[TriEmbed] Power Questions

[Jeff Highsmith]

Chip,

If you could run the ATTiny84 directly from a battery (with a voltage
within the '84's specs), you wouldn't have to lose power as heat knocking
the 9V down to 5V.

Another way to save significant power would be to have the ATTiny use it's
low power modes. This won't be effective if there is still a linear
regulator in the circuit. If you had a sensor that acted like a switch or
had a digital output, you could have it wake the '84 upon sensing. The '84
could log a bike and go back to sleep. You would use the interrupts or even
the reset pin for this. A year or two ago, I made my sons' a traffic light
using three LEDs (green, yellow, red) in Radio Shack's smallest project
box. The circuit is just an ATTiny85 (maybe the low voltage version)
powered by two AA cells. The "on" button is actually tied to the reset pin,
so it resets the '85 when pushed (I was in too big of a hurry to learn
interrupts that day). The program tells the '85 to run the traffic light
for two minutes (average toddler attention span) and then go into low power
sleep. In the couple years of play and (mostly) standby, I can't remember
ever changing the battery.

I'm not sure what would happen to the memory when the battery eventually
drops below the '84's minimum, but maybe you could gracefully remove power
completely at that point using some discrete components.

Jeff :)


On Oct 24, 2013, at 10:24, Charles McClelland <chip at mcclellands.org> wrote:

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<https://www.dropbox.com/sh/9qdzturvffr66do/3QMS0SvYHP> 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<http://www.lygte-info.dk/pic/Batteries2011/All18650/Capacity-0.2A.png>).
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<http://www.intersil.com/en/products/power-management/switching-regulators/integrated-fet-regulators/ISL9110.html>
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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