Hardware and firmware

After giving it some thought, I eventually designed this schematic for the logger:

As you can see, the sensor outputs of the acceleration sensor are connected to the A/D-inputs of the AVR via an 1K resistor. The datasheet of the sensor specifies this: in combination with a capacitor this forms a low-pass filter which filters out the high-frequency crap of the internal oscillations going on in the sensor. I decided on leaving that out and replacing it with software: measuring each pin a few hundred times and averaging the measurements would have the same effect and such a software routine would be lighter than any capacitors. The resistors stayed, though: I wasn't sure if the sensor needed them and I could abuse the resistance to implement a computer interface over the same pins as the sensor used. More on that later.

The rest of the schematic is quite simple: a 3.6V LiIon-battery will feed the circuit through an on/off-switch, the piece of EEPROM is connected to the microcontroller and a small decoupling capacitor is in the power supply line to suppess transients. Looks easy enough: let's build it!

As the whole package had to be small, I decided on starting with the acceleration sensor PCB and putting the other components on there without using a PCB, dead-spider style:

And then I remembered I still had to find a power-supply. I used a LiIon-cell from an old picture frame I had. While the cell is really small, it's still huge when compared to the circuit!

I wish I had a smaller cell or perhaps the time to invent something using a gold cap or something, but time was of the essence and this was the best I could come up with.

For added sturdyness, I glued everything together using cyanoacrylate ('super glue') and globbed some hot glue over all that. In hindsight, the hot glue did make the construction quite a bit more heavy, so I'd probably be better off using some other way to fortify the device.

And this is what it looks like when connected to a laptop, for size. The laptop is an Acer A110 netbook.

The software in it is written in assembly and quite simple; on bootup, it will emulate an autobauding uart on pins PB3 and PB4. If I want to read out the device or start a new logging session, I connect these to an USB-to-serial converter on which I've bypassed the MAX232-like chip so it directly outputs 3.3V signals. As soon as the AVR receives a char on the serial line, it'll autobaud on that and dump the output of its EEPROM to the attached PC. A new character received will then initiate a new logging session. On the PC side, I've written a really simple program to convert the hex readouts into a nice decimal file which can then be loaded in gnuplot or a spreadsheet program. The software is, as usual, licensed under the GPLv3 and downloadable here.

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