Project sundial is a thought I've been having for a while to build a
self-powering, wireless weather-station and ntp reference clock.
The name sundial is derived from the fact that it is a clock
which uses the sun. Just somewhat indirect.
The parts weather-station and ntp reference clock come
from hardware I already have lying around:
And the self-powering part comes from seeing weather-monitoring
stations in Alaska and Canada that use a solar panel and/or windmill
for power. And the wish for complete electrical isolation from the
in-house network (data transfer will have to be done using wifi).
- a conrad weather station kit with serial readout
- a gpsclock gps receiver
At this moment, this is in the stage of a thought that will take some
calculation (to check feasability) and if it can work out I may even
build it. I decided to create and maintain a web-page about it so
others don't have to duplicate my research or someone with some
valuable input might stumble upon this.
At first I was seriously considering solar power but lately I'm
leaning more towards wind power or even a hybrid solution: both
solar and wind. This is the reason the calculations for solar power
are already on this page.
I guess the cost of the solar panel will be higher than that of the
electricity saved unless I calculate over a reallly long time. But
the added lightning protection also has a value. And the fact that
this is an interesting project to prepare and do (if I actually do
the building). Wind power might be cheaper.
Parts that need thought, calculations and work:
- Choosing the underlying computer: something that can work with
12V low power, and can run an OS with ntp capabilities, and has 2
- Choosing a solar panel, solar power regulator and battery that can keep
the computer, weather station and gps unit powered up even during the
winter. Or choosing a windturbine, regulator and battery for the same power
- Choosing a casing that can keep the computer and other electronics
dry and warm enough and keep the weather station exposed to the elements
enough for correct readouts.
- Choosing a location that will allow enough power generation (solar, wind)
and get reasonable weather readings. We live in the city, which makes usable
weather readings hard.
I'd like to have the weather station running, monitoring the outside
weather. The sensors are: temperature, humidity, light, windspeed,
winddirection, rain. All those sensors will have to be mounted in such
a way that their readouts are usable.
Weather sensors 1.0: Conrad weather station set
The weather station is a Conrad 108677 weather station set, containing the
Conrad 108740 base, Conrad 108731 temperature, light and relative humidity
sensors, Conrad 108707 air pressure sensor, Conrad 108685 wind speed sensor,
Conrad 108693 wind direction sensor and Conrad 125202 rain measuring
The strange thing is that the temperature, humidity, light and air pressure
sensors are built in a watertight and almost airtight case. This does not
help for measuring according to 'the rules' which say that the temperature
and humidity sensor should have an open airflow to the outside air.
Power use of the weather station according to the manuals:
max 10 mA for the base module,
5 mA for the temperature, humidity and light sensor module, 9.5 mA
for the air pressure sensor module, less than 1 mA for the wind
direction sensor, 5 mA for the rain sensor.
The sensors have the option to be powered down when no measurement is imminent
to save even more power.
Total: 30.5 mA.
Power budget for the weather station at 12V: 0.366 Watt.
That is not a lot.
Weather sensors 2.0: 1-wire sensors
Update: after the first tests with the Conrad weather station outside I
decided to go for a bit 'hybrid' solution: I'll use the Conrad weather station
with its sensors in the shed and I'll use a more weather proofed 1-wire
sensor somewhere outside. Netley Marsh 1 wire weather station has ideas on sensor housing and
software for this.
Current monitoring is with Hobby Boards 1-wire sensors for humidity,
temperature and air-pressure. These sensors are in a small wooden 'weather
house' on the north-facing wall of our shed, with ventilation. The interface to
1-wire is the USB interface and I use the w1retap software to read the data.
Lightning is measured by the 1-wire sensor
in the attic of the house. Since the shed has fluorescent lights too, moving
the lightning sensor to the shed will probably give some interference too.
GPS ntp clock
The location will be fixed, but that means the time can be measured
quite precise. The GPS antenna will have to be mounted with a clear
view of the sky.
Since the wardriving hobby means I use gps units and give them some wear
down there are multiple gps units available
Option: gpskit unit
The gpskit unit will be the gps
time receiver. The rockwell unit uses 200 mA, the active antenna 15 mA,
the support print an unknown value (but not much). Guesstimate: 220 mA, at
5V: 1.1 watt.
Power budget for the gpskit gps: 1.1 Watt.
Option: rikaline 6010-x5
Another option is the Rikaline 6010-x5 which I still have lying around.
This GPS unit was used on the wardrive box
and it needs some work to the cables to make it usable. According
to specs it uses 70 mA at 3.3V.
Power budget for the rikaline gps: .231 Watt.
LinuxPPS NTPD support
Something low-power. Not much need for CPU power, just for a ticking
clock. Accepting the data from both weather station and gpsclock means at
least two serial ports are needed, preferrably three to keep the serial
console available (this is very rare). Boot from flash and run in ram,
no moving parts. A wireless network card for the connection to the rest
of the network. It would be nice if the computer board included voltage
sensors to measure the state of battery and power input (solar or wind).
Update May 2007: it seems things are changing because AMD Geode based systems
- PC Engines wrap board does
list a second serial port as an option (via LPC bus) but I can't find
any mention of this other than 'an option'.
