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Dec 06, 2004, 09:37 PM
Thread OP

Inexpensive magnetic flux (gauss) meter

In addition to being fun to play with and educational, this could be useful for working with CD ROM motors. You can use it to:

- Check the North-South orientation of magnets after positioning them.
- Identify and count the poles in a plastic ring magnet.
- Check the relative strength of permanent magnets.
- Determine if a stator is wired as Wye or Delta.

This flux meter uses the Hall sensor from a CD ROM motor assembly. You can probably get a Hall sensor for free from junk equipment, or you can get a new motor with 3 sensors for $4 (which is what I did).

Parts required include the Hall sensor, a 1.5 volt battery, and a digital volt meter. Two of the four sensor wires go to the battery, the other two go to the meter. That's it!

The sensor appears to be fairly linear at low flux levels (like the Earth's magnetic field), but it still gives usable readings at the very high flux levels associated with motor magnets (although the response is nonlinear at high levels).

Edit: 12/24/2004 Added Schematic. 2/2/2005 Updated schematic.
Last edited by Warske; Feb 02, 2005 at 11:18 AM.
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Dec 06, 2004, 09:39 PM
Thread OP

Obtaining a Hall sensor

My Hall sensor came from a $4 motor that I bought from BTW, I'm not affiliated with them, other than as a happy customer. If it is in stock, you can find this motor at
Click on "Motors" and select "Stock CD-ROM Motor Price: $4.00"

Apparently this type of sensor is widely used in such motors and you can probably find one in a junk CD ROM drive, but I haven't tried it myself.

As you can see from the pictures, there are three sensors on the board, and they each have four pins. I don't have a datasheet, but lets assume the pins are numbered 1 through 4 as shown on the photo.

From the traces on the board I saw that all pin 1's were connected together, and also all pin 3's were connected together. I figured these must be power and ground, I wasn't sure which was which, so I experimented. Using a 10 ohm series resistor, I slowly applied voltage, first in one direction, then the other. I found that it works either way! I also noticed the current start to go up at around 2.75 volts, so I figure that is probably the max voltage it should have.

It was possible to do the testing with the chips still in place by soldering wires to the traces on the left side of the board.
Dec 06, 2004, 09:40 PM
Thread OP

Removing the circuit board

With the $4 motor, you can remove the can by grabbing the rotor and pulling hard. Then you can get to the motor wires and unsolder them from the board.

To remove the metal plate, peal back the tape and separate the circuit board from the metal plate using a screwdriver. I used two pliers to twist the plate back on itself and the motor and circuit board dropped right off.

[Edit 12/10/2004] Added pictures, updated description.
Last edited by Warske; Dec 10, 2004 at 04:39 PM.
Dec 06, 2004, 09:41 PM
Thread OP

Removing the chips from the circuit board

There are different ways to unsolder the chips. I built a little tool from hookup wire and used it with an 800 degree tip on my soldering iron as shown in the pictures.

When the solder melts, the chip comes away in the tool because of surface tension. You need to be ready to flick it into a can or remove it with tweezers before it cooks to death.
Dec 06, 2004, 09:43 PM
Thread OP

Wiring the probe

I soldered wire-wrap wire to the pins on the chip:

1 +V (1.5 volts)
2 -Out
3 -V
4 +Out

With this hookup, the meter voltage will be positive when the chip is sitting face up on the north pole of a magnet. That is, its legs are pointing down at the magnet.

I used an AA battery holder from Radio Shack. The current drain is about 4 ma. I didn't bother with a switch: just remove the battery when done. Using a single rechargeable battery, I figure it won't hurt it if I forget and let it discharge all the way.

I used brass tubing for the banana jacks, and heat shrink, CA, and epoxy to put it all together on a stick of wood.
Dec 06, 2004, 09:44 PM
Thread OP


I am using the probe with a Fluke 8060A multimeter. It has a 200 mV scale which is useful for playing around with small magnetic fields like the Earth's. I get about .5 mV for the Earth's field. At these lower values, the sensor seems pretty linear, but I haven't plotted it yet. The sensor is directional, so you can use it to observe the declination of the Earth's field.

For strong magnets the Hall sensor puts out as much as .5 volts. It isn't saturated, so you can still use it to compare magnets and see which has a stronger field. But it isn't linear that that level. A magnet that is twice as strong might increase the reading by only 5%.

I started playing with a Helmholtz Coil for calibration. I found this page which describes it:
Dec 06, 2004, 09:46 PM
Thread OP

Other links and information

After building the probe, I started looking on the Web for information.

This page shows a diagram of a 4 wire Hall Sensor and some information on measuring magnetic fields.

This is a similar project (written in Dutch), incorporating a 3v battery, current limiting resistor, and push button switch. Looks much better than mine!

Another meter using sensors made by Allegro.

I had wanted to do more with this, but I'm out of time. In fact, I will be out of town for a few days and I won't have access to this forum until I'm back. If you have more information on this subject, please feel free to post it so others can benefit.


