I’ve made some refinements to the Backcountry Logger firmware, based on my experiences in the field last weekend. I’ve decided to bring the Logger Mini with me on my upcoming John Muir Trail trip, rather than the Logger Classic that I brought to Mount Langley. The Mini (shown here) is smaller, lighter, and has a higher-resolution screen than the Classic. Earlier I was concerned about the durability of the Mini, but I constructed a fairly rugged carrying case for it out of a dental floss container!

The biggest problem I had on the Langley trip was accidental button pushing while the logger was in my pocket. The first photo shows my solution to this problem: both the PREVIOUS and NEXT buttons (but not SELECT) must be held in order to unlock the device when awakened from sleep mode. I also got fancy and threw the BMOW logo onto the unlock screen for fun. So far this approach to unlocking seems to have conquered the random button pushing problem nicely.

The extra resolution of the Mini’s 128 x 64 displays really helps the graphs — they look much cleaner than with the Classic. I boosted the OLED display brightness to the maximum, but unfortunately it still looks washed-out when viewed in direct sunlight.

The Mini has a SQ-SEN-200 omnidirectional tilt/vibration sensor, which the firmware uses to detect movement. If there’s movement for four consecutive minutes, it concludes you’ve begun a trip, and starts the trip timer. After 6 consecutive minutes with no movement, the trip timer stops. In practice the movement detection with the SQ-SEN-200 works very well, but the time thresholds may need to be tweaked. The 1:19 trip shown here was my walk down the hill to the store and back.

To protect the Mini, I built a storage case out of an old dental floss container. I cut off the lower half of the container to reduce its height, and then hot-glued a plastic scrap over the bottom opening to seal it. The resulting case is very sturdy, and fits the Mini perfectly.

The firmware now almost completely fills both the program and data memories on the ATmega, which I think is my clue that development is done. Save for possible bug-fixes, then, any further Backcountry Logger development will focus on hardware improvements such as different displays or batteries, and simplifications and cost-reductions of the circuitry.

The backcountry logger and I climbed Mt. Langley this past weekend, providing its first real test in the wild. Langley is a 14026 foot peak in the southern Sierra Nevada mountains of California, about five miles from Mt. Whitney. I brought both the classic logger and the new mini logger for the trip, but ultimately left the mini logger in the car due to concerns over its fragility. The classic logger worked very well, and accompanied me through all the miles of trail and rocks to the summit.

The drive from my home in San Francisco to the trailhead near Lone Pine passes right through Yosemite National Park, so I was able to test both loggers in the car. As we traveled through Tioga Pass on the highway, I was thrilled to see cool elevation profile graphs with numbers in thousands of feet– my first direct proof that the logger works at elevations beyond spitting distance from sea level. You could clearly see each smaller pass on the graph as a series of humps on the overall upward trend. I don’t know why seeing a real-time elevation profile of your travel is so awesome, but it is.

During the drive, I found that the mini logger’s OLED screen didn’t hold up as well in sunlight as I’d hoped. While it looks beautiful indoors, outside in the sun it has a decidedly washed-out look. It’s still readable, but the basic LCD of the classic logger is much better for reading in the sun. I think the OLED offers some brightness or contrast configuration options that may help a bit, so I’ll research those.

We arrived at the 10000 ft trailhead, where I left the mini logger safely in the glovebox. Upon returning two days later, it showed some cool graphs of the 65-degree day to night temperature swings in the car. Meanwhile I packed the classic logger in my backpack’s waist pouch, and my hiking buddy and I hit the trail. Despite being in high mountain country with near 360-degree views, the GPS failed to get a satellite lock, so we relied on the logger’s elevation info plus the topo map to measure our progress on the trail. Spotty or non-existent GPS reception plagued us all weekend, so we were very happy to have the logger to help measure progress during our climb.

