(Chute) Reefer Madness

I've been working on this on and off (mostly off by far) for about two years. I began using a 3.3V Arduino Pro Mini and tried various non-contact thermal cutting methods for the reefing line, the cutting being done within a small-as-possible reefing unit through which Spectra/Dyneema fishing line would be threaded (it has a considerably lower melting point than the more common nylon monofilament line). Opposing PWM controlled glow plugs were fine at the low voltage used, but weren't durable. Ceramic enclosed vaporizer elements were durable, but required a 2 cell (7.4V) lipo which was physically thicker than I wanted and more difficult to charge from a USB port (requires a DC-DC converter).

I recently discovered the very handy Adafruit Trinket and decided to use the 3.3V version of it along with a miniature, low voltage but high torque linear servo. That combination is shown in the video below. The magnetic buzzer uses 30mA when in action (the rattling in the video is due to the PCB not being secured and vibrating on the breadboard - the buzzer is LOUD). The servo uses 130mA when active under no load. Quiescent current of everything is about 7 mA and will be even less when I remove the indicator LEDs from the Trinket before encasing everything.

The 280mAh lipo has an overcharging/overdischarging circuit on the cell and will be charged via the charging plug using a toy grade quadcopter USB charging dongle. The PCB is homebrew and has some SMD components on the side opposite the side seen in the video. The user's thumb is used to cover the phototransistor window to accomplish the various things that need to be done to set and use the device. It works on a time delay which hopefully should suffice to prevent most bad things from happening from early or late ejections. The use of a small drogue chute in simulations can be used to approximate apogee to surface time, allowing a conservative delay time to be set, especially on first flights of a new rocket.

[video=youtube;x2bdK6PW6ls]https://www.youtube.com/watch?v=x2bdK6PW6ls&[/video]

That's a neat little project! I love the PCB, I'm a fan of designing special boards for projects. I've never homebrewed them myself, though. I've gotten quite fond of ordering from OSHPark though. Moreover, I hadn't looked at the Trinket before, but it does, indeed, look like a handy little board.

Martin Jay McKee

martinjaymckee said:

That's a neat little project! I love the PCB, I'm a fan of designing special boards for projects. I've never homebrewed them myself, though. I've gotten quite fond of ordering from OSHPark though. Moreover, I hadn't looked at the Trinket before, but it does, indeed, look like a handy little board.

Martin Jay McKee

Click to expand...

Thanks. The Trinket is a very handy board when you don't need many ports. For PCB manufacturing, I've had good luck with this fairly well-known outfit:

https://www.seeedstudio.com/service/index.php?r=pcb

They're cheap and make a high quality product. They're in Asia, though.

I've used both Itead Studios and Seeedstudio in the past. Since I rarely need more than one or two boards, and I am almost always way under the 50mmx50mm size, I find that OSHPark is the same price ( or much less, I've gotten a number of designs for ~$2 delivered! ) -- plus, the two weeks rather than three or four is nice. If you really need a half dozen larger boards and you can wait though, you really can't beat the prices of some of the Asian proto-houses.

I did get one board ( about 100mm square ) manufactured by Advanced Circuits, in Colorado. Wow were they beautiful quality.... They blow everything else I've seen out of the water. They better though, for what I paid for them.

Out of curiosity, about what is the all-up weight of the system as it stands going to be? Looks like it could pretty easily be under a couple of ounces.

Martin Jay McKee

martinjaymckee said:

I've used both Itead Studios and Seeedstudio in the past. Since I rarely need more than one or two boards, and I am almost always way under the 50mmx50mm size, I find that OSHPark is the same price ( or much less, I've gotten a number of designs for ~$2 delivered! ) -- plus, the two weeks rather than three or four is nice. If you really need a half dozen larger boards and you can wait though, you really can't beat the prices of some of the Asian proto-houses.

I did get one board ( about 100mm square ) manufactured by Advanced Circuits, in Colorado. Wow were they beautiful quality.... They blow everything else I've seen out of the water. They better though, for what I paid for them.

Out of curiosity, about what is the all-up weight of the system as it stands going to be? Looks like it could pretty easily be under a couple of ounces.

