4" Electronics Bay Design for my Level 2 Cert Flight

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Your work and attention to detail is truly impressive and quite informative. Thank you for sharing.

Two questions:
1) I assume the leads from batteries are soldered to the backside of the custom PCB?
2) Where are the switches located in the circuit? Between the battery and the altimeters or between the altimeters and the charges?

One thought I had for tidier wiring on the J3 and J4 connector. Not sure if those are crimped or soldered leads on the connector but after soldering the wires onto the altimeters a printed wire guide could assist with trimming the wires to length before the connector is attached. Not sure if that's possible but that is what popped into my head.
 
wsume99 said:
Your work and attention to detail is truly impressive and quite informative. Thank you for sharing.

Two questions:
1) I assume the leads from batteries are soldered to the backside of the custom PCB?
2) Where are the switches located in the circuit? Between the battery and the altimeters or between the altimeters and the charges?

One thought I had for tidier wiring on the J3 and J4 connector. Not sure if those are crimped or soldered leads on the connector but after soldering the wires onto the altimeters a printed wire guide could assist with trimming the wires to length before the connector is attached. Not sure if that's possible but that is what popped into my head.
Click to expand...
Thank you for the kind words.

I've attached the schematic as a PDF, but the switch breaks the V+ going to the altimeter. So the altimeter is completely inert when the switch is pressed (assuming the switch is enabled via the strapping 0 Ohm resistor). You're correct - the mating leads that go to the battery are soldered in the back side of the PCB.

JST connectors are GREAT, but if there's a downside it is that their crimp tool is horrendously expensive. Typically in the $600 range per contact family, which is impractical for a hobbyist. They sell "jumpers" of different configurations and lengths, which is a much more cost-effective way of getting the wire with the proper connectors crimped by their factory using hydraulic presses. Much better than I could do by hand even using their tool! So my only practical degree of freedom is to alter the length of the wire that is soldered into the Quantum altimeters. The first one I did was a "little" rough. The second was far from perfect, but much better than that first! ;) I'm very comfortable flying with them as they are. There's a finite number of desoldering / resoldering operations that can be done on PCBs, and I'd hate to lift a pad from the board in the search for perfection.
 

Attachments

  • Electronics Bay Sled.pdf
    29.9 KB · Views: 48
Thanks for the schematic.

Ok, I'm not an electrical engineer but I did take EE classes in college and I do understand basic circuit design. I am not trying to be critical but just want to make sure I am reading the schematic correctly.

Looking at the primary circuit, I assume J1 is the power connection from the battery and pin 1 is V+ and pin 2 is V-. V- is always connected to the altimeter (J3, pin 6). Regardless of the state of S1 V+ will be applied to pin 6 on J3 (altimeter). I'm assuming R2 >>> R1 so the switch is limiting the flow of current to the altimeter. You are inerting the altimeter when the switch is open by limiting the current available (via R2) to be safely under the no fire current level of your igniter?
 
wsume99 said:
Thanks for the schematic.

Ok, I'm not an electrical engineer but I did take EE classes in college and I do understand basic circuit design. I am not trying to be critical but just want to make sure I am reading the schematic correctly.

Looking at the primary circuit, I assume J1 is the power connection from the battery and pin 1 is V+ and pin 2 is V-. V- is always connected to the altimeter (J3, pin 6). Regardless of the state of S1 V+ will be applied to pin 6 on J3 (altimeter). I'm assuming R2 >>> R1 so the switch is limiting the flow of current to the altimeter. You are inerting the altimeter when the switch is open by limiting the current available (via R2) to be safely under the no fire current level of your igniter?
Click to expand...

It's totally okay - no worries at all! I gave you a schematic that would not pass professional muster by any stretch!

You're right, J1 and J2 are the battery inputs to the board. Pin 1 on both J1 and J2 represent V+ (battery positive), and Pin 2 on both J1 and J2 represent V- (battery negative). The two batteries are not connected together in any way.

The resistors shown in the schematic are used as "stuffing options" in electronics manufacturing vernacular. R1 OR R2, and R3 OR R4 will have a 0 Ohm resistor installed, which is a direct connection from one side of the resistor footprint on the PCB to the other. In this case, I will install the 0 Ohm resistors for R1 and R3, and R2 and R4 will not be populated so they are an open circuit. This enables the switch to control whether voltage goes to the altimeters. The design uses a snap action switch with a lever arm. When the switch lever is pressed, the switch breaks the connection between its pins 1 and 3 (NC means "normally closed", so when the switch lever is NOT pressed, the NC pin on the switch will be connected to COM pin on the switch, and when the switch lever is pressed that same connection is broken). I will have a relatively thin rod inserted into the rocket's electronics bay through one of the three static pressure ports, which will press both switch levers and opens the path from each altimeter to its battery V+. This keeps each altimeter disconnected from its respective battery. When the rocket is placed on the launch rail, I will pull the rod out of the electronics bay through one of the three pressure ports and the switch will be released. At that point, pins 1 and 3 of both switches will be connected, providing power to both altimeters.
 
