# Simple magnetometer circuit

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In case anybody in motivated, I developed a simple magnetometer (based on an Allegro chip) for use in measuring the field strength of guitar pickup magnets. Note that this circuit is intended to drive a high-impedance load like a digital voltmeter or the like.

Joe Gwinn

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Thanks Joseph!

Wes

-- "Additionally as a security officer, I carry a gun to protect government officials but my life isn't worth protecting at home in their eyes." Dick Anthony Heller

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Very cool.

Thanks Joe!

--Winston

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Thanks. The predecessor circuit, never published, was far more complex.

It occurs to me that a gradiometer circuit might be useful for detecting nearby field generators at the expense of distant field generators.

If one uses two hall-effect chips separated by a known distance and oriented the same way, if one feeds the outputs into opamp plus and minus inputs, the output will be the field difference.

In my simple magnetometer circuit, if one replaces the 15K-500-15K divider string with a second hall chip oriented the same as the first, the difference in outputs will be reported by the voltmeter.

One can implement a differential sensor with two hall chips glued to one another back-to-back. This may be useful because the Micronas HAL401 may be hard to buy in small quantities.

Analog Devices offers one linear hall chip, the AD22151, which appears suitable as well, and should work in my simple circuit in place of the Allegro chips (although the pin numbers will likely change). The AD22151 is a little SOT-23 surface-mount chip, but I bet one can easily solder magnet-wire leads to it, and make a nice little probe with it.

Joe Gwinn

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What is that "6k2" in the schematic?

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6.2K

Similarly 3M3 = 3.3M

4R7 = 4.7 ohms

A more common convention in Europe and the YouKay.

Best regards, Spehro Pefhany

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The 6.2K? I think it's part of the voltage feedback network.

--Winston

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European for 6.2 Kohms - saves a whole decimal point, and avoids the comma versus period debate. I also like the European approach to resistor symbols - just a rectangle, much easier to draw.

Joe Gwinn

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Joseph Gwinn wrote in news:joegwinn- snipped-for-privacy@news.giganews.com:

Also avoids losing the decimal all together in publishing, copying, artwork, etc. Less chance for errors in magnitude to creep in.

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Any recomendations for the trimmer pots? Single turn, muliturn, preferences?

Wes

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One could put that circuit, the battery and a switch in say, a Pomona # 3602 box. Replace one of the BNCs with a stereo jack for the Hall effect probe and you are all set. Nifty.

--Winston

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Wes fired this volley in news:cePcl.174683\$ snipped-for-privacy@en-nntp-09.dc.easynews.com:

Usually (not always) the term "trimmer" sort of implies a multi-turn pot. (trimmer caps are usually 270-to-300-degree single-turn devices).

Since they only run a buck-plus at Digi-Key, that's the way I'd go.

Precise, reliable, and good for a couple K's of settings before you wear out the wiper.

LLoyd

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"Lloyd E. Sponenburgh" fired this volley in news:Xns9B97C442499FBlloydspmindspringcom@216.168.3.70:

I should have added, "or screw-compressed devices with no particular number of turns end-to-end".

LLoyd

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For value it's hard to beat the 3/8" multi-turn trimpots. Sealed, 25 turns or so, and they have a clutch mechanism. They start at around \$1 USD from Mouser or Digikey. (eg. Murata PV36 series).

Best regards, Spehro Pefhany

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And actually -- the trimmer caps which I know the best are 180 degrees from max to min or back.

Enjoy, DoN.

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Multiturn is essential. Single-turn will be impossible to set accurately enough, and too sensitive to mechanical disturbance.

Other than that, it isn't critical. I used 25-turn cermet pots packaged by NTE that are sold by the local electronics parts store (U-Do-It in Needham, MA, off Rt 128 near Boston).

Joe Gwinn

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