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.
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).
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.
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.
- 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)