Aby kolega nie pomyślał że, jestem dupkiem bo naskoczyłem na kolegę wcześniejszym postem, postanowiłem troszkę "osłodzić" atmosferę. Zamieszczony poniżej opis , dotyczy najprostszej konstrukcji magnetometru ( miernika wykrywania pola magnetycznego ). Zapewniam że zabawa jest przednia. Krótko o niuansach technicznych urządzenia: 1. cewka - to zwykły transformator 230/12V ( sposób połączenia uzwojeń jest widoczny na foto ) wszystkie uzwojenia w szereg. http://obrazki.elektroda.pl/9254445000_1456330362_thumb.jpg 2. schemat jest stosunkowo prosty, PCB łatwa do odzwierciedlenia. http://obrazki.elektroda.pl/6188136900_1456330476_thumb.jpg http://obrazki.elektroda.pl/6694426500_1456330478_thumb.jpg http://obrazki.elektroda.pl/5070771800_1456330479_thumb.jpg I na koniec ciekawostka, polecam cewkę zawiesić na sznurku długości 2-5m i ok. 1m nad ziemią, ciekawe rzeczy momentami sygnalizuje ten miernik. I UWAGA, TO TYLKO ZABAWKA i DO TEGO CELU SŁUŻY PS. znalazłem w zbiorach oryginalna publikacje z Elektora
PROJECTS
MINI-PROJECT
Magnetometer
Detects even the smallest changes
Rev. Thomas Scarborough
The circuit described in this
article is incredibly sensitive
to changes in the magnetic
field. It can be used to detect
earthquakes, but it can also
function as a car alarm or for
theft prevention. The
construction is
straightforward and only
standard components have
been used in the design.
The author, who lives in Cape Town,
South Africa, originally designed this
circuit to detect small earth tremors
that could be possible precursors to
more violent earthquakes. We know
that earthquakes only occur very rarely in Western Europe, but this circuit
also lends itself for use in several oth-
Figure 1. This oscilloscope trace shows the signals generated when a magnet is moved nearby (see text).
62
er applications. The circuit in question
is fairly simple and it uses an ordinary
mains transformer as a sensor coil. It
is capable of picking up minute changes in the magnetic field strength. It is
so sensitive that it can detect a passing train at a distance of two kilometres. Before we look at the principle of
operation we’ll take a look at several
possible applications for the circuit:
- Theft prevention: fix a neodymium
magnet to your laptop or briefcase and
the magnetometer will immediately
warn you when it’s picked up.
- Car alarm: when the car is moved and
changes its angle to the Earth’s mag-
elektor electronics - 5/2007
7
2
1M
IC1
LM380N
IC2A
C2
6
1
IC2B
R5
100k
2
1
R2
330k
3
4
1
R6
100k
IC2C
5
IC2E
R7
100k
6
1
R4
R3
220k
9
1u
16V
1
8
IC2 = 4069U
C6
C3
C5
470n
100u
16V
C4
470n
470n
IC2D
C1
10u
16V
4
5
3
R1
470k
47k
SENSITIVITY
P1
11
1
10
P2
10k
C7
CENTRE
P3
100k
100u
16V
+12V
R8
10R
1
100u
40V
3
C10
10k
+12V
S1
2
C11
C16
4
C17 IC3
100n 100n 100n
11
14
IC2
7
RESET
C13
C14
C12
100u
16V
100u
16V
3
10
11
12
13
14
15
16
17
18
1
D3
D4
9
D5
10
Trigger
IC3C
8
D6
D7
P4
100k
D8
V-
DIV LO
12
4
78L12
R12
IC3=TL074
IC5
1
IC3B
LED10
LED9
LED8
LED7
LED6
IC4
LED5
LM3914N
LED4
8
LED3
REF ADJ
LED2
LED1
5
IN
6
DIV HI
7
REF OUT
2
1
5
R11
100k
1N4148
IC2F
13
D11
14
IC3D
7
R13
470n
47k
12
13
6
470n
D2
V+
100n
C9
+12V
D1
R10
9
C8
47k
IC3A
3
MD SEL
2
R9
100k
D9
D10
C15
100u
16V
050276 - 11
Figure 2. The circuit diagram shows the large number of amplification stages used. They ensure that even the smallest variations in the magnetic field can be detected.
netic field it will be detected by
this circuit.
