[TriEmbed] Trail Traffic Counter Update and Lessons Learned

John Vaughters jvaughters04 at yahoo.com
Tue Nov 26 16:34:32 CST 2013


Here is the basic concept of a transistor amp, but it probably does not even need to be that complicated for what you are doing.

http://webpages.ursinus.edu/lriley/ref/circuits/node4.html#SECTION00044000000000000000


I am going to back down from I2C being better on power than SPI, Google has shown me otherwise. It appears SPI is better, so well played on your choice `,~)
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On , John Vaughters <jvaughters04 at yahoo.com> wrote:
 
Here is the basic concept of a transistor amp, but it probably does not even need to be that complicated for what you are doing.

http://webpages.ursinus.edu/lriley/ref/circuits/node4.html#SECTION00044000000000000000


I am going to back down from I2C being better on power than SPI, Google has shown me otherwise. It appears SPI is better, so well played on your choice `,~)



On Tuesday, November 26, 2013 4:54 PM, Charles McClelland <chip at mcclellands.org> wrote:
 
John, 

Thanks for the response - the help I have received from this group is fantastic.


I may have not laid out the sequence correctly, I was using the DC-DC converter on an earlier rev with the 18650 batteries (2 in series - 7.2 V) and the 5V Pro Mini.  There is a 3.3V version of that converter as well.  I just thought it was neat to have a converter that was pin compatible with the 7805, required less components, and was much more efficient.

I looked at using a transistor to switch on a full-scale signal but ran into an issue with that.  I need to be able to quickly and easily adjust the sensitivity of the sensor and did not see an easy was to to that once I moved from an analog to a binary input.  This is probably a shortcoming on my part for not being better at analog electronics.  If there is a link with information on a switching transistor circuit with variable sensitivity, I would be interested in learning more.

I will have to look at the I2C vs SPI power issue but, the micro SD card had me committed to the SPI bus already.  

Thanks,

Chip


On Nov 26, 2013, at 3:49 PM, John Vaughters <jvaughters04 at yahoo.com> wrote:

Charles,
>
>
>That is truly awesome to see everyone come together and help you find a solution. I appreciate the feedback on your project. I am a bit confused on your DC-DC converter, it showed a 7-36V input, yet you are only providing 6V. I wonder if that affects your efficiency? Or  maybe you linked to the wrong model converter? Anyway, not important, but curious to me.
>
>
>On your sleeping and sensitivity issue, consider using a switching transistor to amplify the signal. You could design it as an on/off for full scale 3.3v.
>
>
>Also, I am not sure how much more power you use with SPI than with I2C for your Real Time Clock. I think SPI is a heavier power drain, but I wouldn't bet my life on it and the difference may be negligible.
>
>
>Just a few thoughts.
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>
>John Vaughters
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>On Tuesday, November 26, 2013 2:39 PM, Charles McClelland <chip at mcclellands.org> wrote:
> 
>To all, 
>
>
>I am sorry I missed last month’s meeting where there was apparently some good discussion on approaches to this project.  I am hoping my travel schedule will allow me to make the December 2nd meeting.  I have done a fair bit of work since my last note and wanted to share some progress and lessons learned (and perhaps get some more good ideas from this group).
>
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>1) Features:  In talking to my friends from the local parks and trail managers from bike clubs, it became clear that using the EEPROM on the ATTiny was not user-friendly enough for them.  In addition, they wanted real time date and time logging not just a running millis() count.  So I added a micro-SD card for data logging and a real time clock.  Since the micro-SD card was an SPI bus device, I switched from the 1307 I2C real time clock to the fancy (and expensive) DS3234 based clock which could sit on the SPI bus and had temperature compensation which is important since this device was intended for outdoor use.  This decision, in turn, forced me to abandon the ATTiny for the Arduino Pro Mini.
>
>
>2) SPI bus frustrations - I lost a fair bit of hair trying to get both the real time clock and the Micro-SD card reader to co-exist on the SPI bus.  I found out that in addition to the chipselect wire, I also had to change SPI Mode as each device has a favorite mode for SPI communications.  The Mode is selected in the SD-Card library itself so I missed the fact that is was switching to MODE0 each time I called a function from that library.  I chose not to use a library for the real time clock (just SPI) so I was able to change it in my code as I called the two devices.
>
>
>3) Battery life.  For version 3, my goal was to get to a full week running on standard batteries.  I believe I have achieved this and here are some of the trials and successes:
>- Moved away from the 7805 voltage regulator.  Initially I tried this pin-compatible alternative that is 85% efficient and does not require external capacitors.  Eventually, I chose to simply use the regulator built into the Pro Mini board.
>- Changed from 18650 LiON batteries to 4 AA alkaline batteries in series.  Not sure if this is any better than 3 AA batteries so I plan to test that next.  Thoughts?
>- Moved from 5V to 3.3V - this helped on two fronts.  1) The micro SD card is a 3.3V device so I eliminated some components. and 2) I was able to use the voltage regulator on the Pro Mini for more of my battery pack’s useful life
>- Put in a switch to turn off the LED - it turned out that the indicator LED I was using for setup / testing was a major drain - added a latching push button switch to turn it off for normal use
>
>
>4) Sleep mode - I played with this but was disappointed.  It turns out that the millis() count is stopped in all but the lightest sleep mode.  This was a real problem before I moved to the real time clock but I could also not get the interrupt working consistently with the piezo sensor (too transient a signal?).  May revisit but it looks like I hit my goal for battery life without this step.
>
>
>5) Sensor - I heard there was discussion about using an accelerometer instead of a piezo for sensing movement.  I would be interested in this but have a concern about using a $10 for the most exposed part of the system.  I recently bought a lot of 10 piezos from eBay for $6 and my expectation is that this is the component which will get the most wear and abuse.  I am looking at a few alternatives to the basic piezo - a larger version, one with a mass attached (as I am looking for low frequency signal), and perhaps a vibration switch which could solve my interrupt problems.  Would be interested in any thoughts folks have on this.
>
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>I will plan to bring the board to the next meeting.  I have a trail manager in Connecticut who is asking to be a beta tester so I plan to send him one and once I get his input.  Then, I hope to move to move to a PCB design and avoid the extra cost of the breakout boards.  If anyone is interested, happy to share sketches and designs.   Also, if there is a local park or community who wants to get some hard data on trail use, I would be happy to hook them up in return for their input on the design.
>
>
>Thanks again for everyone’s help.
>
>
>Chip
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