- The Soekris net4501 does
list a second serial port as an option with some differing opinions on the
usability of this port although the 4501 manual
on the Soekris website (pdf) mentions it is really standard.
The Soekris net4501 draws a maximum of 10W from a 6-20 volt power supply.
Normal use seems to be 5W.
Price at kd85.com : 140 eur.
- The Soekris net4511 has no usable second serial port (it is hidden below
the pcmcia connector).
- The Soekris net4801 lists
a second serial port with a standard 10-pin header.
The Soekris net4801 draws a maximum of 15W from a 6-20 volt power supply.
I've been looking at other options for small PC systems but I keep
returning to Soekris because other systems either use more power (bad), use
5V (12V is standard with solar power), only have one serial port (bad) or
are just way more expensive than a Soekris (it's a hobby project!)
Update January 2008: Just ran into the Alix 1.c board
at linitx, which has the right amount of serial ports and a
separate console (vga+keyb) and can run at 5W of power!
Power budget for the computer: 4.8W
I'd like the weather station to be in our shed (and the sensors outside),
where electronics are in semi-outdoor conditions (temperature will get
below 10 degrees or above 40 degrees, humidity will get over 70%). A
case which is at least semi-outdoor sounds best.
Mini-itx is 170 * 170mm (6.7 x 6.7 inch). Other stuff which will need to be
inside the case: the usb - 1-wire interface.
Power system 1.0: off the grid
In the longer run, this choice was not feasible/too expensive for the
environment I want to run this in (where a power socket is readily available)
but I decided to leave the thoughts and calculations here for others.
The power system will consist of either
The power system will need to have enough capacity to keep the complete
system running when there is not enough input. The battery
will take care of short-term energy shortages (days) but can't for example
store enough energy to survive the winter.
- A solar panel, a regulator and a battery. The battery will have to take
care of energy storage for nights.
- A wind generator, a regulator and a battery. The battery will have to take
care of energy storage for periods with not enough wind.
A regulator is needed to make sure power flows in the right direction: only
from source (solar panel, wind generator) to the battery and not back. It
also protects the battery against deep discharging by the application. The
application simply gets cut off when power is running down.
Power calculations for solar
Power calculations for solar panels are somewhat complicated. What you
see when a solar panel is sold is the maximum power rating, under ideal
circumstances (direct sunlight shining on the panel from above). But the
real world is: the sun only shines part of the day, and the weather isn't
always completely sunny, and the sun rotates over the sky. Searching for
'solar power calculator' found me this one:
Solar power calculator geared towards RV usage a simple one which assumes horizontally mounted
solar panels which is logical for the roof of an RV. But, it's a start to
see what kind of daily power budget comes from what type of panel.
One factor is the angle of the panel and the orientation. Paraphrased and
translated from [ url expired ] zonnestroom hellingshoek and [ url expired ] zonnestroom orientatie the correct orientation is south and the
correct tilt angle 65-70 degrees. This is optimized for
delivering maximum power with minimum sun (in winter) because the battery
is for keeping the system running for a few days.
Complete calculations can be found at Power - Calculations for Solar Arrays and Battery Backup, a quite helpfull page with all the
formulas. Now, to find the correct numbers...
Solar power maps by the advanced energy group
the worst-case ESH (equivalent
solar hours) for the Netherlands is 1 in December. That's
Given this, for an average usage of 5.4 watt (5.397 rounded up),
5.4/12V * 24 hours = 10.8 Ah/day. Correcting for efficiency,
10.8 Ah / 0.8 = 13.5 Ah. To get that from a solar array needs
(at least) a 13.5 Ah / 1 ESH = 13.5 A (or 162 Watt peak) output array.
Simply put: given for example solar module AS 80 from Conrad I
would have to buy 2 of those, invest 1170 euro and give up 1m20 * 1m05 roofspace.
A 6 watt load would cost 0.144 kWh/day costs 0.0314352 euro per day which
would make the payback time for the solar panels over a 100 years.
Power calculations for wind
This being the Netherlands, wind might be the better option.
Found out the calculation is two-part: first find the average effective
windspeed for the location, terrain roughness and rotor height. Our neighbours
probably would not like a 10 meter high tower, so this limits yield. I need
(given the 5.4 watt) 47.33 kilowatt/year. Installing a windmill like the
Windgenerator Air-X 400W/12V from Conrad on the roof of the garden shed would
only give me about 1/4th of the needed power.
Hybrid could be the best solution: both wind and solar. But I could not find
solar + wind battery charge controllers. Until I visited the Elmuseet in Danmark and peeked at their display of a setup
with multiple different solar panels and a wind generator, all feeding one
solar controller input. So it had to be simple! I searched again and this
time found that a few Schottky diodes are enough to make sure power only
flows solar to controller and windgenerator to controller and not 'back'.
Schottky diodes typically have a very low voltage drop of about 0.45 V. The
major disadvantage is the low maximum reverse voltage, typically 20V. Enough
for this application.
Power system 2.0
A 12V power supply in a power socket.
Comments about this page and updates are welcome. E-mail is the preferred
Koos van den Hout
The Virtual Bookcase