Editing 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 sinusiodal 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 only solution is to get the batteries away from the compass, or otherwise change the existing solution.  After some brainstorming, we settled on the idea of 


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.

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.