Aloha and welcome. The first thing you need to figure out is what you're trying to measure. If you're measuring an magnetic field, then a hall effect sensor might work.
Here's one example for about $5. this kind of keeps it real that you're not dealing with an expensive component if the measurement meets your requirements.
There's a datasheet here.
I imagine you need to figure out how sensitive you need the magnetic field measurement to be and what the middle of the spectrum is defined as. You could look at Gauss measurement units. Are you looking at detecting a 25 gauss field or a sub-one field like a 0.6 gauss field. If you need accuracy below 1.0, then part above is not sensitive enough, but you may be able to find more sensitive parts.
There are a whole range of magnetic field measurement devices here.
Another way to figure out what magnetic field strength you need to detect is to do a Google search on shark magnetic field and see what comes up. You may be able to quickly find values in Gauss units based on other research. From there, you can start to figure out the sensitivity of the equipment you need.
The other sensor you might need is a motion detector.
To summarize, the first step is to figure out what you need to measure, then find the sensor parts that can provide that measurement.
Ultimately, you may find that an off-the-shelf system might work better. Or, it might be easier to use an Arduino or a Raspberry Pi for your application. The projects in this community focus on the Renesas Synergy Platform, which is more difficult to learn to use than a Raspberry Pi or a Arduino, but provide a cleaner path to a production (large scale) product. I'm just trying to help you out, so you don't waste a lot of time learning something that might be not what you're looking for.
Whatever platform you choose, you'll still need to identify what sensors you need.