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
I
moved to California because of some economic changes
that have occurred during the last few years. The sunny
area in which I currently live has little rainfall between
April and November. Thus, it’s kind of fitting that
I would consider building a solar-powered data collector
whose main job is to record sunlight exposure.
Many
people in these parts have solar collectors (or are
planning on installing them). My Sunlogger is a device
that helps you decide where to install solar panels.
In addition, you can use it to monitor the efficiency
of your main solar collectors.
SUNLOGGER
SCHEMES
The
Sunlogger must be placed outside, so it makes sense
to make it rugged and as self-sufficient as possible.
After considering several design ideas, I settled on
using a waterproof plastic enclosure (see Photo 1a).
To reduce the risk of leakage, there shouldn’t be a
physical connection between the electronics inside and
outside the enclosure.
|
(Click
here to enalrge)
|
Photo
1a—A clear case like this is designed to be waterproof.
A Gortex membrane blocks water while passing air
in order to let the case breathe when there are
pressure changes. b—The solar panel fits perfectly
in the selected case. |
It’s
easy to conceive that the unit should be solar-powered
so that external power supplies and, more importantly,
cables aren’t needed. But how do you get the data out
of the unit? In one scheme, the unit would collect data
for a long time, until it is retrieved, opened up, and
read out via a cable. Although this would be an easy
solution, imagine if the unit is on the roof of your
house. It would be a hassle to climb a ladder each day
to get the data.
Another
scheme would be to use some kind of wireless connection.
I have looked into using 900-MHz RF modems (like those
from Linx Technologies), but although certainly usable,
they require a protocol in order to read the data out
of the unit. You also need to build a base station for
your PC or laptop that needs to be compatible with the
RF modulation in your modem. These modems are advantageous
because they are relatively low-powered. Because I can
design my own protocol, there is a lot of flexibility.
In
the end, I settled on Wi-Fi. Although not particularly
low-powered, Wi-Fi has other advantages. Off-the-shelf
Wi-Fi cards cost about as much as a nice duplex 900-MHz
modem module. They are standard and can interoperate
with any other Wi-Fi card or access point. Also, many
people already have a Wi-Fi network at home, so the
cost of integrating a Wi-Fi-based data logger in your
network is pretty much zero in comparison to running
cables or using proprietary RF modems.
Let’s
dive into the details by reviewing some information
about solar panels. Later I’ll describe the data logger
design and implementation.
