Digital Chess Board: Hardware Design Idea #2

Last time, I've gone through a basic idea of a digital chess board using RFID tech and figured out that with 5+ dollars per reader chip, the cost of the board would be too high. Furthermore, most cheap RFID chips feature serial (SPI or UART) interfaces, which cannot be easily multiplexed to allow the master to talk to multiple slaves. So, what are other options for detection?

RFID works great because it contains two pieces of information: chess piece presence and unique chess piece identifier. The former one is essential because the computer needs to be able to detect the presence of a piece. The latter one allows the computer to know which piece is present on a tile. This info CAN be stored on the piece itself, but it can also be deduced by tracking the state of the board. We know the initial state of the board, and we know how every single piece moves, which means that by keeping track of how the pieces move, it is trivial to calculate all legal positions for the next step.

That poses the question: what can provide us with a yes or no answer to "is a piece present on this square?"? The answer is magnets and hall effect sensors. DigiKey sells SI7210-B-03-IV, which is an I2C enabled hall effect sensor. This sensor, unfortunately, runs in to issue of address clashing as there are only 4 available addresses. Thankfully ALS31300EEJASR-500 exists, with 16 different addresses, configurable by the ratio of voltage between A0 and A1 pins or 64 different addresses stored in the register. This is a rough diagram of how the sensors would be laid out.

Naturally, the board should also have an indicator for each square on the board. Whilst it might be possible to introduce an RGB LED which is configured using a single wire interface, the easiest choice is to have single color LED light up. To reduce the number of wires required, we can design it as an LED Matrix - 1 wire actives a column, and 1 wire activate a row. As long as LEDs are turned on and turned off at a frequency of more than a couple of hundred hertz, players should not be able to notice the switching.