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Secondary release mechanism

Page history last edited by PBworks 15 years, 7 months ago

Purpose

If all else fails, we need for the craft to return to earth no matter what. This device will automatically release the payload after a set period of time (probably 3 hours).

 

Mechanism

New method:

For the second AHAB launch, I made a circuit that would time and trigger the Nichrome directly. It was much lighter and ran off a single CR123 battery. Schematic and source code are at weak.org/ahab/trigger.

 

Old method:

A 4 inch length of 32 gauge Nichrome wire is wrapped tightly around the cord that connects the payload to the balloon. One end of the wire is connected to the positive terminal of a series pair of CR123 lithium batteries. The other end of the wire is connected to the collector of a TIP31 NPN transistor. The emitter of the transistor is wired to the negative terminal of the batteries.

 

The base of the transistor is wired to the positive speaker output of a kitchen timer (which runs off it's own AAA lithium battery). When the timer goes off, the transistor shorts the camera batteries through the nichrome wire, which then heats up and severs the cord. Note that because the timer pulses the speaker, the transistor only gets about 0.9 volts average, which is barely enough current to activate the transistor.

 

Alternatives to Nichrome

 

Nichrome wire works really well, and is proven time and again. However, if you're unable to get it, you can use a low value resistor. You'll need much more current, though. An 18 ohm 1/4 watt resistor will get smoking hot with 6 volts (or about half a watt), and seems to be fairly reusable.

 

 

 Also see the Secondary Abort Device Test Plan.

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