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I looked up "attention to detail" in Wikipedia and there was a picture of g.pitts. :D:D:D

I especially liked how you 3-D printed your contact info right into the sled - brilliant!

Lots of TRF'ers are attending your "Advanced Av-Bay school" and learning a lot - I'm one of them. Thanks!:cool:
 
@g.pitts Thanks for the explanation, very helpful. Really appreciate you sharing the build in this thread.

I am curious what switch you are using? I looked at Omron thru-hole style snap switches and the highest capacity switch in that style I found was rated for 2A DC. I'd prefer something capable of at least 5A.
 
wsume99 said:
@g.pitts Thanks for the explanation, very helpful. Really appreciate you sharing the build in this thread.

I am curious what switch you are using? I looked at Omron thru-hole style snap switches and the highest capacity switch in that style I found was rated for 2A DC. I'd prefer something capable of at least 5A.
Click to expand...
Yes, the smaller switches in particular tend to be a little light in terms of current rating. Any reason you are looking at 5A? That seems a bit high to me, but then your application may be different than mine. In my application, I'm using low-current e-matches so the maximum current I'd expect the switch to carry is around 3A (excluding the power-on inrush, which I can't do much about). Inrush will no doubt lead to degradation of the switch contacts over time, but the number of launches this rocket will ever see is tens of flights so I'm not terribly worried about that.

I ended up selecting the C&K ZMSL03130T10LLC, which has silver contacts and is rated at 3A for 100K cycles. This particular variant of the switch also has a slightly higher operating force spec of 195 grams or 6.87 ounces. I figure a little higher operating force on the switch should translate to greater resistance to inadvertent switch actuation during rapid deceleration events.
 
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I'm wanting 5A because I planned to use a 500mAh 2S/20C lipo battery to power my altimeter. So the battery can source up to 10A constant power. They're rated for 30C rush, so max 15A. I'm assuming the ematch will have approx 1 ohm of resistance so that 2S battery would produce ~8A of current when the ematch is energized, unless I put a second ematch or resistor in parallel. In reality the circuit will only be connected for a brief moment so I don't know there is much to be concerned about but having the switch rating closer to the expected current (however brief) seems like a good thing to me. Feel free to tell me I'm wrong or nuts.
 
wsume99 said:
I'm wanting 5A because I planned to use a 500mAh 2S/20C lipo battery to power my altimeter. So the battery can source up to 10A constant power. They're rated for 30C rush, so max 15A. I'm assuming the ematch will have approx 1 ohm of resistance so that 2S battery would produce ~8A of current when the ematch is energized, unless I put a second ematch or resistor in parallel. In reality the circuit will only be connected for a brief moment so I don't know there is much to be concerned about but having the switch rating closer to the expected current (however brief) seems like a good thing to me. Feel free to tell me I'm wrong or nuts.
Click to expand...
If you go the route of adding a second e-match or a resistor, you want to put it in SERIES with the initial e-match. If you put it in parallel, it will lower the resistance seen by the battery, and the current will go up rather than down.

Generally you can exceed the rating of a switch, but it comes at a reduction of its specified life since the contacts are stressed. Just my opinion, and I'd bank my certification flight on this, but I believe that the brief duration you're looking at for exceeding the switch spec is pretty negligible in terms of reliability. It will impact longevity somewhat, but do you think you'll launch this electronics bay 100,000 times? ;) If you want to play it safe and use a switch that is rated at 5A, there are quite a few out there. Were you thinking of making a PCB, or would you mount the switch on a plywood or fiberglass sled? I ask because some switches are available with wire leads, which would make your wiring/soldering job easier if you don't want or need a through-hole mounted device intended for a PCB. If you want to pursue that, PM me and I would be happy to help you make a selection that meets your criteria.
 
curtisheisey said:
I moved away from JST connectors and over to Amass XT30U connectors, available on many 2S lipos and easier to solder. You can also mount it right on the PCB.


https://www.amazon.com/Finware-Upgr..._3?keywords=Amass+XT30U&qid=1581250970&sr=8-3

Tattu 4 Packs 450mAh 7.4V 75C 2S LiPo Battery Pack with XT30 Plug
https://www.amazon.com/gp/product/B07K18DJG5/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1


View attachment 406069
Click to expand...
Yes, these are nicer connectors than are provided with the lower capacity LiPO batteries. However, they're physically larger so as to increase the contact surface area for greater current carrying capability. Unfortunately, that works against me in my effort to keep things as small as practical. Life's always a tradeoff, isn't it? :)
 
g.pitts said:
If you go the route of adding a second e-match or a resistor, you want to put it in SERIES with the initial e-match. If you put it in parallel, it will lower the resistance seen by the battery, and the current will go up rather than down.

