been detected entering the stratosphere. When it’s time to
cut the load line, the PICAXE records the final time, air temperature, and air pressure into memory. It then fires the relay
for 10 seconds to melt the line before ending the program.
BUILDING THE PICAXE TERMINATOR
Like the camera timers above, the first components
soldered to the PICAXE Terminator PCB should be the
lowest lying components (that is, the resistors and diode).
Solder an eight pin socket to the PCB instead of the PICAXE
to protect it from damage. Use snappable headers for the
programming header and shorting pins. The components
sensitive to orientation are the diode, voltage regulator,
regulating capacitor, LM335, SM5812, and ultimately, the
PICAXE. Be sure to double check their orientation before
soldering them into place. Since the circuit is housed inside
a Styrofoam box, the temperature sensor and power switch
must be mounted to cables in order for them to reach the
outside of the box.
The terminator’s nichrome coil mounts to the two holes
at the bottom left of the PCB with #2-56 hardware. Use a
three inch length of thin gauge nichrome wire for the coil
(my spool of nichrome wire is about 30 AWG). Wrap the
nichrome wire around a 3/16 inch diameter dowel to form
the coil (I use a jeweler’s screwdriver). The ends are curled
around the #2-56 bolts and firmly bolted to the PCB.
CONNECTING THE PICAXE
TERMINATOR TO THE NEAR
SPACECRAFT
Since I’ve not had a chance to test the PICAXE
Terminator in flight (only ground tests have been performed
to date), I haven’t finalized its mounting design. However, I
envision mounting the terminator to a sheet of Sintra
plastic or thin modeling plywood. This mounting plate will
be large enough to hold the terminator PCB on one side
and two batteries, the logic battery (9V transistor battery),
and the nichrome heater battery (either a rechargeable or
camera battery) on the back. The logic battery will be a
lithium 9V battery and the nichrome coil battery will be
either a 7.2V rechargeable lithium or a non-rechargeable 6V
lithium camera battery.
Attached to the mounting plate and directly in line with
the nichrome coil are two brass or aluminum tubes, with
one above the PCB and the other below. The load line runs
through these two tubes to align the load line with the
nichrome wire coil. The tubes prevent the load line from
pulling the coil out of shape or free of the PCB.
The bottom of the mounting plate attaches to the apex
of the recovery parachute with a split ring that’s also
positioned in line with the nichrome coil. If the ring is not
mounted directly underneath the nichrome coil, then
the load line will twist up around the nichrome coil as the
balloon rotates in relation to the parachute.
There are two things to note here. First, it’s the
NEAR SPACE
mounting plate that attaches to the parachute (via its split
ring) and not the terminator PCB. Second, the load line
from the balloon doesn’t tie to the mounting plate. The load
line from the balloon only passes through the terminator
and its mounting plate. Since the load line ties directly
to the parachute apex, the PICAXE Terminator and its
mounting plate do not experience stress from the weight of
the near spacecraft.
To protect the batteries and nichrome coil from the
cold of near space, a two piece Styrofoam box (a front and
back face) will cover the mounting plate. The front face has
holes that the LM335 temperature sensor, power switch,
and Commit tag pass through. The box is held closed
around the mounting plate with rubber bands. Rubber
bands tolerate the cold and ultraviolet of near space well
while allowing the balloon crew to quickly and easily open
and close the box before the balloon is launched.
To make it easier for the launch crew, pass the load
line through the PICAXE Terminator the night before and tie
it to the parachute apex. The code for the PICAXE
Terminator is also available for downloading on the Nuts &
Volts website.
Well, that’s all for this month. Next time is a report
on the Spaceward Game that my girlfriend Rachel and I
attended. Until then,
Onwards and Upwards,
Your Near Space Guide NV
January 2008 95