This little project is a Gauss Meter, or Flux Meter, or Magnet Polarity Indicator. Basically, it senses magnetic fields. Using a hall sensor, the meter can measure the Gauss/Flux density and polarity of a magnet. It only needs a few parts, so can be built without a circuit board.
A Gauss Meter is handy when you want to know what end of a magnet is a North or South pole, and when you want to test magnets for strength, especially if they may have been heat damaged.
The heart of the meter is a UGN3503U or similar hall sensor. The UGN3503U is a linear hall sensor, meaning its output level changes with magnet Gauss changes, and the device can be sourced from most electronics suppliers, like Jaycar, Altronics, Farnell, etc..
Gauss Meter – [Link]