Edge light acrylic is often used for signs and is readily available. Basically, shine a light onto the surface of the plastic and it defracts in such a way that the edges appear to glow.
Now already we're thinking, since we decided to stop worrying about the cost of making our board game, adding a few more pence to the cost in order to add some surface-mount LEDs might be a cool idea. We could drill a hole in the centre of each square, connect the LED to the hall effect sensor (so it triggers when a playing piece is detected over the hole) and have the LED light up the base of the playing piece from underneath, which in turn is made from edge-light acrylic.
We've yet to find out if a hall-effect sensor can provide enough current to light an LED and hold an input pin on a shift register high (it should be able to, but we really need to try this out for sure) but in preparation of receiving our latest batch of goodies from Farnell, we've come up with a PCB layout:
(a 2mm hole drilled through the centre of each pair of SMT LED pads will allow us to mount an LED "upside-down" on the board, so that it shines through the hole when viewed from the top side)
Each of the seven hexagon shapes (we've drawn them as circles just because this was easier in ExpressPCB which we still prefer to use for knocking up quick prototype boards) has an SMT LED in the centre, with a tiny hall effect sensor slightly offset. The hall sensors we're using are these:
They're absolutely tiny - about 2mm by 3mm in total! In fact, these may prove to be a little bit too small for hand soldering, but they were the cheapest non-latching SMT-format sensors we could find and we'd ordered a load before even checking the dimensions, so these are the ones we're going to try out on our first prototype!
Every board has it's own shift register. We're trying these parallel-in-serial-out (PISO) shift registers:
Because we need to be able to hold the PE (parallel enable line) low and send a clock pulse to get the data from the input pins into the shift register, then pull it high to get the shift register to "pass the data along" on each clock pulse, we're having to send a few of these pins to a connector on the board.
So as well as the power and ground pins, every board also needs a serial data in line, a parallel enable line, a clock enable and a clock pulse line too. That's a 6-way connector. The easiest method we could think of, to allow all the boards to be laid out in any number of configurations, is to include a surface mount 0.1" pitch connector on the underside and to simply daisy-chain them together with some custom-made IDC cables.