Monday, 20 January 2014

One last try for mechanical switches

Having visited the BRT Group in Hove this dinnertime, it was time to put Steve's theory to the test. Our latest design allows for small holes between two PCB pads, into which we'll drop our ball bearing, bridging the contact, and informing our controller that piece has just arrived or been moved away from each square on the playing surface of our board game.

Everything has looked promising, as the ball bearing "borrowed" from a tilt switch worked just fine. Having just an hour to work in, I quickly etched and drilled some boards for testing. Here's the ball bearing from the tilt switch triggering a simple circuit (the LED is connected to 2xAA batteries, with the ground from the battery going to the two contacts then to the cathode of the LED).

I tried a number of "drop tests" and when the ball bearing finally settled in the hole, the LED lit up every time (it flickered as the ball bearing wobbled about in the hole). Things were looking good.

Sadly, with the (chrome-plated) steel ball bearing, the results were less encouraging. Just like earlier tests with washers, the LED would only light up if the ball bearing was pressed firmly into place. Given that everything else is the same, only the bearing has changed, it makes us believe that the coating of the bearing (or the material the washer is made from, in earlier tests) has a dramatic effect on the reliability of the switch.

So there we have it. Mechanical switches, whether made from washers, ball bearings, steel, copper, chrome-plating and so on are all just a bit too flaky for us. I tend to agree with Steve - there's a simple solution to this somewhere, we're just not seeing it. Because we're focussing on using easily-accessible materials, such as off-the-shelf washers and bearings, if the solution involves sanding or grinding these to get them perfectly flat, they're not really the answer we're looking for.

In fact, after some discussion, we're beginning to wonder if mechanical switches are going to be suitable at all. Even if we had them working, no-one has actually addressed how to stop the exposed copper from oxidising (tinning the contacts with solder also reduces the surface conductivity) or how reliable the contact might be after a few thousand open/close operations. In short, perhaps it's time to give something else a try...

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