- Vehicle detector: approaching cars or trains
can be detected over
a large area around
the magnetometer
due to the vibrations they cause.
- Extremely sensitive vibration alarm:
minute vibrations in
the vicinity can be detected, such as a bouncing
ball on a wooden floor tens of metres away.
- Magnet sensor: the circuit obviously
reacts to nearby magnetised objects
as well, such as a magnetised screwdriver half a meter away, or even an
‘old-fashioned’ 3.5-inch floppy disk.
- Cat flap opener: attach a magnet to
the cat collar and when the cat comes
close to the cat flap it will be opened
automatically by the circuit.
5/2007 - elektor electronics
Concept
There are basically two types of magnetometer: ones that give an absolute
value of the magnetic field strength
and others that show the change in
the field strength. This circuit detects
the variations.
Figure 1 shows an oscilloscope trace
of the output of the circuit, when a
strong loudspeaker magnet was moved
at a distance of about a metre away
from the sensor (an old mains transformer). The magnet is first tilted one
way (at 0.5 s), then the other way (at
2.5 s), then the magnet is shaken backwards and forwards (from 5 to 6.5 s)
and finally the magnet is slowly rotated. It is interesting to see that you can
tell from the shape of the waveform in
which direction the field changed.
When this circuit was first designed
the author wanted to create a seismometer that was inexpensive and
could operate in a stand-alone fashion
(i.e. without the use of a PC or data
logger). This resulted in a fairly simple
circuit that used standard components,
including a mains transformer as sensor and an LED bargraph as indicator.
There is also a trigger (alarm) output
that turns on when the full scale of the
LED bargraph is reached.
Practical circuit
The most important part of the magnetometer is the detection coil. In the
prototype a mains transformer was
used (230 V/12 V, 2 A), but in theory
nearly any transformer or coil could be
used. The author found that the abovementioned model worked well and
gave the circuit a very good sensitivity.
The primary and secondary windings
of the transformer were connected in
series (and in phase) to increase the
sensitivity.
The coil is connected to the inputs of a
type LM380 opamp (see Figure 2). This
is really a power-amp IC that can deliver 2.5 W, but it turns out to be just
right for this circuit because it has a
fixed gain (50 times) and the output
automatically settles to half the supply
63
PROJECTS
MINI-PROJECT
Figure 3. A PCB has been designed for the circuit to make the construction easier
voltage without the need for separate
bias resistors at the inputs.
The low-frequency signal is then amplified further using a number of gates
from an unbuffered 4069UB CMOS IC.
An unbuffered CMOS inverter can be
made to function as an amplifier with
the addition of a resistor between the
input and output. In this case four inverters have been used as sequential
amplifier stages (IC2A/B/C/E) with
passive RC low-pass filters in between
(R5/C3, R6/C4, R7/C5). This provides
an enormous gain to the output signal
from the LM380. All the filter stages
(another two follow later on) reduce
frequencies above about 20 Hz, mainly
to suppress interference from mainsborne signals.
Next, IC2D adds another dose of gain
to the signal, where the DC offset to
the input of the gate is provided by po-
tential divider R4/P2/P3. After another
RC filter (R9/C9) the signal is buffered
by IC3A and fed to a halve-wave peak
rectifier (D11/C13), which supplies a
DC voltage to the input of the LED bargraph circuit. In this way a peak-hold
function is implemented, which shows
and holds the largest measured value
on the display. Pressing S1 resets the
LED display. If you don’t need this
peak-hold function you can replace D11
with a wire link and leave out C13 and
S1. All changes in the signal level will
then be shown on the LED bargraph
display.
The rectified signal is fed via a buffer
(IC3B) and a final RC filter (R11/C12) to
the input of the well-known LM3914
(IC4), a much used LED driver IC that
contains all the electronics to drive a
10-segment LED bargraph display (D1
to D10).
The reference input of the LM3914 has
been set such that the signal strength
is indicated relative to LED D5. LED
D10 is on continuously to indicate that
the circuit is powered up; it may be left
out of the circuit if not required.
