Have you ever wanted to find out how strong a magnet really was, or how the strength of the magnetic field varied as you changed the distance from the magnet. Devices used to measure the local magnetic field are called magnetometers or gaussmeters. There are commercially available meters for this, but they are usually a bit expensive for some experimenting.
The availability of inexpensive hall effect sensors have made is possible to build your own magnetic field meters cheaply. Measure Your Magnetism and Build your own Gaussmeter articles have a nice example circuit using hall effect sensor.
I decided to make my own circuit to do the same using a little bit different sensor component. I used Allegro A1302EUA-T hall sensor that I bought from Elfa (Hall-anturit SIL-3, A1302EUA-T). That IC is powered from 5V power source (needs 10 mA) and has output of 1.3 mV/G (= 13 mV/mT). The sensor hall sensor A1302EUA-T costs around three Euros. High precision in output levels is obtained by internal gain and offset trim adjustments made at end-of-line during the manufacturing process.
The circuit below is my version of magnetic field measurement. The V letter inside the circle means voltage meter. I connected my digital multimeter there. The hall effect sensor is on the right. The resistor on it’s output convert the 13 mV/mT voltage output to 10 mV/mT that is more practical reading on multimeter screen (you can easily figure out the mT value from multimeter reading without doing calculations).
On the left side that 2 kohm trimmer is for zero adjust. The hall sensor output is at around half of the operating voltage when there is no external magnetic field. By setting the trimmer to the same voltage allows you to get exactly 0V reading on the multimeter when there is no magnetic field. Now it is easy to read the magnetic field strength and the polarity from multimeter. When you set the multimeter to millivolts DC range, the reading directly shows you Gauss reading. Just leave out the last digit (millivolt), and you get the the reading in milli-Teslas. Simple and easy. The measurement range is according to datasheet at least +-140 milli-Teslas (1400 Gauss). The adapter can be also used to measure varying magnetic fields by connecting the output to oscilloscope (hall sensor has 20 kHz bandwidth).
I decided to power the circuit with stable 5V power supply because the quiescent voltage output and the magnetic sensitivity, Sens, are proportional to the supply voltage. To get the stable 5V power for the circuit I used a modified version of my Simple 5V power supply. I used LM2936Z-5.0 regulator IC. It is a low-dropout (LDO) 5V regulator in TO-92 case (pretty similar to 78L05 but works with lower input-output voltage difference). I first though of using 9V battery for the power, and I even had a nice case for that battery. But after some thinking I decided to use two 3V lithium batteries as the power source and build the entire circuit inside that 9V battery case. Here is how my project turned out. The banana connectors are designed to be connected to the multimeter.
Here is the view to the dirty details inside the project case.