The first night we made camp a bit over 11000 ft, and rose early the next morning for summit day. My immediate thought was to check the overnight temperature graph, which showed a nice curve with an overnight low of 30.5 F around 1:00 AM. Ice in my water bottle confirmed the sub-freezing temperature. By 6:52 when this picture was taken, the sun was up and it had already warmed into the mid 40’s. After more experimentation that day and the next, I found that the temperature readings are easily skewed by sunlight or human hands. When left sitting in the shade or dark, the logger traces a nice smooth temperature graph. But move into a patch of sunlight, and the temperature reading can easily jump 10 degrees or more in couple minutes. Just holding the logger in your hands and pressing its buttons can have a similar effect. To get a good temperature reading, it’s necessary to leave the logger on the ground in the shade for a few minutes, then press a button once to wake it and immediately read the temperature.

I had planned to calibrate the logger’s altitude reading every time we came to a pass or peak whose altitude could be determined from the topo map. In practice, though, this wasn’t possible. The real trail wasn’t always quite where the map showed it, and often it wasn’t clear exactly where the trail was at all. Above about 11500 ft, the route to the summit was more of a network of faint use trails converging and diverging, rather than a single well-defined path. Only when we reached the summit was I able to calibrate the logger, where I found it was off by about 300 feet.

This isn’t a mountaineering web site, so I won’t say too much about the ascent, other than that it was a lot tougher than I’d expected. We climbed New Army Pass, then hiked several miles across a desolate plateau at around 12000 ft, before reaching the foot of the summit block. Even in mid-July, there were still several snowfields that we had to traverse or detour around. At about 12500 ft, the trail faded away completely, and we were left with 1500 vertical feet of picking our way through refrigerator size boulders, trying to find a good route towards the top. I’m in pretty good physical shape, but the high altitude combined with the steepness of the slope reduced my pace to a crawl. I wore the logger on a lanyard around my neck, and checked it often to see how many more vertical feet remained.

Here you can see a couple photos of the logger next to the USGS survey marker at Langley summit. The graph in the second photo shows how we had to descend several hundred feet after New Army Pass, before resuming the climb towards the summit. On the return trip to camp, that extra 300 feet of climbing on the way “down” made an exhausting day all that much harder.

Overall the logger worked well, and I was very happy to have it along. One need became obvious after a day of real use, however: some kind of “lock” state. I was constantly pushing buttons accidentally, especially when I wore the logger around my neck, giving rise to unexpected beeping noises. All the random button pushing normally left the logger on some random screen or menu when I went to use it again. While annoying, this was generally harmless, except on the last day when the logger locked up completely. I’m not 100% certain the lock-up was due to random button pushing and not some other failure, but I was unable to revive it without removing the battery to reset. I plan to make a software mod that requires the PREV and NEXT buttons to be pushed simultaneously, while SELECT is not pressed, in order to wake the logger from sleep.

This weekend I begin the John Muir Trail, a three week and 220 mile journey through the Sierras in terrain similar to Langley. The logger will be along for the trip!

Who’s the best solderer in the world? I am. That’s right. All you other soldering fools just want to taste my sweet victory because the Logger Mini works!

OK, in reality I may be in the bottom decile of the soldering class, but after assembling this bad boy, I’ll never fear soldering anything again. All surface mount parts, all crammed in way too densely, all impossible to debug when it doesn’t work… but it does work. Me and my big, bad soldering iron have tamed this beast. Most of the assembly wasn’t too bad, and I was able to get the majority of the parts soldered within a couple of hours. The fine-pitch ICs didn’t give me too much difficulty this time either. But as always, there were a few twists and turns along the way.

As I predicted in my last post, there were clearance problems between the battery holder and several components. I unwisely designed the battery to sit on top of a few smaller components, and it quickly became clear that wasn’t going to work. I cut away part of the holder’s body with a knife to make things fit, but the real solution will require a different style of holder or a slighly larger board. I also struggled with a missing component when I discovered I’d ordered a 390 Ohm SMD resistor instead of 390K Ohm. Rather than string 1000 resistors together in series, I cut up a 390K resistor pack I had on hand, bent the pins to size, and shoehorned it onto the SMD pads.

The BMP085 pressure/temperature sensor also gave me a nasty surprise. Its connection points are underneath the device, but for some reason I thought they also extended outward to the edges of the device and up the sides, providing access for the soldering iron. Nope. All the connection points are completely hidden underneath, inaccessible. Fortunately the pads on the PCB extend out about 1mm beyond the perimiter of the chip. What I ended up doing was tinning all 8 pads, then placing the BMP085 on its footprint, and reheating each of the tinned pads in turn while I pushed down on the chip from above. The idea was that the solder would transfer heat under the chip, melting the solder underneath and bonding it to the connection point sitting on top of it. It seemed to work.

