Sunday, 29 September 2013

Digital board game - time to get serious

After spending hours and hours working on our digital board game, we've come to the conclusion that we're going to need some help getting a robust, working two-layer board.
So far we've tried to concentrate on:
a) a board that can be built cheaply using easily sourced components
b) a board that can be manufactured quickly and easily by anyone with no specific skill requirement.

To date we've managed the first, but failed miserably on the second.
We tried using copper tape to make a keypad membrane, but it was far too fiddly to lay down consistently well enough to be robust. So in an attempt to make it more robust, yet keep it easy to manufacture, we knocked up a prototype using conductive ink (one of those silver pens from Maplin). The pens are quite expensive, so we've blown the first requirement of making our boards - but for now we're concentrating on addressing the second!

We drew the "switches" as two halves of a touch-to-make switch. After drawing the conductive traces, we placed a paper separator layer on top. Finally we stuck a top layer on with bits of copper tape to bridge the two halves of the switch, when pressed.

During testing this worked fine, but only when the buttons were pressed by hand. Because the board is copper, not ferric-based, our neodymium magnets are not attracted to it. So we tried sticking some steel nuts to the underside of the board. Our only question was whether or not the magnet would be strong enough to be attracted to the nuts, through the thickness of the copper PCB board (about 1.6mm) ?

It turns out that the magnet is plenty strong enough! There was quite a pull between the magnet and the playing surface. So we did some testing using our fully printed (and plastic laminated) top layer.

here you can see the resistance across the switch is measured at about 13 ohms when closed by the magnet, which is attracted to the nut glued to the underside of the copper board

By a bizarre coincidence, the height of our steel nut raises the top of the copper board to be exactly flush to the top of  a sheet of 5mm thick foam-core FoamEx board.

So with this in mind, our current plan for making the electronic board game sections is to have a PCB made by a professional manufacturer and use some steel (it may even be regular nuts and bolts!) on the underside of the board. The PCB could be edged using FoamEx (or maybe some 5mm acrylic) to give it a finished look, and the top, printed layer, could be glued over the whole thing, covering the joining edges.

Professional manufacturers prefer to work with Gerber files, and our favoured ExpressPCB is only good for making toner-transfer images as PDF files - it doesn't really export the final PCB layer in any format - so we downloaded and installed a previously tried PCB program: DipTrace.

For 30 days, DipTrace is fully featured (you can download a 300-pin-limited free, non-commercial version) so we've got a couple of weeks to try this out and see if it works. DipTrace is actually a really neat suite of programs for making PCBs - we might even go crazy and spend some serious money on getting a full licence once this project is out of the way!

After a few hours of shifting traces around, we managed to make an 11x8 keypad matrix on an A4 sheet. Impressively, DipTrace has a 3d model viewer, to see how your PCB will look once assembled:

Our board is pretty basic. We've used "fingered" touch-switches on the top surface (each half of the two-part switch is interleaved and the printed playing layer on top will have conductive dots under each square on the board) which are routed to a series of pins on a PIC 16F722 on the underside. The notches on the edges of the board will house tiny 3mm magnets which we'll use to connect the edges of each board section to the next one.

The advantage of the PCB-based approach (we originally started off like this, and gave up in favour of a "cheaper" alternative many months ago) is that each board section will be good and robust. It also means that we won't have to mess about with micro-wires, taking the traces from the copper-track-switches to a custom PCB on the underside - since each board section will effectively just be one big PCB, it means that assembly will involve soldering a single 28-pin SOIC chip and sticking a few magnets with some no-solder conductive glue.

With the PCB already designed, all we need to do is find a manufacturing house with a fast turnaround, so we can give the idea a go!

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