Nuts and Volts - January 2008

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.