[TriEmbed] Power Questions

[Charles McClelland]

First, 

I want to thank you all for the help I have received so far.  I am working with a group that is building and maintaining trails in the woods around the RDU airport.  We are talking to the state and county parks folks and need some solid data on trail utilization to support your requests.  I am building a simple sensor to count the bike traffic per hour on a trail in the woods over a week.  

Thanks for Paul’s suggestion, I went with a Piezo sensor attached to the underside of a wooden bridge and write the counts every 3.6M clock cycles to EEPROM.  Each count takes just one byte (never more than 253 bikes in an hour) so the setup should last for 21 days before taking up all 512 Bytes of memory.  I tested it last weekend and it worked like a charm - here are some pics and a short clip of the sensor in action.  

Now the problem and a request for help.  I am currently using a 9V battery (typical capacity 310 mAH) and a 7805 linear regulator.  With a fresh battery, I am only getting about 24 hours of operation.  I have some high capacity 18650 LiON batteries which operate at 3.6 volts (voltage curve here).   Which I can put in series using a simple holder.  The problem is that I will not get nearly the full capacity of the batteries (3400 mAH each) before I am below the drop-out voltage of the 7805.  At the PCB Carolinas conference yesterday - I asked around and was delighted to hear about a “high efficiency buck boost regulator” like this one.  However, I can’t seem to find one in a through-hold package.  

I also considered putting a diode in series with the 18650 batteries to knock off a volt.  However,  looking at the voltage curves, it seems that I would exceed the 6V maximum the Atmel data sheet specifies for the ATtiny84 when the batteries are freshly charged.

I want to keep this circuit cheap and small as it will be buried in the dirt for a week at a time and may well get stolen.  Any suggestions would be appreciated.

Thanks, Chip

PS - thank you for the PCB Carolina’s suggestion - made some great connections to local suppliers and enjoyed the free food.
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