# Hall based Magnetometer

The magneto meter

I had a Hall sensor in my junkbox waiting for some fun project. I had it as a sample from Hoeben Electronic and my plan was to make a P6042 probe to complete the probe amp I had. But short after that I found an original.

Looking for some parts I found the sensor and decided to do some tests with it. I was checking the working of an ESC but the current to the motor was to high for my P6042 so 1+1=2 and I decided to make a magnetometer. The P6042 combines a Hall sensor with a current transformer so I tried if a Hall sensor alone was sensitive enough for current measurements. Turned out is could be usefull but if you want it to be sensitive without ferite  you need more then one sensor.

The insite

Current through a conductor causes a magnetfield around the conductor. That is not very strong so a current probe uses ferite to kinda “focus” the B field. The strenght of that field is given in Tesla’s or Gauss. one Tesla is 10000 Gauss.

current as a vector

the B field round a conductor (source wiki)

1 Gauss is 1 maxwell per cm/squared. There is a connection to current. 1A per meter is 4.pi.10E-3 oersted.

To give you an idea about the values some examples. A small refrigiator magnet is 50 gauss (0,005T).  a very strong Neodymium-iron-boron magnet is 2000G (0,2T) and the Earth magnetic field is 0.31-0,58G. My sensor outputs 100mV for 0.1T with 1mA bias. It should have a bandwidth of several MHz but is guaranteed for 100 kH. But no data on lowest sensibility.

The schematic is rather simple. A currentsource made with a LT1001 (opa277 in schematic) and a LT1031 (LT1021 in schematic) 10V Vref. (They were not not in my Eagle lib) Then an instrumentation opamp based on two low noise audio amps. If you want a high bandwidth you can place an other good opamp. The power supply is a bipolar  +12 and -12V version

The sensor

So i was curious about how it would perform. I made a 0-1000x amplifier with to opamps and a 1 to 5 mA adjustable currentsource. to bias the sensor. It is rather sensitive and directional. So you get a positive output for North and negative for south. a small refrigorator magnet caused a good output when I had the probe on it but moving a loudspeaker magnet above it at 30 cm caused a nice reaction. When I hold it on top of a small switching powersuply adapter I catched the signal so I hooked up a generatot to a resistor but the 50 mA was to small. A coil would be better so I tried a loudspeaker that was hooked up to a function generator

Loudspeakertest

Aloudspeaker

The yellow trace is the amplitude measured over the speaker coil. The blue trace is the magnetometer.

I made a enclosure and a probe with a 3D printer. The probe looked better in Freecad.

probe design

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