by Marvin Mallon
Do you wonder what altitude your model
Cessna made it to on that last flight?
Need to know how high up the face of
Half Dome you’ve climbed? Wonder how far
down it is to the swirling Pacific as
you hang glide over Torrey Pines?
Sounds like you’re a candidate for a
Zlog miniaturized recording altimeter.
This intriguing device is the
brainchild of Matt Woolsey, sole
proprietor of Hexpert Systems located
in Grass Valley, CA. Principally created
as an accessory for hobbyists engaged
in model aircraft flight, the concept of
a recording altimeter lends itself to a
number of applications. This article
explores how it works and how it can
be adapted to your own project.
The fully assembled unit weighs
only eight grams and measures 1.57”
x 0.92” x 0.37” or in metric terms, 40
mm x 23. 4 mm x 9. 4 mm. The device
is a stunning example of state-of-the-art electronic miniaturization. It was
designed to provide a lightweight,
compact device for measuring and
recording altitude over time. It is
specifically for use onboard a radio
controlled (R/C) aircraft, but should
be useful in other applications, as
well. ZLog provides live altitude
information in the field via its
self-contained digital display, and
recorded altitude data through a
How It Works
The ZLog module uses a high-resolution barometric pressure sensor
system to detect the minute changes
in air pressure that occur due to
changes in altitude. It is sensitive
enough to detect altitude changes
of less than one foot. Since it is so
sensitive, it is also subject to changes
in weather and local pressure
variations. Long-term altitude readings
58 November 2008
will vary considerably due to varying
atmospheric conditions. It is best used
to measure relative changes in
altitude. Key features include:
• Displays altitude information in
• Tracks maximum altitude.
• Records altitude data for later
• USB PC interface built in for
configuration and data download
• Local controls allow configuration
in the field.
• Records altitude either periodically
or when triggered from an external
• Firmware upgradeable.
• Weighs only eight grams.
The host system requirements are:
• A PC with Win98, Windows ME,
Windows NT, Windows 2000, or
• CD-ROM drive.
• USB port.
• Two megabytes hard drive space.
Figure 1 shows the ZLog module.
Two pushbutton switches are used to
configure the device and to select
either a current or maximum altitude
display. A three-pin connector
connects to the model aircraft’s
onboard receiver to obtain power,
but for non-R/C use a battery can be
attached to the servo pass-through
connector. Input voltage can range
from 3. 7 to 30 volts DC. Current drain
is only 13 milliamps. The backlight
draws another three milliamps. The
unit operates over a temperature
range of 0° C to 60° C. Within this
range, the pressure sensor has an
accuracy of ±1.5% of its measured
output to the microcontroller. If the
temperature dips to – 10° C, the
accuracy is reduced to ± 3.0%. These
deviations are non-linear and are
dependent on altitude.
The unit can operate up to
52,000 feet but will only display 9,999
feet, yards, or meters. The recorded
data permits altitude readings to a
maximum of 32,767 feet. Conversely,
the minimum absolute altitude for
operation is –2,000 feet. This figure is
academic since the device is hardly
waterproof, but if taking a reading at
the bottom of the Mariannis Trench is
your thing, then attach it outside your
bathysphere next time you go diving.
At that level, it will display –999
feet/yards or meters and record (either
relative or absolute) up to – 32,512
feet. Negative altitude range is
required to measure altitude in
low-pressure zones (e.g., in a storm).
Water pressure increases much faster
with depth than does air pressure.
Either above or below sea level, the
unit has an altitude resolution of one
foot (or one meter, whichever is the
chosen measure). These parameters
are summarized in Figure 2. Figure 3
is a block diagram of the ZLog unit.
Here are descriptions of each section:
• Integrated Pressure Sensor — Silicon
micro-machine sensor with integrated
signal conditioning, amplification,