Hack Notes CVA 090618
North Paw Hack Notes, June 18th, 2009
From Skory's Email
So turns out a Triac is basically a transistor for AC circuits, i.e., not what we want. Bash recommends just going with two transistors like in the circuit I made yesterday. Seems like that costs the same and it certainly works.
"Total dissipation" is including load, I don't think we need to worry too much about it.
Lunch resulted in more than anything a clear discussion of exactly what we want so Bash can take that to his contact at AKM (who was apparently unconvinced that their chip could solve our problem). Basically, there are inherent limitations in 3-axis magnetometry that mean you need to pair with an accelerometer. We don't really want to do that, nor do we really care about full 3-axis magnetometry, what we want is less tilt sensitivity without needing to add an accelerometer. Now that this is more clear maybe we can get some good results from these guys!
Also from a conversion with Skory - progress is being made on the schematics. He has removed the EEPROM - cool in our protounits, but not necessary for the kit market, and just increases our board size. Reorganized to put the programmer pins and the comout pins next to each other - people who really want to use both will find a way (e.g. top and bottom headers). Added transistors and a 100uF cap to the motor wiring. 100uF cap is to provide the start up current, especially necessary now that the power regulator and it's caps are gone. Eric will see how large a cap we can get in a small footprint. Compass has been rotated, second power rail added to motor headers. Switch still needs to be moved to a place where it will be accessible from inside the enclosure/holding strap.
Finding New Parts
The "audio jack" idea for power plug was a bad one: these things can short as you plug them in, which is bad. So new parts, actually barrel jacks rather than mini audio jacks:
CP-035D-ND PJ-035D 1.35mm x 3.5mm RA barrel jack $0.41 CP3-1003-ND PP3-002D plug 1.35mm x 3.5mm $0.93
Transistors will serve as our "switches" for the motors, lets find some cheap ones. PN2369A-ND is a less capable version of the 2n2222 (200mA instead of 800mA), it's also about half the price at 19 cents.
Short high uF cap found:
P962-ND ECE-A1AKS470 5mm tall 6.3mm DIA 47uF 10V electrolytic cap $0.102
It's wider than it is high, not sure how acceptable that's going to be. The harsh truth is that all of these squat ones are fat, to get down to even 5mm DIA we must drop both the voltage and the uF-age. On the up side, the price sure is right :-)