NEARSPACE
APPROACHING THE FINAL FRONTIER
■ BY L. PAUL VERHAGE
SOME SENSORS FOR YOUR
BALLOONSATS AND THE GREAT
PLAINS SUPER LAUNCH
NOW THAT YOU HAVE A BalloonSat flight computer (see the November ‘06
issue of Nuts&Volts ), what are you going to measure with it? Well, this month
we’ll look at two sensors: a miniature near space weather station and a
temperature sensor array. With these sensor arrays, your BalloonSat can chart
environmental conditions from the ground to near space and measure the
temperatures of your experiments. Afterwards, I’ll give you a short report on the
Great Plains Super Launch. So, fill your balloon with helium and let’s get started.
NEAR SPACE WEATHER
STATION
The miniature weather station
measures air temperature, pressure,
and relative humidity along with the
internal temperature of the BalloonSat
(Figure 1). I’ve flown this sensor array
on several BalloonSats and I’m very
happy with the way it functions. To fly
as a BalloonSat experiment, the weather station must be small and lightweight
NEAR SPACE WEATHER
STATION PARTS LIST
• Two LM335 temperature sensors
• SM5812 absolute pressure sensor
(0– 15 PSI)
• HIH-4000 (or the equivalent older
HIH-3610) relative humidity sensor
• Two 1K 1/4W resistors
• 0.1 µF capacitor
• Weather station printed circuit boards
(see the diagram in this month’s
column)
• 24 AWG stranded wire
• Headers (0.1” spacing)
• Receptacles (0.1” spacing)
• Thin heat shrink
20 September 2007
(because BalloonSats are typically
limited to one pound of weight). I like
the weather station for its compact size
and also for its affordability (around
$50, as I recall). Now, the only thing
that this weather station can’t measure
that typical home weather stations
can is the wind speed and direction.
However, since balloons are forced to
move with the wind, the GPS receiver
onboard the near spacecraft that is carrying the BalloonSat will fill in the wind
data. Aside from near space use, this
miniature weather station could be part
of a traditional home weather station or
part of a Mars or Titan model rover.
■ FIGURE 1
Most of these items are available
from electronics companies like
Jameco and they don’t need further
explanation. But we will take a closer
look at the sensors at the heart of the
miniature weather station.
The LM335 is a voltage controlled
zener diode in a TO- 92 form factor.
So, it looks like a small transistor (e.g.,
the 2N3904). The LM335 drops a
voltage that’s proportional to its temperature. Specifically, it drops one volt
per one hundred kelvins and (ideally)
zero volts at zero kelvins. The kelvin is
equivalent to a degree Celsius, but the
Kelvin temperature scale begins at
absolute zero — the coldest temperature possible. You can download
the datasheet for the LM335 at the
National site (
www.national.com/pf/
LM/ LM335.html).
The HIH4000 is a three pin SIP
relative humidity sensor that is manufactured by Honeywell. The sensor
produces a voltage that is linear with
respect to relative humidity. At 0%
RH, the sensor produces an output of
0.8 volts and it maxes out at 100%
relative humidity with a voltage of 3. 86
volts. That makes its voltage output
