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Offline kayakpirateTopic starter
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« on: August 09, 2009, 12:01:47 pm »
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 I came across this while looking at magnetic sensors to build a differential magnetometer.
 They cost around $12 each on fleabay.
 If I buy 2 of them and set them on an old broom handle (perfectly aligned) and stuck a $6 meter on it, that would be a $30 mag.  Wink

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http://www.memsic.com/data/products/MMC212xM/MMC212xM.pdf


 Tell me if you think this would work and if anyone can give a schematic as to how & where to hook up meter, etc...

they seem to have the reset circuit and some offset correction built in, but how to combine two of these with a meter, etc.. is not clear to me.

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Offline GoldDigger1950
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« Reply #1 on: August 09, 2009, 02:56:57 pm »
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It is an interesting device and quite sensitive but cannot be easily put to use. The device uses the I2C bus (pronounced "Eye Squared See") to communicate data which in turn requires an embedded processor to control. You would not be able to hook them up with a meter and use them without some serious circuitry and programming.

Sorry.

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It's all about that moment when metal that hasn't seen the light of day for generations frees itself from the soil and presents itself to me.
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« Reply #2 on: August 09, 2009, 08:54:56 pm »
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 Thanks for raining on my parade. lol
 I was wondering what that I2C was, now I know.
 Good thing I didn't order them. But they are interesting and it's strange that the entire assembly costs half as much as a comparable magnetic sensor.

Thank you for the information.

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Offline GoldDigger1950
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« Reply #3 on: August 09, 2009, 09:12:00 pm »
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Oh, I do like them and will probably source them here in Australia or order from America. Retired electronics engineers never really give up designing, you know. I am well versed in I2C control for memory and real time clocks so I am going to look into this device more seriously. Rather than use a wooden pole, I believe I will think about using aluminum channel or tubing. See? The wheels are turning and if you look closely, there is a bit of smoke coming from my ears.

I hate it when that happens.

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« Reply #4 on: August 10, 2009, 01:08:47 am »
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kayakpirate;

I will look at the PDF and get back to you..... IF I can find this spot again, if it doesnt get removed....?

I should have read further down and seen the I2C... then I would have KNOWN it was an April Fool joke. But I wont say its impossible, with reservations.

I have read about Hall effect devices being used, but they were not I2C, you might have the germ of an idea.... by accident. Build a PIC processor to  accept the I2C data and use it!

Speaking of I2C, it never started out as a serial protocol, it started out as a type of semiconductor function, or as related to function. Example: the 7400 quad NAND, when it came out, was touted as having I2C inputs, as opposed to DTL.... Diode/Transistor Logic. Now, it means something else, altogether. (No, TTL does NOT mean /time to live,/ as one PIC author thinks.... Transistor/Transistor logic.)

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« Reply #5 on: August 10, 2009, 09:50:24 am »
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Hi,

The specs of this chip in its datasheet say that its sensitivity is 512 counts per Gauss.
Used as a Gaussmeter, that should be a good choice but,as a magnetometer to be used to measure the earth field, it is much, much too insensitive.
One Gauss is 10-4 Tesla or 105nT (or Gamma), thus, one single count represents 100,000/512 = 195nT. This is at least one hundred times too high to be of any use in the field.

Willy

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« Reply #6 on: August 11, 2009, 05:58:41 am »
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willy;

Are you sure?  Funny

Do you mean per bit or the full 9 bit count?? Waveing

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« Reply #7 on: August 11, 2009, 07:21:39 am »
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The ADC returns a count of 12 bits for each of the vectors X and Y (see page 9). This full range of values covers the range -2G to +2G (=4 Gauss) (see bottom left of page 1). 12 bits are able to represent 2048 distinct values, 2048counts/4G = 512 counts per Gauss (or 100,000nT).
Thus, it is not possible to distinguish magnetic field variations of less than 100,000nT/512 = 195nT.
Sorry to disappoint you but these calculations are right according to the specs of the datasheet.

Willy

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« Reply #8 on: August 11, 2009, 07:36:29 am »
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Please note also that this is the specs @ 25?C and 3V. Any slight variation of these two parameters varies the measurement values as it is for fluxgate sensors. The variation in temperature is +- 1100ppm/?C.
These (annoying!) dynamic characteristics do not exist on the PPM principles.
Willy

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« Reply #9 on: August 11, 2009, 08:18:32 am »
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Willy;

Why not a GRAVITY sensor, instead of magnetic field.

I have a better idea, reflect a laser off a mirror, a meter away and measure the phase change... as related t gravity change, there should not be any temperature effect, but then the gravity variation may not affect phase, enough, or at all....so, any body got info on how gravity affects coherent light??

Willy?

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