Internal Active Cutdown v1

Version Details
deadmanv1 Original design using I2C
deadmanv1.1 similar to v1, but uses simpler serial-only communications

This device is designed to melt through a synthetic support rope using nichrome wire. It will listen for a heartbeat signal from the communications module, received from mission control. If the heartbeat signal stops arriving within a certain timeout period, then the cutter automatically fires. The cutter should also fire in response to direct cut command from any module.


Carl Lyster




The device should be made as small and lightweight as possible.


The device will not bear, nor transfer, any load. A single rope line will be run through the device, and when it cuts, both rope ends must reliably entirely exit from the device upon being cut. If both rope ends do not exit, the device will fail to cut down the balloon.

The device will be loosely mounted to allow the line to shift slightly. Batteries should be secured to device. Auxiliary power input connection, or removeable battery securement (such as cable ties) to replace battery, should be provided for bench and cryo testing the device. Solder connection to battery terminals is fine.

Thermal Insulation

This module will not be mounted in an insulated payload box. All electronics for this flight program should be designed for operation at -55C ambient temperature. For the cutdown heater wire, please create a small insulated chamber only around the heater element to minimize airflow both and weight.


Hardware interface spec
There will be GND, and two I2C connections to make. An 8 pin Molex Connector of Type (Molex PN: 22232081) will be used. Pinout is available here. This connector has .1” Pin Spacing. If these connectors aren't immediately available, simple .1” pin header holes may be drilled conforming to the standard pinout.


I2C White Star Balloon bus protocol. I2C slave address for cutdown: 0b0000110 (Hex 0x06) and Device Type are specified.

For flight SpeedBall-1, the Cutdown module need not conform to the initialization and discovery spec listed at the above link. The Cutdown module should be designed with firmware upgradability in mind to accomodate these in the future.

The Cutdown Module needs to respond to several commands:

Command Description
0x12 0x34 0xDE 0xAD 0xBA 0x11 Cutdown NOW
0xAA Reset Uplink Dead-Man Timer.
0x01 0xNN Set Dead-Man timer interval length to NN, and reset it

CutDown NOW should immediately activate the cutdown system.

Reset Dead-Man's Switch should reset the Cutdown Module's predefined (Programmable) interval before cutting the balloon out of the sky.

Timer Interval

The timer should be adjustable via software command, and the last setting should be retained through power-cycling by storage in Non-volatile memory.

Timer adjustment will be via software command argument, in the form of a 0-255 value. 0 should be the shortest interval, and 255 the longest. These should roughly approximate actual minutes, so 0x0F would equal ~15 minutes.

The actual timer intervals will be calibrated while in a cryogenic test by the White Star team.


Power should be provided within the module, from Energizer Ultimate Lithium batteries. Note: This differs from cutdowns in the past that have used 2CR5 battery packs. This deviation is a result of testing many battery packs at -55C. The 2CR5 packs did not work at -55C, whereas the AA Lithiums did.

This is the only module that will be self-powered. This is for safety/redundancy. It should have enough power to run the onboard microcontroller for 96 hours, and still cut down the balloon at the end of that time. Please provide us with the lowest operating temperature of the battery used so we can plan where to mount/insulate/heat it. Cutter should have enough energy for 2 times as long as required to melt the rope at -50c in still air. (will be shielded from wind)

hardware/cutdown/deadmanv1.txt · Last modified: 2018/07/29 17:58 (external edit)
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