Edit 2/7/2005 - Added
Links into this thread (by post number):

Gauss Meter
What is a Hall sensor & how does it work 122 127
Linearity & saturation 13
...Single turn coil 79
...Earth's magnetic field 119
Offset 119
Regulated power supply 59
Flux meter with detachable probe 69
Comparing magnets 15 59
Measuring a solenoid 83
Measuring Earth's magnetic field 119
Destructive testing of permanent magnets 20
Other gauss meters/sensors. Measuring stronger fields 111 115
Magnetic field calculators 111

Magnetizing Neodymium Magnets
Magnetizing Neodymium 30
Forming capacitors 32
Resonant frequency of an LC circuit 45
Storage oscilliscope alternative 48
Schematic and WinSpice3 transient analysis of pulse 51
Optimizing the coil 55 62 74 118
Capacitor ESR 57
Flyback diode 66 70 100
High voltage power supplies
...Electronic flash units 81 89 91 94
...Variac and transformer 118
...High speed mechanical switch 48
...SCR 100 118
Converting Gauss to Oersteds 83
Measuring capacitance 95
Field required to magnetize Neodymium (graph) 104
Last edited by Warske; Feb 07, 2005 at 10:13 AM.
Dec 06, 2004, 10:38 PM
I'm out flying on the hill
MAVA's Avatar

Flux Meter


Thank you for your post.

Very informative.

Dec 07, 2004, 06:01 AM
Registered User
Andy W's Avatar
Now, can you make a flux capacitor?

(good posts, great info!)
Dec 08, 2004, 10:25 AM
Fidler & twidler
empeabee's Avatar
Originally Posted by Warske
There are different ways to unsolder the chips. I built a little tool from hookup wire and used it with an 800 degree tip on my soldering iron as shown in the pictures.

When the solder melts, the chip comes away in the tool because of surface tension. You need to be ready to flick it into a can or remove it with tweezers before it cooks to death.
Very good poor mans ' branding Iron ' to remove the chip'
Very good job - I'll be building one RSN

Dec 08, 2004, 11:42 AM
WOW, thanks!!
Dec 10, 2004, 05:09 PM
Thread OP
Thanks for all the feedback. I updated post #3 with pictures to better show how to remove the circuit board from the motor.

If anyone finds a datasheet for the hall sensor, please post the info for the rest of us. Also if you find other sources for cheap analog output hall sensors...

This is off topic, but if you are also interested in what to do with the motor, you might want to check out these links:

RC Groups Discussion > Airplanes - Electric > Power Systems
Brushless CD-ROM Motor SURPLUS
I think the above is the genesis of

RC Groups Discussion > Airplanes - Electric > Power Systems
One hour CD_ROM motor construction

Dec 13, 2004, 11:42 PM
Thread OP

How linear is the Hall sensor?

I ran a test to check the linearity of my GoBrushless Hall sensor. As you can see from the graph, it is quite linear to 150 mV, and fairly linear to 300 mV.

Considering the Earth's magnetic field is about 0.5 Gauss, and the sensor reads the ambient field in my living room as 0.58 mV, I figure this gives me a usable linear range of up to about 250 gauss (0.5 * 300 / 0.58).

I've included the data if you want to play with it. Note that the sensor was tested at an applied voltage of 1.3987 V and the sensor has an offset of 3.1 mV.

Of course, the sensor is still very useful outside its linear range to check magnet field direction and to determine if one magnet is stronger than another.

Now that I know what the linear range is, I'm thinking it would be interesting to test the temperature sensitivity of the N45 magnets that came with my GoBrushless kit. What temperature causes them to degrade, and how much do they recover when they cool down?
Dec 13, 2004, 11:44 PM
Thread OP

Kitchen table physics

Let me mention how I tested the sensor so you can test your own or come up with a better idea and post it here!

The concept was to use an electromagnet and a variable voltage power supply to provide the test field. Unfortunately, if I used an iron core to intensify the field, I might be measuring the saturation of the core instead of the sensor.

To get a coreless coil, I took apart a dead fluorescent light ballast. This worked great up to about 0.7 amps, but that only got me to 110 mV of sensor response. At higher currents, the coil starts to get hot, its resistance goes up, and the current isn't stable enough for a good reading. Plus I didn't want to heat up the sensor and be reading its temperature coefficient.

To get past that problem, I took the readings in 5 stages. After the first stage with 110.1 mV at 0.7 amps, I set the current to zero and biased the sensor back to 110.1 mV using a nice big rare earth magnet. I then recorded the response up to 0.7 amps again and repeated the procedure. Since magnetic fields add together in the absence of saturable material, this should work and the graph suggests that it did. Five stages let me get up to an effective current of 3.5 amps, which would have vaporized my coil. BTW, I figure it is important to make the test platform quite ridged so that the magnet won't move toward the coil as current is applied.

It was surprisingly easy to bias up the sensor to the exact last reading. I just positioned the magnet with blocks of wood to give a slightly high reading, then shifted it to one side to get the exact number.
Last edited by Warske; Dec 13, 2004 at 11:46 PM.
Dec 16, 2004, 07:44 PM
Thread OP

Checking your magnets

Do you fly with matched batteries? How about your motor magnets? Are they all the same strength? How do you know if one of them is a dud?

Now that we've established the linear range of the flux sensor, its time to get to some practical applications.

As you know if you've looked at the CD ROM motor building threads, its important that you put the same number of turns on each tooth so that the ESC doesn't get confused. But what if one of the magnets is (gasp) *weaker* than the others? (This is what the marketing folks are into when they talk about creating a need where none previously existed. But no, I'm not selling these sensors. You have to build one yourself! )

Using my usual cardboard and masking tape, I rigged up a test jig. This allowed me to keep the sensor at a fixed distance from the magnet (a distance where the flux density is within its linear range), and let me accurately place the magnets relative to the sensor.
Last edited by Warske; Dec 16, 2004 at 07:47 PM.

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