8 days ago •
A bunch of the sensor boards arrived this week! I confess that holding them in your hand is a lot different than seeing them in Eagle CAD -- these new sensor boards are tiny.
Modular Sensor Boards
The first three (of five) sensor boards arrived, with the radiation sensor board (and the touch interface board) arriving a few days later. The spectrometer board is the only sensor board that still has to be designed, and I'm waiting on laying it out until I have a better idea of how the radiation sensor board fits on the back. The spectrometer is the largest sensor in the device, but if there's room I might be able to lay it down and make the entire device even thinner -- definitely worth holding off a few weeks on.
The first thing that struck me is that these sensor boards are *tiny* -- much smaller than any of my previous boards. They're about the size of the tip of my finger! I think the idea of utilizing as much real estate on the outside of the device as possible for sensors (rather than just having them all face towards the front) is really a fantastic design idea for keeping the device small.
These boards are so tiny that they're a bit of a challenge to attach a solder stencil to. Since invariably many of the parts I use end up having very fine pitches (0.4 to 0.5mm), my favorite method of stenciling is to clamp the board and stencil in a clamp, adjust it so it's aligned, then squeegee on some solder paste.
Here, the sensor board combining the Honeywell HMC5883L 3-axis magnetometer (left) and the Invensense MPU9150 9-axis accelerometer/gyro/magnetometer (right) is being assembled.
Here's the board, with solder paste. Looks good! The paste on the MPU9150 is a little misaligned by a few tenths of a millimeter, but it tends to sort itself out during the reflow process. I usually adjust some of the more worrisome looking misaligned paste around with a pair of tweezers before placing the components.
Here's the board with paste and after the components have been placed. Time for reflow!
I tend to reflow most of my boards in a $20 toaster oven. It sounds crazy the first time you hear it, and you feel crazy the first time you do it, but it usually works out very well. Sparkfun has a tutorial on converting toaster ovens into reflow ovens, but I live a little more dangerously and just set the heat to maximum and bake -- watching the board like a hawk until it reflows, and then popping it out of the oven immediately after.
These sensor boards are /so small/ that they will fall through the grill of the toaster oven, so I've layed this one on an old coaster -- er -- breakout board from another project. I'm also trying not to snap pictures for too long, since I have to pull it out as soon as it reflows or the parts will cook!
The finished magnetometer and inertial measurement unit sensor board. Not shown was attaching the sensor board connector to the back of the board -- a 20 pin double-row 2mm-pitch male connector.
I hadn't yet soldered on the sensor board connectors to the motherboard, in part because they were low on stock, so I could only order enough for a few boards. These have alignment pins that go through the board (and make routing a little more challenging), but it's worth it -- they align in exactly the correct, orthogonal orientations.
I confess that one of my largest anxieties about this design was the sensor board connectors. I literally looked through thousands of connectors on Digikey searching for one that was both right-angle, board-to-board, medium-density, strong enough to firmly mechanically support the sensor boards, and larger than a 0.5mm pitch for easy alignment and soldering. There were not many options. The ones I settled on looked like they would have /exactly/ the right mechanical clearance if the board could be routed within 10 mil of the connector footprint, which is a little tight and the first time I've had such a tight clearance, but it ended up working out famously.
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15 days ago •
A very exciting post after a weekend of beginning assembly!
The bare motherboard PCBs and the rest of the components arrived this weekend, and I'm very happy to say that there's now a partially assembled prototype. There's of course still lots of hardware verification and software to write, but moments after finishing the motherboard I wrote a simple test for the OLED display, and took this quick test video. Of course my cat decided it was the perfect time for sits...
I remember nearly a decade ago when I first started designing and populating boards with surface mount parts, I started off like everyone else manually placing paste on each pad with a solder syringe, and then "fine tuning" these globs of paste with tweezers to make sure that there wasn't too little (which would cause a pin to fail to solder), or too much (which may cause a bridge with a neighbouring pin). Recently folks like OSHStencils have popped up, which make very inexpensive laser cut solder paste stencils. This not only saves a great deal of time, but cuts down on bridges or unsoldered pins. I'm still refining my technique -- I usually find for tight-pitch TQFPs with a bunch of pins I'll still prefer to manually solder them to reduce the chance of bridges (which can be difficult to repair), but for passives, small QFNs, or other parts with large pitches, these inexpensive solder stencils are absolutely great!
A set of tweezers and some time later, the parts on the bottom of the board are populated and ready to be reflowed.
When I first read about reflowing boards in a toaster oven, I wasn't sure what to think, but with a few inexpensive boards to practice on for your first few tries you can usually get fairly good results! For large boards sometimes you'll find the heat is a little uneven, and parts in one location of the board may reflow well before others. This certainly leads to cooking parts, especially delicate sensors, but if you watch it diligently you can usually minimize this. (Or, at least have a good idea what parts to replace when the board doesn't power correctly ;) ).
