and exploration using BalloonSats if
the flight computer is large enough.
An Extreme BalloonSat
Flight Computer
As mentioned, the core of the
BalloonSat Extreme is Parallax’s
BASIC Stamp 2pe. The BS2pe has
16 memory banks of 2 kb each for a
total of 32 kb of memory. So, there’s
lots of room for programs. Since
BASIC is the programming language
of the BS2pe, the learning curve for
new students is pretty shallow.
The power of Parallax’s version
of BASIC means that in most cases,
only a single 2 kb memory bank is
sufficient to operate a near space
mission. This leaves the remaining
30 kb of memory available for data
storage. That’s enough memory for
15 separate experiments.
However, a flight computer can’t
collect and store data unless it can
interface with a wide variety of
sensors. So the BalloonSat Extreme
flight computer is designed with five
external ports.
First is the flight computer’s
analog port. The BS2pe doesn’t have
built-in analog-to-digital conversion
capability, meaning it can’t process
sensor voltages directly into digital
values. Therefore, it relies on a
MAX186 IC to provide this service.
This ADC is pretty slick. It has
eight channels and digitizes voltages
with 12 bits of resolution (for a total
of 1024 voltage bins). Its maximum
voltage is capped to 4.096 volts, so
with those 12 bits of resolution, it’s
capable of measuring voltage
changes as small as one millivolt.
To make interfacing analog
sensors easier, each channel of the
analog port provides a connection to
+5V, ground, and one channel of the
MAX186. All it takes to operate an
analog sensor is to plug it into one
channel of this port.
The second external interface is
the servo port. This port has three
male headers that connect servos
using the standard Futaba connector.
The BalloonSat Extreme uses a
separate battery pack for the servos.
The reason why is a separate battery
pack prevents a stalled servo from
draining the main battery and
crashing the flight computer due to a
low voltage brown-out.
Next is the camera port where
three of the BS2pe’s I/O pins
connect to relays. These relays take
the place of a camera’s shutter
switch, and allow the flight computer
to trigger up to three cameras to take
a picture. With three cameras, a
BalloonSat can simultaneously record
images in all directions.
The fourth port is the digital port.
Five BS2pe I/O pins are assigned to
this one and like the analog port,
each channel in the digital port
provides ground, + 5 volts, and an I/O
connection to the BASIC Stamp. The
digital port can operate experiments
like a Geiger counter — a sensor that
produces a five volt pulse every time
a cosmic ray passes through.
The last port is the GPS port.
This port is a male DB- 9 connector
firmly bolted and soldered to the
printed circuit board (PCB). It
connects to one I/O pin so the
BS2pe can collect data like mission
time and altitude. The port uses one
of the DB- 9’s pins for serial data,
pin 4 for positive five volts, and pin 5
for the ground connection. Because
of the connection to power, a GPS
can start providing position data as
soon as it’s plugged into the port.
There remains one more external
connection: the commit pin. It’s the
method used to start the BS2pe
program operating experiments and
collecting data. That way, the
BalloonSat Extreme doesn’t record a
significant amount of data on the
ground.
Construction
There are three steps to
assembling the BalloonSat Extreme:
making the PCB; soldering
components to the PCB; and wiring
the flight computer. I’m fond of using
DALPRO (now CamCon Chemical)
products to make PCBs. However,
you may be more familiar with other
products. Whatever the method, use
the bottom copper pattern available
at the article link to make your PCB.
Drilling the PCB
The majority of the holes in the
PCB are for component leads, but
here are 28 holes you need to drill in
a larger diameter.
When I taught electronics in high
school, I and my students hated
APPROACHING THE FINAL FRONTIER
July 2014 63
Four AAA cell holder
Three AAA cell holder
Two sub-miniature toggle switches (use toggle switches with solder lugs)
24-pin IC socket 0.600 inches wide
20-pin IC socket 0.300 inches wide
Male DB- 9 header (PC mount)
Female DB- 9 header (PC mount)
LM2940 five volt voltage regulator
Three reed relays (use Jameco item #1860088)
100 µF or larger electrolytic capacitor (tantalum is preferable)
Two 4. 7 µF electrolytic capacitors (tantalum is preferable)
Two 0.1 µF capacitors
0.01 µF capacitor
Three 1K resistors
10K resistor
3x5 female receptacle
3x8 female receptacle
3x3 male header
BASIC Stamp 2pe
MAX186 12-bit ADC
PARTS
LIST