This is a simple magnetic field flux meter I designed and modified over the time. This final version is the simplest I could get to.
The reason why I designed it is very simple and lays on the fact that if there’s some uncertainty about effects on the humans of the high frequency electromagnetic fields generated by celllular phones, much less uncertainty is about the damage caused by low frequency magnetic field generated by the mains (220/110 Vac) surrounding us (LFMF).
I wanted something that could gauge the field with some good approximation but most importantly I wanted to spot “hot points” around my home so I could decide where I could place safely our beds, as an example, without worrying of LFMF causing in the long run related disease. I’ll discuss these issues later on.
One of my previous designs included a small meter but then I decided I would use a regular multimeter.
The principle of magnetic filed flux metering is simple and based on basic physics : a variable magnetic field induces a voltage at the ends of a coil ( Faraday’s law of induction ) of electric wire. I used this principle to gauge the field and through amplification meter it on a regular multimeter. Total cost : very low, a simple operational amplifier, two (or one, see over) a few resistors, a plastic case and switches and that’s it
The circuit is very simple : a better view of the schematic.
The coil voltage is amplified by a simple instrumentation amplifier built around an Analog Design AD708 : a very crude but effective design as the coil is close to the amp. AD708 is an excellent AmpOp, other models are fine as long as they stand the +/- 9V supply : a regular LM358 should do just fine.
I designed two ranges of 0.1μT/V and 1μT/V which are pretty sufficient to gauge any field around home. A regular multimeter set to 10 Vac full scale is fine. Given a voltage reading of say, 3Vac in the 0.1μT/V scale equals a flux of 0.3μT while in the 1μT/V scale it would equal a flux of 0.3μT.
The coil is actually the most tricky part of the circuit yet it is simple to design.
I used a JVC loop antenna that came with my stereo. I never used the loop antenna with my stereo and decided to design my magnetic flux meter coil around it but any piece of plastic some wire can be wound on is fine. The size of my loop antenna is 0.1m x 0.12m circa.
It must be of plastic and no large metallic parts should be close to the loop to interfere with the field.
First I measured the loop size and put it into the equation (see “how I designed the coil” at the bottom of this post) to find out the number of turns I needed to wind over the loop antenna plastic core.
The number turned out to be 132 : we have 50Hz mains here in Europe, the number of turn depends on the area of the loop and the frequency of the field to measure as well, that is the frequency of the mains.
I removed the existing wire turns from the JVC loop antenna and wound the new turns with 0.2mm diameter wire. The gauge of the wire isn’t really an issue as what matters is the area enclosed by the loop and the number of turns.
The circuit fits easily a small chunk of perfoard. The OpAmp is on an 8 pins socket as well as the switched capacitor voltage inverter. This latter part of the circuit is necessary to generate the negative supply voltage for the AmpOp and is not required if two 9V separate batteries are used. I went for the 1 battery+switched capacitor voltage inverter option.
This is a closeup of the coil enetering the plastic box : the thin copper wire is connected to a chunk larger insulated twisted pair. It must be twisted to null-out the effect of direct coupling of the field to the wire.
A few notes on what to check around your home.
First, I’d check those walls you are not sure what’s on the other side. A large electric oven ? A washing machine (well, you might guess by the noise) or some other large appliance that radiates when you’re sleeping ? Maybe the back of a large crt TV.
You might experience that regular new crt computer monitors don’t radiate much from the front (keyboard) side. The back isn’t that good.
Very old monitors and TVs perform very badly from this point of view but it is not a good reason to dispose of them : just find a better position for them so as the field goes where no one stands for long time (co-workers, say).
How I designed the coil
From Faraday’s induction law 
a variation of the magnetic flux through a closed loop of wire generates a voltage at the loop’s ends that is tied to the time rate of change of the flux.
In the equation above, 
is the product of the component of 
along the perpendicular to the loop, times the value of the surface. In our case I assume the field is perpendicular to the coil’s surface so is 
where 
is the frequency of the field (in Europe 50Hz, in other countries 60Hz) and t 
is the time (in seconds) but we don’t have to worry about these two latter variables. is the product of the surface times the perpendicular component of the filed that is 
With a square loop the surface 
, 
being the sides of the square loop in meters. The loop can be any reasonable shape : if it is a regular round loop it is 
;again, r must be in meters.
