desired maximum pressure altitude.
Otherwise, the PICAXE Terminator
waits until the programmed
maximum time has elapsed.
To cut away the balloon, the
PICAXE fires the relay for 10 seconds.
The current flowing through the relay
heats the nichrome coil to a dull red,
melting the line running through it.
The line separates letting the near
spacecraft drop and the balloon rise
by itself. Without the weight of the
near spacecraft holding it back, the
balloon quickly ascends to an altitude
where air pressure will burst the
balloon. It’s guaranteed to burst
even if it was previously in a neutrally
buoyant condition.
THEORY OF OPERATION
The ADC in PICAXE pin 1 and
pin 2 digitizes the voltages of the
two sensors in the terminator. Pin 1
connects to the output of the LM335
temperature sensor. This sensor
produces a voltage of 0.01 volts
per degree Celsius. Theoretically,
the sensor produces zero volts at
absolute zero, or 0 kelvins and maxes
out with five volts at 500 kelvins or
227 degrees Celsius (that’s 440
degrees Fahrenheit).
PICAXE pin 2 connects to the
output of the SM5812 (or its
equivalent) pressure sensor. This
sensor produces 0.5 volts in a
vacuum, 4. 5 volts at one atmosphere
of pressure, and its output is linear
with atmospheric pressure. PICAXE
pin 3 connects to a pair of pins that
are pulled up to five volts with a 10K
resistor. While the shorting block is
on the pair of pins, pin 3 is shorted
to ground. pin 4 connects to the coil
of a 5V reed relay. When set high,
pin 4 energizes the relay’s coil,
switching six volts to the terminator’s
nichrome coil which gets hot and
melts the load line between the
balloon and parachute.
The programming header not
only allows the program in the
PICAXE to be updated, but also allows
access to data stored in PICAXE
memory. On each mission, the
PICAXE records eight, one byte
environmental records into memory.
By reviewing these records, the
terminator’s program can be tweaked
for better performance. Here’s a table
of conditions that the PICAXE-08M
stores and their memory locations:
Condition Location
Air pressure at commit 0
Air temperature at commit 1
Time to tropopause 2
Air pressure at tropopause 3
Air temperature at tropopause 4
Time at cutdown 5
Air pressure at cutdown 6
Air temperature at cutdown 7
■ FIGURE 7
After powering up the terminator, the PICAXE waits in a program
loop for pin 3 to go high (five volts).
The transition to five volts takes
place when the launch crew
removes the Commit tag which is
connected to a shorting block like
the one described above for the
camera timer.
After the tag is removed, the
PICAXE first dumps the data in its
first eight bytes of memory (use
the PICAXE programmer’s terminal
feature to read them). Next, the
current air pressure and temperature
are recorded into memory. Since it
takes a few minutes to finish raising
the balloon before its release, the
PICAXE goes into a short wait loop
before it will begin monitoring the
time, temperature, and pressure.
Then, the PICAXE enters into a one
minute loop where the time is
incremented and the pressure and
temperature are digitized and
recorded as needed.
Air pressure decreases with
increasing altitude, however, air
temperature only decreases with
increasing altitude until the balloon
enters the stratosphere. Once in the
stratosphere, the air temperature
begins increasing with increasing
altitude because of the ozone layer.
When the air temperature begins
increasing (which occurs between
40,000 and 50,000 feet), the PICAXE
records the elapsed time, the air
pressure, and air temperature as the
time, pressure, and temperature of the
tropopause — the boundary between
the troposphere and stratosphere.
The one minute program loop
continues until the maximum time is
exceeded, the pressure begins to
increase, the air pressure reaches its
minimum value, or the air temperature
begins dropping after the balloon has
94
January 2008
