Level is not a reading

Four answers, and none

The scan has five results: left, top, right, header, and none. It is tempting to read none as level. It is not. None means the roller is joining no pair, and that happens for reasons that have nothing to do with being level.

Level keeps the last side

Tip the board left and the roller rolls to the left side and joins A and B. Now set the board level. Gravity no longer pulls the roller anywhere, and nothing in the switch pushes it back to the middle, so it stays. The scan usually keeps reading left.

This block's old page said the opposite: that a flat board closes nothing and that this is the neutral state. On this switch, a level board reads whatever it read last. Sometimes the roller does stop between two contacts and the scan reads none, but that is luck, and a sketch cannot depend on it.

A corner can read nothing

Tip the board towards a corner, say with the left and header edges both going down, and the roller heads for that corner. It may land on the left side or the header side. It may also come to rest against the one contact in that corner, touching nothing else, and one contact joins nothing. The scan reads none.

So a none can mean "tipped towards a corner", "level and resting between contacts", or "rolling across right now". The scan cannot tell those apart, because the switch cannot.

Power-up is a guess

At power-up, the roller is wherever it was left. If the board is level, the first scan reports that: a side, or none. A sketch that wants to know the board's state at start-up has to wait for someone to tip it, or has to be mounted so that rest is always one side.

That last is the practical answer. Mount the block tipped a little towards one edge, or on its edge, and rest becomes a reading you chose. A tip-over alarm stands it header-down for that reason.