Hack Notes CVA 090616
 Hack Notes, June 16th, 2009
 Battery Messes Up Compass!
So apparently having AAA batteries next to the compass (we measured effects starting about 2" away) really distorts the reading.
reading real -37 135 28 180 60 225 80 270 94 315 124 360 124 360 221 405 278 450 323 495 388 540
Basically it imposes a sinusoidal distortion of about 50 degrees amplitude. But of course it's more complicated that just that. I tried to model an equation which would allow us to compensate for this distortion, but it would be very complicated. Also, we have no confidence that the distortion pattern will be constant enough for any "calibration" scheme to work well. Different battery types, different states of charge, etc will all make it read different. The compass chip has onboard calibration system as well, but (a) we don't have code to use it and (b) as before, calibration may not stay constant. Furthermore it would be extra work to flash the calibration unto all the compass chips before putting them in the kits...
The only solution is to get the batteries away from the compass, or otherwise change the existing battery solution. After some brainstorming, we settled on the idea of moving the batteries to a closed enclosure on the other side of the anklet, and using barrel jack and wires to bring the power over:
New battery solution: SBH-431-1A-ND 3xAAA batt holder with cover, leads $0.75 CP-M2509N-ND 2.5mm submini audio jack $0.38 CP3-1004-ND 2.5mm submini plug $0.76
The old battery holder was $1.68, so actually this solution is only like 20 cents more.
An additional benefit of this is spitting up the considerable weight and bulk of this solution, balancing the armature out a little bit.
 Onboard Charger Idea
Charger / FTDI chip: 296-9320-5-ND 8DIP NiMH batt charger IC $2.13 WM17115-ND USB Mini Jack $2.19 768-1011-1-ND FTDI programming chip $4.50
The cost isn't so bad, but the total footprint for this stuff is pretty high. Also, the FTDI chip needs a pile of passives not listed here. I'm actually thinking these 3 on a sub-board would make a pretty sweet "expansion kit". But I can also make a "convenience" case for putting the charger / USB jack on the board. For 5ish bucks, people don't have to move the batteries back and forth every night, just jack it into their computer. That makes the device a lot more usable.
 Pillow Material Sourcing
Eric did some searching for a suitable "velcro-compatible" pillow material. I found some great stuff called "Display Loop", it's "100% Nylon Fabric with Polyester Foam Core and Nylon Tricot Backing". It is the "hook" half of velcro. Unfortunately, I've only been able to find it in 60" bolts, minimum 5 yards - comes to about $60, plus enormous shipping no doubt.
My favorite supplier so far is Buy Hook and Loop, but you can also get some at Levitt Textiles, and at Hook and Loop (which by the way offers custom sewing into straps - looks familiar!). I'm going to call Buy Hook and Loop at 781-767-4511 sometime soon, and ask if we can get it in 3/4" strips/ribbons, and ask about samples.