November
2004, Issue 172
Wi-Fi
Sunlogger
Ingo’s
Wi-Fi Sunlogger is a solar-powered data collector
that records sunlight exposure. This RCM3400-based
device is the perfect tool to have on hand when scouting
out sites for solar panel installation.
by
Ingo Cyliax
POWER
CONSUMPTION
The
system’s power consumption needs to be on the average
less than the solar production minus the inefficiency
of charging the battery (66%). This leaves an average
power consumption:
or
approximately 72 mWh per day. At 3.6 V, this works out
to be approximately 20 mAh per day. If your system needs
to run 24 h per day, this comes out to be less than
0.833 mA of constant current draw, which is not very
much.
My
system consists of a processor, A/D converter, and wireless
communication system. The Wi-Fi card alone uses 300
mA when transmitting. How can you make this work? The
answer is by reducing the duty cycle. The system is
in one of three states: it’s low-power Sleep mode, it’s
collecting data and storing, or it’s bringing up the
communication system to talk.
You
can reduce the amount of time needed to communicate
by communicating only when absolutely necessary (i.e.,
when downloading the data that has been collected).
If you communicate 30 s per day [30/(3,600 × 24)], the
following is true:
You
have to make sure the system uses less than 0.729 mA
of constant current for the rest of the day. Or, to
put it another way, running the Wi-Fi card for 30 s
per day uses up more than 12.5% of the available power.
These
types of calculations are necessary for any kind of
solar-powered system, including the Mars rovers. Communication
is expensive and only allowed for short bursts or at
low data rates to conserve power, especially if you
plan on doing other useful work.
So,
now you have a rough idea about what the power budget
is going to be. Let’s look at ways to make this work.
