The placement of parts for the Smart Cutdown.
you’re not careful. In fact, the radio I used to test the
Smart Cutdown had one failed channel and I’m not certain how or when I damaged it. So exercise some care.
The Smart Cutdown PCB is designed for the miniature
radio receiver to be mounted to it, but I connected my
radio through a six wire harness in case the radio needed
isolation for the PCB. The motor outputs from the miniature radio receiver connect to the inputs of the two
optoisolators (the 4N25s) on the PCB. If the near spacecraft operates the desired button on its toy transmitter, it
energizes an LED inside the Smart Cutdown’s opto. The
LED’s light causes the NPN transistor inside the 4N25 to
conduct, letting current flow to the PICAXE for detection.
The four wire pairs (main power, radio power, cutter
power, and audio beacon) connected to the Smart
Cutdown PCB are strained relieved by passing them
through holes drilled in the PCB. The cables then exit the
PCB from beneath it.
The miniature radio receiver’s voltage depends on the
toy R/C car you purchase. In the Smart Cutdown pictured
in this article, the radio uses a single 1.5 volt AA cell. Main
power can be a six volt lithium battery as can the cutter
power. However, test the cutter power on the ground
before a mission. Some lithium cells will not source
enough current to melt a load line running through the
nichrome coil as a safety feature. This can occur with
rechargeable cell phone batteries, but I was able to use a
non-rechargable lithium camera battery in a prior test.
Speaking of the cutter, the hot wire cutter is three
inches of 30 gauge nichrome wire. It’s wrapped around a
1/8 inch dowel for shaping and then bolted to the PCB
with 2-56 hardware. Below is the PICAXE code I used to
test the completed Smart Cutdown. In it, a BASIC Stamp
signaled the cutdown with two on-off pulses of one second each. In a real mission, the flight computer would signal with one radio channel and then verify with the second (say by alternating pulses).
Well, it looks like we’re running out of space in this
Since the Smart Cutdown enclosure is not subject to
much force, a thin walled Styrofoam box provides all
the necessary protection for the nichrome coil. The
enclosure hooks to the parachute’s apex, leaving the
load line free to slide through the nichrome coil. Do not
tie the load line to the enclosure or Smart Cutdown.
month’s issue. Next time, I’ll include a sidebar on reefing
parachutes with a second electronic recovery aid. If you’d
like to receive a copy of a Smart Cutdown PCB, contact
me. I’d be happy to make one at nominal cost. You can
reach me at my new email address at NearSys@gmail.com
Onwards and Upwards,
Your near space guide NV
November 2008 83