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January 17, 2010
News - Crowdsourcing Algorithms
about a month ago
I did a preliminary test using my Nexus 7 tablet with an app to capture accelerometer data. I had the screen facing up and tapped on the screen. There were significant jumps in values (possibly implying my assumption below that common mode noise is filtered to be less than ideal). So this scheme seems workable. But there are some issues remaining:
 using FIR/IIR or even DFT/FFT filtering is out of the question, since the data sampling is not truly periodic. The closest thing to use for aperiodic data is something called a periodogram, that is not really useful for harmonic analysis.
 The “notes” state that sampling is approximately every 2 ms, but does not state what the accelerometer’s registers are setting (e.g. are they at defaults?). Register settings also affect whether or not the chip uses internal high-pass filtering (to eliminate DC offset) and some other issues.
 From my data (before I looked at Nate’s data) it looked like I could “see” my knocks and reminded me of a nonlinear “threshold” filter I did for work. The Nexus data was sampled every 5 ms (vs. Nate’s 2ms) and I could see each knock as “damped ringing” of the MEMS accelerometer data with about a 10-20 sample duration. I need to compare against Nate’s data to see if it too “rings” and with what periodicity (preliminary answer is it rings with a period of about 50-100 samples).
I’ll get back (soon?) with an update soon.
So this setup is really a “knock” sensor. If the accelerometer were IN the bag, then it would be a punch sensor. Most accelerometers are designed to reject mechanically coupled noise, so what this is trying to do is measure the ineffectiveness of the manufacturer’s mechanical isolation (which could be digital if there was another knock sensor onboard to do common-mode rejection).
I think a microphone is a better sensor for this application. Maybe one of your MEMS microphones.
News - Hackster Project of the M…
about 4 months ago
 A more literal translation is: “Is my elevator here yet?” Yours would translate to: “Wo ist mein Aufzug?”
 Since it works in the elevator, THAT elevator is not a (good) Faraday cage.
I just got one of these. I wanted to add the “optional” on-board 23LC1024 SPI SRAM, but I can’t seem to find out whether the board footprint is SOIC or TSSOP. Which is it? The pjrc web site didn’t have anything useful either. I guess I could order both types, but that sucks.
about a year ago
What do you mean by “three pairs of items”? I see only single pairs (two of 1 or 1 with 2).
That’s got to be an “oops”.
about 2 years ago
So where are the accuracy specs? Someone still working on that?
about 2 years ago
Your GitHub link for bus pirate is dead.
No public wish lists :(
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