For a coil made of 
N turns the final equation becomes
Evaluating the derivative yields :
is the value to gauge: we want to measure it via
.
I’ll set the top value for to 1μT that is 10-6 T and want this top value to generate = 500μV at the ends of the coil.
My coil has sides equal to 0.1m and 0.12m approximately.
f is 50Hz in Europe and 60Hz in other countries.
Solving for N:
where
The value of 50Hz must be changed where the mains’ frequency is 60Hz (US, as an example).
With my JVC loop antenna I get
turns of insulated copper wite.
The wire gauge isn’t much important. It is 0.2mm diameter in my case
23 responses so far ↓
DIY: Magnetic field flux meter | The Kevin Pipe // May 17, 2008 at 7:12 pm
[...] version, but that isn’t any fun. All the schematics and build information can be found on the 5volt website. If you decide to make one, don’t forget to check out the “How I designed the coil” page. The [...]
Daily DIY Network - Science Projects Plans Guides » Blog Archive » DIY: Magnetic field flux meter // May 17, 2008 at 7:13 pm
[...] version, but that isn’t any fun. All the schematics and build information can be found on the 5volt website. If you decide to make one, don’t forget to check out the “How I designed the coil” page. The [...]
DIY: Magnetic field flux meter // May 17, 2008 at 7:14 pm
[...] version, but that isn’t any fun. All the schematics and build information can be found on the 5volt website. If you decide to make one, don’t forget to check out the “How I designed the coil” page. The [...]
DIY: Magnetic field flux meter » Developages - Development and Technology Blog // May 17, 2008 at 7:27 pm
[...] version, but that isn’t any fun. All the schematics and build information can be found on the 5volt website. If you decide to make one, don’t forget to check out the “How I designed the [...]
Nick // May 18, 2008 at 7:08 am
Wireless World published something very similar a few years ago – I built it and was surprised at just what did have a strong field associated with it – there’s less talked about spurious fields and their effect on helath these days, but it wouldn’t hurt to be careful !!
Richa // May 18, 2008 at 7:59 am
Some useful information at the end……
but overall,i dont feel it was that useful for me, as i was looking for a college project and this wasnt exactly that.. although i might consider using it at my own home.. worthy of it..yeah!!
Abrar // May 18, 2008 at 11:06 am
This one is an excellent project and ill definately try it out once ……..
regards,
Abrar.
chu ma // May 19, 2008 at 9:38 am
Thats zero zero at the end of the letters.
What if I don’t know the symbols you are using?
will this work with DC current I live near a subway that is above ground (sounds contradictory but it’s true.
also how about laptops, or computers between your legs?
admin // May 19, 2008 at 10:10 am
Richa, in what the project isn’t exactly what you where looking for ? Need more directions ? details ?
admin // May 19, 2008 at 10:15 am
Chu ma, I don’t really understand your comment on symbols, please be more specific (maybe an example?)
What is supposed to cause illness are AC (low freq) fields. DC fields are OK. But DC fields are AC during the transients : with the underground look at the needle of the multimeter when the cabs are braking/accelerating. These fields shouldn’t be dangerous but it’s interesting to notice physics at work (increasing/decreasing current in trains supply-> variable magnetic field -> variable Ac current).
It is also interesting to hunt for power lines below ground and transformer stations.
Dave // May 19, 2008 at 4:03 pm
Interesting project. I did something similar a while back. For the pickup coil, I used the coil from a relay (with the armature removed). I’m told that a ferrite bar antenna from an AM radio also works reasonable well. Instead of a multimeter, I used a LM386 op-amp to drive a pair of headphones. It made for a good relative strength indicator, although I never calibrated it for absolute accuracy.
Such systems won’t respond to DC fields, though. For that (or, for the absolute value of an low frequency AC field), I like the Allegro A1321 Hall Effect Sensors:
http://www.allegromicro.com/en/Products/Part_Numbers/1321/
With one of those, I can use a sensitive multimeter to get absolute field strength values (Hint: Put the meter across a bridge circuit formed by the sensor and a potentiometer to allow you to null the zero field output.).
