■ FIGURE 19
sensor without the case as it has a
smaller footprint which makes it easier
to mount in most utility boxes. The
AAG module with the case was a bit
too large for my utility box so it is used
as my indoor sensor.
The DS2438 used in these modules has the capability to take temperature measurements. As long as the
module has plenty of ventilation and is
used in parasite mode, it is possible to
use this portion of the chip as a tem-
❑ 202-0004-01 SO/SOP board
❑ 1-Wire to Serial Adapter (DS9097U-A)
uct_info.php?cPath= 23&products_id= 28
❑ DS2450 1-Wire QuadAtoD
uct_info.php?cPath= 26&products_id= 99
❑ DS18S20 1-Wire Temperature Sensor
uct_info.php?cPath= 26&products_id= 93
❑ Barometer Module (B1-R1-A)
Notice that we are
reading the temperature
sensor from the DS2438 chip.
Once we have a temperature
reading and a humidity
reading, we can calculate the
Dew Point. The dew point is
the temperature at which the
air can no longer hold the
moisture that it contains. If the
temperature drops below the
dew point, the moisture (or
dew) will be released. In some
cases, this will be fog.
■ FIGURE 18
perature sensor. The DS18S20 is much
more stable, so I prefer to use these.
Later, we can combine
other readings like wind
speed and temperature to create a
Wind Chill value.
One tip that I can offer is to coat all
the electronics used in your sensors with
liquid electrical tape (as shown on the
Hobby Boards module in Figure 18).
Just don’t coat the HIS-4000 sensor.
Right out of the box I found that
both these modules delivered an
accuracy of 5% which is better than
most home gauges I tested delivered.
They tracked well with my local
What would a weather station be
without a barometer? Once you add a
barometer to your weather station,
you have the ability to predict certain
As before, I have provided a test
program called HumidityGauge, shown
in Figure 19. The program updates the
display once every 500 ms.
❑ Humidity Module (H3-R1-A)
uct_info.php?cPath= 22&products_id= 46
Again, both AAG and Hobby
Boards offer a barometric pressure
module, but unlike the humidity
modules, they use totally different sensors and 1-Wire interface chips. At the
time of the article, I had just received
the AAG barometer and did not have
a chance to create a test or calibration
program. Once I do, I will publish the
results on the Kronos website.
❑ Humidity Module (TAI8540D), Pressure
en-us/ dept_ 2.ht ml
❑ AAG Weather Instrument (TAI8515)
❑ Standard Anemometer
The Hobby Boards pressure
gauge is shown in Figure 20 will not
run in parasite power mode. Due to
the MPXA4115A sensor used, a
minimum of 14V is needed to power
the board. You do have a couple of
choices in powering the board. You
can use a power injector provided by
Hobby Boards or you can use an
AC adapter. The board has its own
regulator for the logic components.
The board is available in kit and
assembled form, and a PCB and
complete schematic is also available
for the do-it-yourselfer.
❑ RJ11 Surface-Mount Box — I used a
GE TL26101. These can be purchased from
most home centers.
For barometric pressure, you can
keep the sensor indoors. I have found
little or no difference between indoor
pressure and outdoor pressure. This
is best since most of your outdoor