BMO: Part 1, The Display

The first hurdle when designing BMO was selecting a display for the face. Without a face, BMO isn’t much fun. The first idea considered was to use an old iPhone 3 that I had laying around. It was self contained and I thought that would be the easiest display to use. The problem with the iPhone is that it is kind of small and since the final size of BMO depends on display size, I would wind up with a small BMO. Plus the aspect ratio is wrong. BMO seems to look best with a 4:3 ratio. You can simply cover some of the display to make a 4:3 ratio but that will give BMO too much bezel on the sides.

The second consideration was an iPad. This is much larger than an iPhone, almost too large. It also has the aspect ratio issue and would require a large bezel. But the real killer was the price. I wasn’t prepared to walk around with a $600 BMO.

Other tablets were considered such as the Kindle Fire and Samsung Galaxy Tab but we were still looking at $200+ for a BMO. And again, tablets have a wide screen.

The best idea should have been the most obvious for what we were trying to do. Just use a digital picture frame. They are cheap and come in a 4:3 aspect ratio. Since all we were trying to accomplish was a slide show of BMO faces, this turned out to be the perfect solution. The frame we chose was a ViewSonic 8″ frame VFD826-70

The guts

The first task was taking the thing apart. It comes apart pretty easily, just unscrew the back and it comes right off.

There wasn’t a lot of reaserch done when selecting this frame but we got kind of lucky. The frame isn’t battery powered, it’s only power is an AC adapter. Tests with a 1000ft extension cord were unsuccessful so we needed some way to power this thing off of batteries. The frame comes with a 5V wall adapter so I figured I could try to wire in some AA batteries and see what happened. I was able to find a power jack that matched the one on the AC adapter and soldered it to the leads of a battery holder. I put in 4 alkaline AA batteries and plugged it in. It worked! Sort of… I noticed the images were washed out, the contrast was off. I didn’t have this problem when powering it with the wall jack so something was wrong. AA batteries are 1.5V so 4 of them is 6V. That’s a little higher than the spec problably says. It’s usually between 4.8V and 5.2V for these things.

Battery holder

At first I thought the backlight was just too bright so I put in a potentiometer to crank down the voltage to the backlight. Although that did reduce the brightness, it had no affect on the contrast. So the 6V was probably too much for the LCD. It turns out alkaline batteries weren’t the best choice anyway. They don’t have the capacity for high drain devices. NiMH are a better choice and they are rechargable. Plus they are only 1.2V so 4 of them is 4.8V which is more in line with the tolerances. I tried those and it worked perfectly. The only question now was, how long will it last? I charged up the pack and plugged it in and let it run. The batteries are 2500 mAh and tests showed the display pulling about 400mA so I figured it should last a few hours. It turns out it lasts about 3 -4 hours on one pack which is fine for conventions. You can swap out the pack when it’s dead and BMO can pretty much last the whole day on the convention floor.

With that part solved, BMO needed a body.

Stay tuned….

This entry was posted in BMO. Bookmark the permalink.

Leave a Reply

Your email address will not be published. Required fields are marked *