Sensor Board
This board had been designed to perfectly fit the 25mm holes under each square of the chessboard, providing both a consistent positioning of the sensors themselves and a clean way to implement all the connections.
Two different mounting options are available.
Reed Switches
The simplest solution is to populate with one reed switch SW1 or SW2 on the back side of the board.
Note
Tests showed that one reed switch is enugh to detect when a magnet is placed above it, however an additional one can be mounted.
On the top side, an optional LED (in combination with a 1kΩ resistor) can be mounted to have a visual feedback when the reed switches are being triggered.
Hall Effect Sensor (not tested)
Alternatively, an hall effect sensor mounted on U1, can be used.
Warning
After early tests this solution had been avoided for a few reasons:
The master board need additional circuitry.
One more cable has to be soldered to each sensor board.
Digital sensors (A3144) seems less sensistive than the reed switches.
Analog sensors (49E) require even more additional circuitry.