has been wound carefully.

Circuit Parts list

R1, R9, R11*, R14 = 10 kOhm
R2*, R5, R7* = 1 kOhm
R3*, R6, R16 = 100 kOhm
R4, R8 = 1 MOhm
R10, R12 = 22 kOhm, 1%
R13 = 100 Ohm
R15 = 6.8 kOhm
R17 = 3.9 kOhm
R18, R19* = 39 kOhm
R20 = 82 kOhm, 1%
* = 1%

C1, C7 = 100 μF, 10 V, radial
C2 = 10 nF, metallized polyester film, 5%
C3 = 4.7 μF, 10 V, radial
C4 = 10 μF, 10 V, radial
C5 = 47 pF
C6 = 2.2 μF, 10 V, radial
C8 = 100 μF, 16 V, radial

L1 = see text

D1-D3 = LED, green, high efficiency
D4-D6 = LED, yellow, high efficiency
D7-D10 = LED, red, high efficiency
D11 = 1N4148

Integrated circuits:
IC1 = TLC274
IC2 = LM3915

S1 = single-pole switch with make contact
PC1-PC4 = soldering pin 1.3 mm dia.
Bt 1 = battery 9 V
Enclosure as suitable

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