Martin Jay McKee

Click to expand...

I've read in a number of places about OSHPark and from your comments I'll have to give them a try. I've never used Advanced Circuits due to the expense. I found them years ago while doing PCB manufacturer searches.

Nothing I've made for many years has been larger than 3" x 4" and that's the size of my stock of pre-cut double-sided PCB material I usually use although I have a stock of much larger PCB material. Using toner transfer with a heated laminator, a 1200dpi laser printer, and an excellent HP paper, I can reliably etch 10mil traces although I very rarely need to and instead use 15mil and up.

The total weight of every flight component shown in the video is 17.5g as they are. Some wires are much longer than they'll need to be and I won't use connectors, just soldered connections. The wires shown soldered to the controller board are just for testing purposes. I don't know yet what the case will weight. Not much. The screws and insulating nylon washers used for the mounting of electronics are all M2; screws are 2 x 6mm self tapping. Very tiny and I haven't weighed them.

I didn't know much about PCB manufacturing when I got the boards made by Advanced Circuits. Indeed, it was the first PCB I ever designed ( and one of the most complicated ). I was learning the software at the time for both schematic design and layout. Doing the layout took me about a month and a half of 20 hours per week, plus some marathon nights at the end. I just had to find somewhere that would get the boards done quickly. They did but yes, quite expensive.

I've wanted to try toner transfer, but I've never really had a good place to do it, besides, I haven't worked with larger things for quite some time. Most of my have traces in the 8mil range, and many have a section somewhere that pushes 6mil, so I figure I might as well stick with commercial production. If you can do it, self-made boards seem like they could be a major boon though, just for ease of respinning and short turn-around alone.

I'm impressed with how small it's all turning out -- less than 3/4 oz, wow ( again ). The trick is really finding a light enough actuator ( that linear servo is impressive ) and battery. At this point, circuitry can be made almost arbitrarily small ( depending upon how much you want to spend for assembly! ). I'm working on a flight computer and it would be easy to be in the same range -- weight wise -- even without the servo.

Again, I think it's a fascinating project. I'm looking forward to seeing it progress.

Cheers,
Martin Jay McKee

martinjaymckee said:

I'm impressed with how small it's all turning out -- less than 3/4 oz, wow ( again ). The trick is really finding a light enough actuator ( that linear servo is impressive ) and battery. At this point, circuitry can be made almost arbitrarily small ( depending upon how much you want to spend for assembly! ). I'm working on a flight computer and it would be easy to be in the same range -- weight wise -- even without the servo.

Again, I think it's a fascinating project. I'm looking forward to seeing it progress.

Cheers,
Martin Jay McKee

Click to expand...

Thanks. The need for a tiny but strong low voltage linear servo and potential reliable mechanical release mechanism design issues based upon its use were the reasons I initially went with a non-contact thermal cutter design, potentially very easy IF one can find an adequate miniature and rugged heating element. An eBay search for such a thing that might be available due to the popularity of vape pens produced a rugged and reliable unit, but, as I said, they require a fat 2s 7.4V lipo for adequate heating. I couldn't know that for certain until I bought a few for testing.

After that most recent heating element quest failure, just by chance I discovered that a linear servo I bought dirt cheap on eBay years ago just to have one for unknown uses that I've stored in my plastic storage bin of aircraft R/C related stuff since then had such an amazing amount of torque and was designed for use with a unregulated single cell lipo, exactly what I needed!!! Luckily, it's not an obscure servo design and is still being sold by US R/C retail sources. I just bought two more so I can build a total of three chute reefers as I have a total of three Trinkets and always panelize any boards smaller than my 3" x 4" PCB material just to use the available PCB area.

I don't have any plans at all to commercialize this unit assuming it even works reliably, so this is going to be as physically small as it gets. If it does work reliably, I might release all details as a "use at your own risk" DIY system.

I am looking forward to seeing your flight computer and data logging launch system, too. One of the concerns I've always had when considering wireless launch systems is link security and a level of safety/reliability suitable for use with potentially very dangerous pyrotechnics.

Final case design (hopefully).