Half the supply at rest

A straight line through the middle

A current runs through the Hall element inside the chip. A field through the element pushes that current to one side and leaves a small voltage across it, proportional to the field and with a sign that follows its direction. The chip amplifies that voltage and adds it to half of its supply. So the output has a middle, and the field moves it either way.

At 5 V the data sheet gives the numbers: 2.5 V with no field, 3.25 mV for every gauss, 1 V at −460 G and 4 V at +460 G. Slide the field and the output follows the line. The gauss is the unit: a fridge magnet is tens of gauss at its surface, a small neodymium disc thousands, and the Earth's own field about half of one.

The middle is not exact

The typical chip rests at 2.5 V. The data sheet allows 2.25 to 2.75 V, shown as the grey band, and at 3.25 mV per gauss that spread is worth about 77 G. A sketch that assumes 512 on an Uno can be out by more than a fridge magnet's worth before you start. So the sketches in this book measure the resting level with no magnet near and subtract it.

The slope varies too, 2.95 to 3.55 mV per gauss at 5 V, about 9 % either way. A reading in gauss from this block is good to about a tenth, and better as a comparison: nearer, farther, stronger, weaker.

Where the line ends

The sheet draws nothing past ±460 G. A small neodymium magnet makes that at a few millimetres, so the reading stops growing before the magnet touches the board. Treat a reading at the end of the range as "strong", and measure from a little farther out when you want a number.