The real challenge proved to be debugging. After finishing assembly, the screen remained stubbornly blank. With no idea if it was a software bug, circuit design error, soldering mistake, or defective part, and no good way to troubleshoot, I was pessimistic I’d ever fix it. This is the kind of situation where I can easily convince myself it’s hopeless and give up, but I tried to be methodical at exploring problem theories as much as possible. Eventually I tried touching an oscilloscope probe to each of the 30 pins on the display connector ribbon, to try to see what was going on, and I discovered that touching one particular pin caused the display to start working! After retouching that pin with the soldering iron, I was in business.

The Logger Mini worked fine for many hours, and I started work on the necessary software changes. Eventually, though, the BMP085 sensor stopped working. The sensor actually lies partly underneath the battery holder, making the problem especially difficult to troubleshoot. Again, I desparied of ever finding a resolution. After many false starts and much hair pulling, I desoldered the battery holder, which was less difficult than I’d feared. I powered the board with a pair of alligotor clips while I poked at the sensor. After retouching all the sensor’s pins with the iron, it sprang back to life. I then replaced the battery holder, restoring everything to factory new condition.

I made a few quick power measurements of the Mini, testing current draw straight from the battery. With the display on, the current is between about 8 to 15 mA, depending on how many pixels are illuminated. With the display off and the logger asleep, the current is 120 uA. That’s a higher sleep current than I was expecting, and some more investigation is needed to determine why. Still, with those current numbers I estimate the battery life will be a very respectable 4-5 months.

The photos here show a straight port of the software from the Logger Classic, and the graphs still only use the 84 x 48 resolution of the Classic. I need to do some more work to fit everything to the Mini’s 128 x 64 display, but development time is running out before the first of my Sierra hiking trips.

While I was traveling, a few goodies arrived in the mail. The most exciting of the bunch are the PCBs for the Backcountry Logger mini version. Pictured here is the mini PCB next to the classic, to give a sense of just how small the mini is. I’ll be assembling the mini prototype sometime in the next few days. Beyond simply being smaller, it also has a few new features, and a completely different battery and power system. Hopefully it works! I think there are going to be some clearance problems around the battery, since I’d thought the battery holder was a pair of end clips, rather than the full-length holder that it is.

Also in this week’s mailbag was the snap-together case for the classic logger. This case was designed using vector drawing software, and the six sides were intended to snap together using beveled tabs on the egde of each piece. It’s a tricky design, because the depth and bevel angle of the tabs must be just right, or else the pieces will be too loose to stay together, or too tight to snap at all. When sizing the pieces, things are complicated further by the need to account for the kerf, which is the thickness of the laser cut. Even when it works as intended, this type of design looks a bit ragged, since the tabs prevent having smooth corners.

I had the case manufactured out of 0.8 mm thick white Derlin by Ponoko. This was my first time using Ponoko, and while the price and quality were good, the speed of the service was pretty slow. It was nine calendar days between when I uploaded the design and when it was acutally cut and shipped. For a simple one-sheet design using in-stock material, I was expecting it to be cut the next business day. My only other laser-cutting experience was with Pololu, which does laser-cutting as a sideline to their main business, and my order from them was cut next day. For a business like Ponoko whose entire premise is custom-cut parts, a nine day lead time seems a bit high. I probably just need to learn to be patient.

As received, the tabs on the case pieces were substantially too big, and the pieces wouldn’t snap together even by applying extreme force. Whether this was because the true kerf was less than advertised, or because my design was flawed, I’m not sure. After half an hour with an exacto knife, I was able to whittle down the tabs and assemble the case you see here. It’s pretty nice, except… where are the buttons? DOH! I guess I should have used tactile switches with a longer plunger, so they’d extend far enough to exit the case. Back to Digikey to order some.

The last mail delivery was some enclosure samples from OKW. These are the enclosures Cesar recommended in an earlier comment, and OKW was kind enough to send me free samples. They look great! I really like the look of the pear-shaped one with green trim, but I think it might be a bit too small to hold the logger mini.