Gaussmeter (magnetic field measurement)

The genesis of the gaussmeter project involves, perhaps not exactly intuitively, the fabrication of suitable enclosures to house the various electronic projects constructed. Well at least those that made it past the breadboard stage. The teardown of old electronic equipment (see the Teardowns Section for examples) not only provides various components to experiment with, but a plethora of thin metal sheeting from the chassis and equipment housing etc. This metal sheeting is an excellent candidate to form custom enclosures for DIY projects. Equipment such as "white goods"(eg washing machines, clothes dryers etc) provide large quantities of such sheet metal.

This then leads to how to cut and fold this sheet metal to produce the desired DIY enclosure. The Enclosures section gives specific details of my experiences, but in particular to the gaussmeter project, a "metal brake" for sheet metal bending is required (don't know why it is called a "brake" when it is a "bender", although in Britain it is more logically generally termed a "folder"). There are a large number of examples of DIY metal brake projects available on the Internet. But I found the need for a mechanical clamp as part of the typical metal brake, even those of the "box-and-pan" variety, to be overly restrictive in the bending/folding that can be accomplished.

This lead to the idea of using magnets, especially electro-magnets, as the clamping force to enable metal blocks to be easily removed, rearranged and strategically placed to enable bending of a single sheet of thin metal into the required minimum number of seperate pieces and necessary screws/rivets/soldering etc to form the final enclosure. Therefore, electromagnet construction (see the Electromagnet section for details), and measuring the resultant magnetic strength would be a necessary intermeditary step. Hence, finally we get to the DIY guassmeter!

Excellent resource in respect to magnets, and gaussmeters, is provided by Rick Hoadley on the "Magnet Man" site (1). This site is where the original idea/schematic for the gaussmeter attachment for a DMM was sourced (2). The Circuit Details section below discusses various alterations and specific components used in the construction of this particular version.

The section in Learning and Component Testing gives details on my various experiments/trials and tribulations with using power supplies and other components/IC's for this project.

The central component of the gaussmeter project is the use of a Hall effect sensor. The Hall effect (3) is the production of a voltage in response to a magnetic field within a conductor that has an electric current. This can be exploited to produce sensors that provide a calibrated mV change per guass change in magnetic field. The particular sensor used in this project is a Honeywell SS496A1.

The Honeywell SS496A1, as per the datasheet, has a minimum magnetic range of ±750 gauss with a sensitivity of 2.5 mV/Gauss. The Hall effect sensor provides a null output (ie the output at zero gauss) of 2.50±0.075 V. Therefore, a voltage greater than 2.5V indicates 'positive' gauss (at 2.5 mV/Gauss) i.e. a south magnetic pole, whereas voltage lower than 2.5V indicates 'negative' gauss i.e a north magnetic pole. The Honeywell SS496A1 is relatively expensive at approximately $5/item (at the time of writing) whereas a perusal of Ebay shows Allegro A1301 sensors have comparable specifiations but can be purchased in the order of $1/item. In any case, I had a SS496A1 available and that is what was used during the project.

The Honeywell SS496A1 is a three-pin device simply requiring the supply voltage on pin 1 (4.5 to 10.5V), ground to pin 2, whereas, the output voltage representing the measured magnetic field strength is pin 3. This means a battery can be connected to the hall sensor as a simple power supply and a DMM used to measure the voltage output on pin 3. The measured output voltage can then be easily converted to gauss by the equation (Vm - 2.5)/0.0025 where Vm is the measured voltage at pin 3.

If only 'one-off' measurements are to be performed, this perhaps would be adequate. Obviously, it would be more convenient to have the measured voltage at pin 3 displayed in gauss (instead of needing to do the necessary calculation). This could be accomplished for example by an ADC interfaced to a LCD or LED 7-segment display, or more easily perhaps, using a PIC mirocprocessor with ADC port and suitable output device.

However, utilising the input/output of the DMM, as shown on the "Magnet Man" site (1) in effect extends the measurement capabilty of the DMM to include magnetic field strength, and avoids the expensive, time and effort of the 'calculation and output' circuitry necessary to convert voltage output on pin 3 of the hall effect sensor to gauss.

Circuit Operation

The central component is obviously the Honeywell SS496A1 Hall Effect sensor which produces 2.50±0.075 V at zero gauss, which with 'negative' gauss (i.e a north magnetic pole) decreases to 0.625 V (2.5±0.075mV per gauss - typical magnetic range ±750 gauss) and with 'positive' gauss (i.e. south magnetic pole) to 4.375V.