I usually try to keep all of the parts that I'd like to reflow on one side, and the parts that I'll hand solder on the other side.
One of the Arducorder mini motherboards, after reflowing. Looks beautiful! There was only one small bridge, on the TPS63001 buck/boost regulator.
Before populating the other side of the board, if possible I like to try and test the components on the first side. For multilayer boards I tend to put the power circuitry on the bottom side, and this allows the opportunity to test the voltages before populating the other side, and help narrow down potential issues if any issues pop up.
Everyone has aspects of design that they're good at, and some aspects that need a little work. Historically I know that I've tended to over-engineer power circuits, and haven't had a perfect success rate with complex buck/boost switching regulators, so I tend to treat these as a learning experience and give them extra attention. Here the FAN5331 boost regulator looks to be working properly, and successfully generating the 13V OLED supply. Great news, and score one for following those recommended layouts in the boost datasheet!
Making your mistakes cheaply
One of the most beneficial life lessons I've ever learned is the idea of making your mistakes cheaply, which is something my mentor in graduate school used to say. I remember before going to grad school, I used to look up at all these researchers and professors and other monster minds and think -- they must be such bright and intelligent folks from all their years of experience, and rarely make mistakes. Ten years later I'm a one of those researchers, and I make ten times more mistakes than I used to -- infact, research is so challenging I sometime say that I'm a professional mistake maker, because most of what I do is try out ideas that might not pan out. The difference is that I've had ten years to learn how to make those mistakes very quickly, ideally without much cost in time or resources, and quickly move onto finding the solution.
I bring this up because in t
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22 days ago •
Another quick update after a weekend marathon of board layout!
Modular Sensor Boards
Recall that in Step 2: Concept and Industrial Design, we decided that the amount of sensing area for a given case size could be maximized by using all sides of the device for sensors. This allows us to design a smaller, more portable device, but also has a bunch of other design benefits, including modularity. By having four separate sensor boards, we can offer our users an upgrade path, and allow folks in the open source community to easily add sensing capabilities without sacrificing the benefit of the existing sensors. From a design standpoint, it also allows us to perfect the design of individual boards without having to do another turn of the entire sensing system, which saves a lot of time. Since the clock is ticking, it's all about making our mistakes cheaply.
I was able to layout the boards more compactly than I had originally thought, reducing their height by 5mm to 15mm -- and ultimately this may reduce the thickness of the entire device by 5mm, which is fantastic. The boards themselves are:
- Atmospheric Sensor Board (left): Containing the ambient temperature, pressure, and humidity sensors, as well as the multigas sensor.
- Inertial Measurement Unit and Magnetometer Board (right): I've coupled these two together purposefully. Inexpensive magnetic field sensors have really advanced in the last decade, moving from simply sensing field strength to giving a 3-axis measurement, allowing you to roughly determine the field direction. The MPU9150 IMU also happens to contain a magnetometer, and I'd like to experiment with advancing this even further by getting range data using the two magnetometers and a bit of geometry.
- Lightning and UV Board (front): The lightning sensor contains an RF antenna and needs a bit of room, so I've placed it on the front. I've also incorporated a UV sensor that's capable of determining the UV index. The UV sensor also contains some very coarse near-field IR distance measurement, and so I've included the LED for this, just incase. This front board also has a little bit of extra width, which was perfect for the MEMS microphone and amplifier circuit.
- Spectroscopy and Thermal Camera Board (bottom): I still have to layout this board, which will house the 3D Printable Mini Embedded Spectrometer, a light source, a low resolution thermal camera, and likely a linear polarimeter. I've had so much luck reducing the height of the rest of the sensor boards, I'm rethinking the layout, and seeing if I have enough room to lay down the spectrometer (saving a good deal of height), and incorporate the polarimeter on the back side of the board the 0th diffraction order so that everything's on the same optical path. That would be very exciting!
In general, I think there are two remarkable things about the sensor board design here. The first is that there's a very low barrier to entry into making your own sensor boards. Three copies of a given sensor board is only around $3 (shipped!) from OSHPark, and only absolute requires a header that's $2 in single quantity, meaning that folks can really and genuinely get into experimenting with rolling their own boards even on a serious budget. Second, the sensor connectors are mirrored, so that you can plug any sensor board into any of the 4 sensor connectors (left, right, front, or bottom), meaning that folks could conceivably mix and match with their own custom designs or upgrade as they see fit with minimal issues. That's great!
Capacitive Touch Interface Board
I like to include something new on each project that I've never used before, to get out of my comfort zone and gain a bit of breadth. In the design concept I decided to use a capacitive touch wheel as the primary user interface mechanism, with a few buttons for a tactile "click" experience when selecting items or moving backwards in the user interface.
After a great deal of research into capacitive touch wheels and parts, I settled on the MPR121 11-channel touch controller. There are fancier controllers out there that will automatically do the wheel calculations for you, but those parts were a little harder to source, and the central-mas
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