Generally you can exceed the rating of a switch, but it comes at a reduction of its specified life since the contacts are stressed. Just my opinion, and I'd bank my certification flight on this, but I believe that the brief duration you're looking at for exceeding the switch spec is pretty negligible in terms of reliability. It will impact longevity somewhat, but do you think you'll launch this electronics bay 100,000 times? ;) If you want to play it safe and use a switch that is rated at 5A, there are quite a few out there. Were you thinking of making a PCB, or would you mount the switch on a plywood or fiberglass sled? I ask because some switches are available with wire leads, which would make your wiring/soldering job easier if you don't want or need a through-hole mounted device intended for a PCB. If you want to pursue that, PM me and I would be happy to help you make a selection that meets your criteria.
Click to expand...
Copy all. I kinda knew the switch would be fine but if a 5A switch is available in the same form factor I'd lean towards that, if not then no big deal either.

I am now planning my first AV Bay build. I've selected the hardware I plan to use. I'm having to adjust my original plan with the new TRA rule requiring a physical switch. I am down to either a fingertech screw switch or a snap switch w/pull pin like you are using. I do have a 3D printer so I planned to print the sled much like you have done. I'm expecting positioning the switch shouldn't be a big problem. I wasn't planning on a PCB but I really like how it cleans up the wiring in the bay. I've never designed a board but I'm not afraid to learn and it doesn't appear to be a very complicated PCB. I like learning new things so I am actually considering it. If not this build then my next one.
 
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wsume99 said:
Copy all. I kinda knew the switch would be fine but if a 5A switch is available in the same form factor I'd lean towards that, if not then no big deal either.

I am now planning my first AV Bay build. I've selected the hardware I plan to use. I'm having to adjust my original plan with the new TRA rule requiring a physical switch. I am down to either a fingertech screw switch or a snap switch w/pull pin like you are using. I do have a 3D printer so I planned to print the sled much like you have done. I'm expecting positioning the switch shouldn't be a big problem. I wasn't planning on a PCB but I really like how it cleans up the wiring in the bay. I've never designed a board but I'm not afraid to learn and it doesn't appear to be a very complicated PCB. I like learning new things so I am actually considering it. If not this build then my next one.
Click to expand...
Good deal! Just in case you wanted to play it safe, I looked up a few switches rated at 5A. Here's one from Omron. These switches also have a spec for activation force, and I set the filter criteria for a minimum of 4 ounces. This particular switch I've provided the link for specs at 120 grams, or 4.2 ounces. I'll PM you on the PCB.
 
It's been a while, and work has gotten in the way of making progress, but COVID-19 has us all "grounded" and I've made a little more progress. Thanks to the feedback of a few folks here, I've broken out the tracker from the electronics bay sled, and the tracker is now going to be situated in the nosecone of my "Level 2" rocket that I'm using for my L2 certification attempt - sometime hopefully soon. I built a primary and spare as a "just in case measure". Here are a few photos of the trackers and receivers, and I'll post the electronics bay sled photos later today.

IMG_3796.jpg
IMG_3798.jpg

IMG_3799.jpg
 
Your 3D printed GPS enclosures look good! You may want to test fit it in the nosecone to see if the sled vibrates (moves laterally) when shaking the cone. If it does, you may want to add a circular "hat" around the forward sled section to prevent excess movement.
 
Nice looking avionics bay.

Be careful soldering wires directly to the PCB. Where the solder stops wicking along the wires becomes a stress point and leads to failure under vibration. A general rule is to always support wires that are soldered to PCBs somehow. You can use elephant snot (silicone RTV sealant) around where the wires interface to the PCB to provide support and damping. Be sure to use the "neutral cure" type that doesn't emit acetic acid during cure. If it smells like vinegar, don't use it ;). This is also useful for supporting large and high components on the PCB, which makes them more robust for boost and deployment accelerations.
 
FMarvinS said:
Your 3D printed GPS enclosures look good! You may want to test fit it in the nosecone to see if the sled vibrates (moves laterally) when shaking the cone. If it does, you may want to add a circular "hat" around the forward sled section to prevent excess movement.
Click to expand...
Thanks, Fred! These things are super snug in the nosecone. There's a PVC screw-in piece that goes behind the back-side of the sled, so it is friction-fit against the mating PVC piece that is epoxied to the nosecone bulkplate (it's the MAC Performance kit for doing this very thing). In addition to that, the large diameter at the base of the sled has no more than about 1/16" of clearance with the PVC threads, so there's really not a lot of opportunity for it to move.

When and if I get to an L3 flight, I have an idea on how I can do this a bit differently - not in terms of the sled, but in terms of how it integrates into the nosecone bulkplate.
 
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OverTheTop said:
Nice looking avionics bay.

Be careful soldering wires directly to the PCB. Where the solder stops wicking along the wires becomes a stress point and leads to failure under vibration. A general rule is to always support wires that are soldered to PCBs somehow. You can use elephant snot (silicone RTV sealant) around where the wires interface to the PCB to provide support and damping. Be sure to use the "neutral cure" type that doesn't emit acetic acid during cure. If it smells like vinegar, don't use it ;). This is also useful for supporting large and high components on the PCB, which makes them more robust for boost and deployment accelerations.
Click to expand...
Yep, totally get that. It's like making stranded wire into solid wire via the solder wicking up into wire beyond the insulation. My L2 certification flight is not planned to be a high-G event (unless something goes incredibly wrong :(). But RTVing any wires is a good best practice I should adopt anyway. Better safe than sorry, right? :)
 
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Here's the electronics bay sled. Fusion 360 image shows that I've designed hole patterns into the sled that will accommodate an Eggtimer Quantum, a PerfectFlite StratoLoggerCF, and a Featherweight Raven4. After the Tripoli pronouncement that we need a mechanical switch into the mix, I decided to keep the altimeters "always on" and break the power going to the charges using the snap-action switches. This way if the power is interrupted due to G-forces, it will not reset the altimeters. And in theory when the charges fire, the rocket will not be undergoing any degree of significant G-force. So that seemed to be the least risky solution to me. Out of another thread here around exceeding the Raven4's current limit, I decided to put a current limiting resistor (1.6 Ohms) in series with each charge output. The e-matches I use require less than 1 Amp to fire and I'm using 2S LiPO batteries.

Annotation 2020-04-04 093109.jpg
IMG_3800.jpg
IMG_3801.jpg
IMG_3802.jpg
IMG_3804.jpg
IMG_3803.jpg
 
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If you are going to scale it up, consider what you don't have to print. Mass scales by the cube of the volume, so goes up quite quickly in larger rockets. I usually start by adding what features are necessary and then provide enough other features to hold them together. This is something I am working to improve each time I do a 3D printed part.
 
Seriously impressive G! Just out of curiosity, what are the link pads J3 - J6 for? I can see that J1 and J2 are your LiPo connections.

This might be a little over the top (sorry OTT!), but I was thinking about securing those wires with a cable tie and anchoring the tie to one of the adjacent screws with a lug. I realise that those wires are short and heavy gauge, but even those could get a jolt under high g accelerations. I probably wouldn't lose any sleep over this though.
 
Voyager1 said:
Seriously impressive G! Just out of curiosity, what are the link pads J3 - J6 for? I can see that J1 and J2 are your LiPo connections.

This might be a little over the top (sorry OTT!), but I was thinking about securing those wires with a cable tie and anchoring the tie to one of the adjacent screws with a lug. I realise that those wires are short and heavy gauge, but even those could get a jolt under high g accelerations. I probably wouldn't lose any sleep over this though.
Click to expand...

Hi Voyager1, thanks, and I knew someone was going to suggest securing those wires! ;) My upcoming flight should result in a whopping 7Gs during boost, so I agree that it is very unlikely to cause an issue.

J3-J6 are installed when using altimeters that only provide only a single output in the form of a MOSFET that opens to ground in order to fire the charge. When installed, those jumpers route the switched battery voltage (when the "remove before flight" flag is removed) out to the appropriate pins on J7 and J8 on either end of the e-bay and ultimately out to one end of the e-matches. It was an easy way to avoid having to "spaghetti wire" the V+ out to the e-matches as is shown in the Raven4 Quick Start Guide:

Annotation 2020-04-04 192104.jpg
Here's a link to the jumpers. They are rated at 10A, so they are more than adequate for this application.

<edit> I've included the schematic to the PCB which may help folks see what's going on there.
 

Attachments

  • Dual Altimeter Sled Rev C.pdf
    25.1 KB · Views: 28
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OverTheTop said:
If you are going to scale it up, consider what you don't have to print. Mass scales by the cube of the volume, so goes up quite quickly in larger rockets. I usually start by adding what features are necessary and then provide enough other features to hold them together. This is something I am working to improve each time I do a 3D printed part.
Click to expand...
You're absolutely right about that! There is way more mass that necessary in what I have here, and I'm sure I could better optimize that. One easy way to reduce it would be to reduce my infill from 80 percent (printing CPE) to something less. Reducing mass in the design phase is the better approach without question.
 
I generally print most of my avbay bits at 15% hex infill, 3 top/bottom layers and 2 perimeter shells. They seem quite strong, but light.
 

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