The HIH-4000 produces a voltage of
0.826 volts at 0% relative humidity
and 3.198 volts at 75.3% relative
humidity. The device’s datasheet also
shows the equation required to
compensate the HIH-4000 for
temperature, and therefore, make it
more accurate.
The final sensor is the MPS-3138-
015: a six-pin surface-mount absolute
pressure sensor. This device is large
for surface-mount components, and
I’ve found it’s easy to solder to PCBs
(printed circuit boards). It’s
manufactured by Metrodyne and is a
MEMS (microelectromechanical
systems) device consisting of a
Wheatstone bridge etched into
silicon. It produces a voltage in
proportion to the air pressure acting
on its port.
When supplied with five volts,
the MPS-3138 produces a voltage
difference between its + and – leads
of zero volts in a vacuum, and a
difference of 100 mV at a pressure of
1.0 ATM (or 1013 mb). Needless to
say, this device requires an op-amp to
produce a usable voltage for a
PICAXE microcontroller. For that, I
decided to use a TLC-272.
The Recording Weather
Station Circuit
I’ve been using the PICAXE
line of microcontrollers for several
years now because of their built-in
analog-to-digital conversion (ADC)
capability. Each PICAXE can
digitize a voltage to either eight
bits or 10 bits. What the PICAXE
does not have in abundance is
internal memory. So, my recording
weather station takes advantage of
the I2C networking capability of the
PICAXE to add additional memory.
In this application, I chose to
use a 24LC256 EEPROM memory.
The circuit shorts the address pins of
the memory to ground, making its
address 000.
The LM335 temperature sensor
is designed to work at standard air
pressure. In the vacuum of near
space, the device is unable to
dissipate the internal heat generated
by its circuitry. For this reason, I
incorporated a relay to control power
to the LM335.
When the PICAXE is not
measuring the temperature, the
device is shut off and allowed to
cool to ambient temperature by
radiation. Just before measuring the
temperature, the relay is energized
and then the output voltage of the
LM335 is digitized. After recording
the temperature, the PICAXE shuts
off the relay and LM335 again.
Two relays are attached to two
separate PICAXE I/O pins so the
recording weather station can record
images during its mission to near
space. Alternatively, one relay could
power on and off one camera and
the second could trigger the
camera’s shutter. To prevent the
recording weather station from
collecting data prior to launch, a
Commit Port is included into the
circuit.
Software in the PICAXE prevents
it from recording data until after the
Commit Port is activated. There are
two ways to activate the Commit
Port: a momentary pushbutton and a
pair of pins for shorting with a
shorting block. Depending on which
method is used, the PICAXE code
waits in a loop until the I/O pin is
APPROACHING THE FINAL FRONTIER
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www.nutsvolts.com/magazine/issue/2017/06.
June 2017 55
Sensors, meet the readers.
Readers, meet the sensors. From
left to right, the sensors are the
LM-335, HIH-4000, and MPS-3138.
All three produce a voltage
proportional to the temperature,
relative humidity, and pressure,
respectively.
The complete recording weather
station schematic. You’ll want to
check the datasheet on the
sensors in order to connect
them to the PICAXE properly.
