Don't forget the 750-millivolt thermocouples found on
gas water heaters and furnaces.  They deliver power for decades as long as
the flame from the pilot light hits the thermocouple.  If one had four or
five thermocouples in series, there would be enough voltage to run a PIC
and other low-current logic circuits.  I don't know the current generated by
such a thermocouple, but older gas furnaces did use them to generate the
control voltage that went through the thermostat to open the solenoid valve
and turn on the main burners.  If the gas went off or a draft through the
vent sucked out the pilot flame, another little valve lost power and closed
to prevent gas from escaping when the pilot was not lit.  While this was a
very inefficient way to generate electric power, it was a case of making good
use of what was already there and that is almost as good as getting something
for free.

Martin McCormick WB5AGZ  Stillwater, OK 36.7N97.4W
OSU Center for Computing and Information Services Data Communications Group