condensation or other contacts with
After using the board several times, we began to
notice that the microcontroller frequently reset. We
eventually traced the problem to radio frequency (RF)
interference caused by an onboard radio tracking
transmitter. We found that the RF was affecting the
potential of the ground plane and thereby the reset pin.
We solved the problem by rerouting several cables,
and “hardening” the reset pin by soldering a 1K pullup
resistor from the reset pin to 3.3V, and a 0.1 µF ceramic
capacitor from the reset pin to ground on the solder side
as shown in Figure 3. We have had no trouble since.
The Sensor Board
The sensor board shown was custom designed by the
authors and other club members using the freeware
ExpressSCH and Express PCB. They are very easy to use
— students became proficient after a few hours of use
with very little guidance. We designed Version 1 as a two-layer board, but have since changed to a four-layer,
making the routing of traces a bit easier as well as
providing a filled power and ground plane.
The dimensions of the sensor board are the same as
the PPTH, so they stack neatly. The base board and
sensor board both use standard 0.1” pin spacing, so you
can easily plug in your own custom circuits using
standard protoboard and point-to-point wiring.
The finished boards arrived in the mail within a week.
Students thoroughly tested the bare boards for pad-to-pad
continuity. In addition to giving students practice using a
multimeter, it also detected some gaps in traces that
escaped detection prior to submitting the files for
The original sensor board mounted on top of the
is proudly displayed by
Maria in Figure 4.
Since then, many
changes have been
made, but the design
essentially the same. A
schematic and PCB
(printed circuit board)
layout of the latest
version (Version 3) are
available at the article
link for your use in
your own near space
Now, let’s go on a
tour of Version 3 of
the sensor board
(Figures 5 and 6).
At the lower right are four light emitting diodes
(LEDs), used to indicate the status of the system. From top
• The green heartbeat LED (LED10) shows the
“pulse” of the microcontroller. It toggles on-off-on each
time through the program loop, telling us that the
program is running. This has been very helpful in
detecting radio frequency interference and battery
• The red SD card LED (LED11) goes on if the SD
card fails to mount successfully at startup.
• The red GPS fix quality LED (LED12) is on if the
number of “locked-on” satellites in less than a value
defined in the code. We require at least four satellites to
assure good values for longitude, latitude, and altitude.
• The red mission box LED (LED13) is on when the
current location is outside of an area defined in the code
by north, south, east, and west, and latitude and longitude
Of course, we are never in the stratosphere to watch
the indicator LEDs, but they are very useful during testing
and just before launch.
Visibility in direct sunlight is acceptable if the LEDs
are shaded. You might prefer clear ultra-bright LEDs for
better visibility. To avoid cross-illumination, be sure to
paint the sides of the LEDs before they are soldered in
place. Current through these LEDs is limited by four 0805
SMD resistors. SMDs (surface-mount devices) give
students some challenging soldering practice.
30 June 2017
■ FIGURE 4. Final product.
■ FIGURE 3. Microcontroller board