Dave
Devin // Jul 3, 2008 at 2:06 am
So is the critical feature for the AD708 and LM358 the low offset voltages? Are there any other requirements for these amps that I would need to know of if I was going to substitute?
admin // Jul 4, 2008 at 10:28 am
Actually the LM358 do not have a low offset voltage (3mV circa) compared to the AD (30uVcirca). I used the AD because I had it in my drawer and it also performs well. The fact is this is not a precision instrument so an amp op with an offset voltage in the range of the mV and a max supply voltage of at lest +/- 15V should do. As we are working with AC the DC offset voltage should be averaged out by the AC multimeter.
For a better instrument I’d use a loop with a smaller diameter and a better 4 Amp differential amplifier or a single monolithic diff amplifier (like and AD620). Guard rings around the input pins should also be considered as some better PCB with capacitors for supply decoupling.
Ciao
cooperman // Jul 12, 2008 at 11:40 am
Brilliant.
Well presented article describing a simple solution to a relatively complex measurement.
Cooperman.
Bryan // Jun 2, 2009 at 11:43 am
Hi, Your post of the low frequency fluxmeter is good. I am trying to do up a project. To read 2 permanent magnets, ferrite and neodynium. Compare them when heated up to 30-40-50 degs.
Using your schematics, can i safely read values from a search coil of maybe 80 turns? The finest wire i can get is from a relay, but i don’t know what is the guage size.
Please do help me.
admin // Jun 3, 2009 at 8:14 am
Ciao Bryan, the circuit measures variable fields. Your magnets must be part of variable magnetic circuit. I might be missing something here of course.
Ciao
admin // Jun 3, 2009 at 8:16 am
@bryan, almost forgot : the gauge can be any reasonable gauge, it doesn’t need to be a fine As possible, just strong enough not to break when winding around the loop. The number of turns can be calculated with the formulas and depend on the loop.
Ciao
A.
Bryan // Jun 16, 2009 at 9:55 am
Thanks for tip yah… Let me bust my brains.
Bryan // Jun 16, 2009 at 9:58 am
Oh… Also just to fill in the blanks. It is named as a search coil, used to detect the total flux of the surface of the magnets… Static field measurement is what i require anyway.
munsif ali from pakistan // Nov 2, 2009 at 1:29 pm
hi, I am munsif ali from pakistan. while studying electro magentic waves , I got an illustration about magnetic and electric field, composing electro magnetic waves. from canadian space agency website, One page explained emmission of electro magnetic waves. and an example of a metal wire was shown.
The definition of flux goes as follows, It is the rate of flow of fluids, energy and particles,
In the illustration, it was explained that when a current oscillates at 5.4 GHZ it produces electromagnetic waves, made up of magnetic field and electric field. where as movement of electrons was illustrated separately in green dots., now I have a question, with regard to illustration of canadian space agency regarding electromagnetic waves, how will you explain magnetic field flux. in that illustration. kindly contact on my e.mail.
admin // Nov 5, 2009 at 12:40 pm
@Munsif Ali thanks for writing.
i’m not sure what’s the picture you mention, nonetheless I made some research at the Canadian Space Agency and found a picture ( http://www.asc-csa.gc.ca/eng/educators/resources/radarsat2/part1_background.asp ) that might be like the one you mentioned.
To me the magnetic flux is the product of component of the magnetic field perpendicular to the surface of the loop (coil) (the blue sinusoidal varying lines in the second picture from top) by the surface of the coil (crf wikipedia http://en.wikipedia.org/wiki/Magnetic_flux ) . In our case the coil is placed verically and perpendicularly to the blue lines and the flux is the product of total magnetic filed entering the coil times the area of the coil.
Hope this helps
Thank you
A.
St.Jimmy // Jan 2, 2010 at 3:57 am
I have that same antenna, so in America, I’d just plug 60 in where the 50 is, right? So, 500*10^-6/2*3.141*60*(.1*.12)*B cos(2*3.141*60*t), or .00000500/4.52304*B cos(376.92*t), right?
But, I don’t understand what B and t are. Can you help me out?
admin // Jan 3, 2010 at 8:41 pm
@St.Jimmy
Hello, yes, correct : place 60 where 50 is in my case.
Also, Bcos(2pi f t) equals 10 exp-6 Tesla
You should get a total of 110 turns with my same plastic JVC core.
Best
A.
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