In order to utilise the voltage measurement and LCD display function of a DMM to form the calculation/output for the Hall sensor, the output from the Hall sensor will be 'offset and scaled' before input to the DMM. This will enable the DMM to display 0V representing 0 gauss (i.e. when the Hall effect sensor produces 2.5V) and ±7.5V representing ±750 gauss.

Therefore, when measuring magnetic fields, the DMM will be able to indicate direction of the field by the voltage being either positive or negative and the strength in gauss will be simply the voltage multiplied by 100. The 'offset and scaling' of the output from the Hall effect sensor indicates an op-amp circuit will be required with a split rail voltage supply (positive and negative voltage).

Power Supply

In order to make the gaussmeter compact and portable, battery operation was selected. A standard 9V (PP3 rectangle shape) cell forms the supply voltage, which is stepped down and regulated by a 78L05 to produce 5V for Hall Effect sensor. The 9V from the battery provides the positive rail for the op-amp, whereas, the negative rail voltage is produced by the NE555 timer circuit (see the Schematic Diagram Section).

The negative rail voltage is produced via a 'charge pump' circuit composed of the NE555 in conjunction with diodes D2, D3 and capacitors C5 and C6. The resistors R2, R3 and capacitors C3 and C4 enable the NE555 to operate as an astable multivibrator (~1kHz). When pin 3 of the NE555 goes positive, capacitor C5 charges through diode D2. When NE555 pin 3 switches to ground, capacitor C5 discharges through diode D3 and charges capacitor C6 to -9V. This means the voltage at the junction of the anode of diode D2 and cathode of capacitor C4 will be always negative with respect to the ground. The downside of this 'charge pump' is that output current is only low (~50mA), but this is more than sufficient for this application.

Signal Conditioning

The LM324 quad op-amp package provides input buffering for the Hall Effect sensor, zero offset, inverting/scaling and output to the DMM. U3:B is the op-amp buffer to avoid loading the Honeywell SS496A1 Hall Effect sensor. U3:C (and associated resistors and potentiometer RV3) provide the 'zero offset' that enables the 2.5V output from the Hall Effect sensor (i.e. zero gauss) to read as 0V on the DMM. U3:A inverts the Hall Effect sensor after 'zero offset' (so that a positive voltage on the DMM indicates a north magnetic field) and scales the voltage so that ±7.5V on the DMM representing ±750 gauss.

Calibration

• Make sure the Hall Effect sensor is not near any magnetic field. Adjust potentiometer RV3 until the output at J3 as measured with the connected DMM is 0V.
• Place SW2 in position so that the potentiometer RV2 is not connected to the op-amp circuit. Set potentiometer RV2 so that the voltage measured at J7 is equal to 4.375V (for the Honeywell SS496A1 Hall Effect sensor, 750 gauss x 2.5mV/G + 2.5V = 4.375V).
• Place SW2 in position so that the potentiometer RV2 is connected to the op-amp circuit. Adjust potentiometer RV1 until the DMM reads 7.5V at J3.
• The gaussmeter is now ready to measure magnetic fields:
• positive voltage = north magnetic pole, negative voltage = south magnetic pole.
• multiple voltage on the DMM by 100 to get magnetic field strength in gauss)

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  Gaussmeter Schematic

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  Gaussmeter PCB - Bottom Copper

Gaussmeter PCB - Bottom Copper

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  Gaussmeter PCB - Top Copper

Gaussmeter PCB - Top Copper

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  Gaussmeter PCB - All Layers

Gaussmeter PCB - All Layers

Initially the circuit was constructed on bread-board to test the basic operation and functionality (see Photographs Section).

A standard magnetic field of known value can be generated using appropriately constructed solenoid's and measuring the applied electrical current. This would then provide a known standard value against which to test the Hall Effect sensor/gaussmeter. However, this 'quantitative' measurement and testing of the constructed gaussmeter is not required for the applications in which it is to be used.

'Qualitataive' testing of the constructed gaussmeter was performed using a permanent magnet in conjunction with a magnetic compass (see Photographs Section).

This enabled testing that the guassmeter reported the correct orientation of the measured magnetic field (north or south pole). While no 'standard' magnetic field was available, the constructed gaussmeter reported a consistent value, which increased and decreased appropriately when the permanent magnet was brought closer or further away.

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ref001: http://www.coolmagnetman.com/magindex.htm

ref002: http://www.coolmagnetman.com/magmetr1.htm

ref003: http://en.wikipedia.org/wiki/Hall_effect