Anthony aka. "Proto G" published this Instructable where he made a useful DIY Gauss meter which is controlled with Arduino Nano.
In this instructable, I will show you how to make a Gauss meter than can measure the strength of magnets so you can compare different magnets you have. It measures the magnets in units called Gauss and has a relatively linear range from 0-4000 Gauss. It will measure beyond that but the numbers beyond 4000 Gauss should only be used for comparison purposes. In addition to measuring the field strength, it also detects the polarity of the magnet and will show North or South, respectively. My favorite part about this design is the ON/OFF switch. It's hidden in the enclosure so the meter can only be turned on and off with a magnet. Since this unit is meant to measure magnets, you're sure to have one on hand. You can use the same latching magnetic switch I designed for many other things like a secret compartment lock. Here's a video showing the complete assembly:
You can see the full construction manual at: