[TriEmbed] Chip's trail activity logger Re: TriEmbed Digest, Vol 7, Issue 16

Fri Dec 20 10:36:00 CST 2013

Charles,

Here is a single transistor inverter. Super simple and low cost, low power.

Nice work, thanks for sharing, I love the entire concept and solution.

John Vaughters

On Friday, December 20, 2013 11:17 AM, Pete Soper <pete at soper.us> wrote:

On 12/15/2013 11:27 PM, Charles McClelland wrote:

Pete,  
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>I think that your idea of a “start to finish” process for making a printed circuit board would be fantastic.  My hope is to be ready to start that process in the coming month or two.  I hope that in addition to making the circuit smaller and more reliable, i can take out some of the high costs of these “breakout” boards I have been buying from Sparkfun.
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OK. 

For all, 
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>I have taken the advice of the group to heart and moved my trail traffic counter from a Piezo electric sensor to an accelerometer - v4.  
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>Here are some lessons learned in this process and a couple areas where I am very open to suggestions:
>- Accelerometers are very cool.  I have created sketches to characterize the orientation of my board and I have a number of ideas for other uses - thank you all for the suggestion.
>- The inexpensive accelerometers like the MMA7361 suggested in a previous post are all analog.  I got it working but saw a few disadvantages primarily that the accelerometer and Arduino need to be close to one another and it tied up three analog lines which needed to be read and summed to create a “trigger” for a knock.
>- The accelerometers which have I2C communications are more expensive but they added the interrupt functions I was looking for.  I went with the MMA8452 which was the lowest priced but also had a “tap” sensor 
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Until you need to worry about optimizing cost the difference between these two chips is surely noise but the convenience difference between the analog vs serial digital interface is immense.  Although I suspect it isn't relevant for your app, the 8452 offers 12 bit resolution vs the 10 bits of the 7361 via your Atmega chip's ADCs, and it may be that without some signal conditioning of some sort you can't even get the full 10 bits.

- I could not find an inexpensive i2C real time clock which could work on 3.3V.  So, I am stuck with the expensive DS3234 on the SPI bus with the SD Card.  Any suggestions here?  A 3.3V 1307 perhaps?
I've been exploring the ST M41T62 which, together with a temp sensor common in microcontroller chips, can be calibrated to 2ppm (5 seconds a month. It can generate a 32khz square wave to run my processor at very low speed when that's desirable, and comes in two packages: a QFN-16 for use with an outboard crystal, and an LCC8 that has a crystal inside it. 

When I get caught up a bit I'm going to make small breakouts for
    both flavors of chip. Here's a rough draft of one layout. I don't know yet if this clock is any good, but I'm going to find out. :-) By the way, the bypass cap in this draft is approximately the same size as the outboard crystal that plays with the QFN-16 version of the clock. (Both boards will be like this one: .4x.5 inches)

- Getting sleep to work is still an issue, turns out the at the Arduino will only come out of sleep when the interrupt goes LOW and the accelerometer’s interrupt goes to HIGH when it is triggered.  Ugh.
>- Unless I want to put an inverter chip on the board, I may be stuck here.  Idle mode allows 
An inverter would not be hard. 

But it looks like the Sparkfun breakout for your clock brings the
    "INT/SQW" signal out. If you don't need the interrupt functionality
    you could program the SQW output to provide an alternate clock
    source for your CPU. If software can reliably switch the CPU between
    it's regular full speed clock and this much slower clock (1hz, 1, 4,
    or 8khz via software config) perhaps instead of "sleep" you could
    set your system to "comatose", where the interrupt handlers start
    off very slow but goose themselves with a clock source change as
    needed. The Atmega chip's current draw will be a direct function of
    the clock rate, so running at 8khz vs some number of mhz would
    translate to a large savings.

- My next “to do” is to enable batch writes to the SD Card reader and see if I can put the card 
Working out power saving strategies is surely a fun of this project. Securely (eeprom?) buffering data until you can pump out an entire SD card "sector" would seem to be one way of saving a lot of power, but I don't know.

reader and clock to sleep between events to save power since my Arduino has insomnia
I'm scheming to use the MSP430G2955 which has 56kb of flash memory that the chip can write to with itself. And the MSP430 was designed from day one to run at low power. Insanely low power. My goal is to get one app that currently uses almost eight square inches of PCB space and six AA batteries down to something the size of a thumb drive and running on a single small lithium coin cell. 

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>Thanks again for all your help.  I was able to finish my v3 board and send off to a friend in Connecticut for field testing so I am making progress.
It's very exciting to follow your project with the msgs to the list and sharing at meetings. Keep us posted!

Coincidentally I'm assembling breakout boards for the FXOS8700CQ
    accelerometer/magnetometer chip (also Freescale). With luck and a
    tail wind I may have one assembled to play with at tomorrow's
    meeting at Splatspace. (Meetup page for this meeting here.) But I'm just learning what it's like to apply solder paste for a QFN-16 package. The first attempt was simply comical, as I couldn't keep the stencil stuck to the PCB and paste went through the holes and underneath the stencil, creating a very neat, roughly square blob over the top of the chip's pads.

-Pete

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>Chip
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