Opamp IC3C provides a trigger output
that generates a logic high when the
LED for the strongest signal level lights
up (D1). P4 is used to set the trigger
level.
The supply to the circuit is provided by
a 12 V regulator, since any mains ripple
on the supply line would be disastrous
for the small signals we’re amplifying.
The power supply can be any mains
adapter that has an output voltage of
about 15 to 20 V DC (50 mA is
sufficient).
Construction and setting up
COMPONENTS
LIST
Resistors
R1 = 470kΩ
R2 = 330kΩ
R3 = 220kΩ
R4,R10,R13 = 47kΩ
R5,R6,R7,R9,R11 = 100kΩ
R8 = 10Ω
R12 = 10kΩ
P1 = 1MΩ preset
P2 = 10kΩ preset
P3,P4 = 100kΩ multiturn preset
Semicondcutors
D1-D4,D6-D10 = LED, red, 3mm
D5 = LED, green, 3mm
D11 = 1N4148
IC1 = LM380N-8
IC2 = 4069UB (unbuffered version)
IC3 = TL072CN
IC4 = LM3914N
IC5 = 78L12
Capacitors
C1 = 10µF 16V radial
C2 = 1µF 16V radial
C3,C4,C5,C9,C12 = 470nF
C6,C7,C10,C13,C14,C15 = 100µF 16V
radial
C8,C11,C16,C17 = 100nF
Miscellaneous
S1 = pushbutton, 1 make contact
L1 = coil, e.g. discarded mains transformer
230 V / 12 V @2A
PCB, ref. 050276-1 from
www.thepcbshop.com
64
With the help of the PCB artwork
shown in Figure 3 it shouldn’t be too
difficult to make a board or have one
made for you. Make sure that you get
the 8-pin package for the LM380 since
the PCB has been designed for this.
Keep in mind that you need the unbuffered version of IC2 (4069UB), otherwise the circuit will definitely fail to
work! Use IC sockets for all ICs to make
the construction easier and to help
with any potential faultfinding. All resistors are mounted vertically. The reset switch is connected to the board
via a pair of wires.
The circuit can be mounted in an enclosure that has suitable cutouts made for
the LEDs, the reset switch and the
power connector.
An old transformer works very well as
a detector ‘coil’. It should have all
elektor electronics - 5/2007
windings connected in series, and you
should take care that they are all in
phase, otherwise the sensitivity will be
reduced. Two short pieces of wire
should be used to connect the transformer to the board.
Once all components have been soldered onto the PCB we can connect the
mains adapter and start with the adjustments. First set the sensitivity control (P1) midway, as well as P2. Now
turn P3 until the centre green LED (D5)
lights up on the LED bargraph. During
normal use, P2 can be used to adjust
the display (you could also use an ordinary potentiometer for this) as and
when necessary. Especially when the
sensitivity is set to a high value you’ll
find that the null-point can vary. When
the sensitivity is lowered via P1 it
should be possible to obtain a stable
setting that shows very little drift.
The final adjustment is the trigger level, set via P4. This isn’t critical, and
should be set such that IC3C switches
reliably when LED D1 lights up and
switches back again when D1 turns
off.
Application tips
At the start of the article we already showed
a few possible applications for this magnetometer. Most of these
are fairly straightforward
and there is no need to
give detailed instructions. It
is important that you should
first ‘play’ a bit with the circuit
to find out how sensitive it is,
what it reacts to and what the
best setting is for P1. Whilst experimenting you should have as few metal or magnetic materials as possible
near the circuit, since they interfere
with its operation.
You can make a simple seismometer by
hanging an old loudspeaker magnet
from the ceiling using a long piece of
string and placing it just above the
transformer. P1 should then be adjusted such that the LED bargraph remains
just unlit. To make a vibration alarm
that can detect passing traffic you
should attach a magnet to the end of a
long ruler. The other end of the ruler
Figure 4. For
the prototype in the lab
we used an old PCB-mounted transformer
with all windings connected in series.
should be fixed to a large surface and
the transformer should again be placed
just below the magnet. You’ll be
amazed by the distance at which vibrations can be detected with this simple circuit!
(050276-1)
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5/2007 - elektor electronics
65