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..
UGN3503U hall effect sensor – Gauss Meter - [Link]