Having decided that we need some steel parts in the construction of our board game sections, it was once again time to try out a new idea.
This time we've tried to focus on easy-to-source materials, rather than trying to focus on the long-game and manufacturing costs and keeping the material costs to a minimum. So the easiest way to get some decent sized steel discs into our board game is with "penny washers". These are 25mm (outside) diameter with a hole for an M4, M5 or M6 bolt (i.e. 4mm-6mm inner diameter).
Because the washers already have a hole in the centre, we decided to use this to house a small pin, and to bridge the pin and the washer using a small conductive circle stuck/printed onto the underside of the top layer: as the playing piece is placed on the board, the magnet in the piece would pull the conductive disc down towards the steel washer, and the pin and the outer washer would be electrically connected.
So it was time to crack out ExpressPCB and put together a design. Then we opened the PDF of our PCB in Inkscape and drew a design for some mdf to house the washers
The great thing about Inkscape is that it lets you open any PDF file and draw over the top of it. This helped us create a design for the mdf which we could guarantee would fit perfectly over the correct place(s) on the PCB
Another visit to BuildBrighton over the New Year Bank Holiday and another PCB is etched and ready for testing!
The laser cutter was in super-smokey cut-and-burn mode for some reason - but it hacked some 25mm circles out of a piece of scrap mdf. We cut some discs of thick photo paper and placed these on the washers, before flipping the whole thing over and hot-gluing the washers in place. The idea is that each washer is sunk into the hole by the thickness of the piece of paper (allowing us to place a top sheet over the whole assembly, without needing to create a separator layer.
Now each pin on the PCB needs to be placed at the same height, so that each is perfectly flush with the washer that surrounds it. Since our tracks run horizontally on the underside, it made sense to connect the washers on the top side vertically just using the copper tracks on some double-sided copper board.
With all the pins and washers in place, the underside looked something like this.
It soon became apparent that the height of the pin head is going to be critical in this approach: too high and the playing surface will quickly become covered in little dipples (where the pin head is pushing through the paper membrane). If the pin is either too high or too low, there's a chance that a copper dot on the layer above won't actually make an electrically conductive connection between the washer and the pin - the height of the pin and the thickness of the washer are critical in this case.
Already we've found that using a magnet to pull a playing piece down onto even the thinnest of membranes is sometimes not strong enough. It seems that the surface tension on the top layer of a membrane (whether manufactured from bits of paper and copper tape, or made professionally by a manufacturer in China) is too great for a small magnet: we need a large magnet and a large piece of steel (the magnetised rubber matting we originally planned on is nowhere near good enough) to make the connection.
Which got us thinking....
If we can't get the magnet to pull the top layer down onto a fixed piece of steel (and given the amount of effort required to get the pins and washers perfectly flush) maybe we could have a small washer "floating" inside a pocket, and use the magnet to pull the washer away from two contacts - when a playing piece is above a square, the washer is pulled up, off the underlying PCB, breaking the contact, and when it is removed, the washer will fall down onto the PCB, bridging the two contacts.
This method seems to offer a number of advantages over our previous attempts:
For a start, the thickness of the mdf layer is not critical, making construction simpler. We can also use smaller (i.e. cheaper) washers, since we don't need a large bulk of steel to pull the magnet down onto. This approach also means we can create our top/printed layer out of any suitable material, we're no longer restricted to keeping it as thin and flexible as possible (in fact, a thicker layer made from printed card might actually be preferable).
This also means that we don't have the problem of trying to attach a top and bottom conductive layer together - everything can be done on a single side of the PCB. In fact, given that we want the conductive side of our PCB to be facing upwards, it's going to be beneficial to work with through hole (rather than our preferred surface-mount) components!
The design above shows how small M4 washers (9mm outer diameter, 4mm inner diameter) could be used to bridge contacts on the PCB. Since we're going to be putting a cardboard/mdf layer on top of this circuit board, we can simply cut a few strips into the mdf to accommodate the soldered pins from the through hole components.
A final, printed top layer over the whole thing, and we should have our first board game section that works with an actual miniature playing piece, not just a heavy duty magnet! It's Thursday today, which means more laser cutting, etching and PCB drilling at BuildBrighton tonight. Fingers crossed.......