timing jumper cables
Tuesday, September 22 2015
Gretchen had something to go to this morning and could only take the dogs for a short walk. So I took them later in the morning on a walk up the Farm Road to the old go-cart tracks and then back via various trails that run west of (but generally parallel to) the Farm Road. Within several hundred feet of home, I put together a pack of firewood weighing 116 pounds, though I also cut longer (non-bucked) sections of trunk and carried them to my biggest staging area west of the Farm Road.
Down in the basement, I eventually determined that the problem with the always-on circulator pump was a consequence of one of the Grundfos PowerTrack valve actuators not being torqued down quite far enough. The way these actuators attach to the valves they drive is just another weakness in their design. If I were to do this all over again, I'd probably use Taco Heat Motor Zone valves, which contain no electromechanical parts and barely even have moving parts (they work on the principle that material expands when heated).
There is still a remaining problem with the solar control system: the Arduino-based controller is always getting a signal that the pump is circulating even when it is off. This is probably because the line running from the 24-volt pump-on signal is being misinterpreted by the ULN2003 that stands between the electromechanical components and an Atmega328. This is probably because it has been damaged over time by transients; I've found my ULN2003s rarely last more than two years. So later this evening, I installed a capacitor (the largest value non-polarized capacitor I could find) across the the relay coil that turns on the circulator pump. I'm guessing, you see, that damaging transients are coming from that coil (and perhaps other places). I used to consider the built-in flyback diodes were enough to protect a ULN2003, but my experience had been that they are not.
In other technical electronic news, for the past two days I've been tinkering with a USBTinyISP AVR programmer. I have two such devices, but only one works. I've determined that the only problem with the defective one is its ATTiny2313 microcontroller. So yesterday I tried to flash a new
USBTinyISP firmware onto a fresh new ATiny2313. This was successful, but there was still something wrong with the ATTiny2313, because it refused to work in the USBTinyISP. Setting the fuses didn't seem to help, though they also seemed to make the ATiny2313 unreachable for further flashing. I was coming into flashing raw Atmel microcontrollers without any experience, and so I didn't know that sometimes when the fuses are set, the microcontroller then requires a crystal for timekeeping (somehow I'd thought the timing always was supplied by the ISP signals when reflashing). Once I realized I probably needed a crystal to get the dead microcontrollers working, I went on a massive search through all my supplies (including my crystal collection and all my piles of scrap boards that I use for cannibalization). I found all sorts of crystals, but the one I needed was 12 MHz and the nearest I could find to that was 11.5 MHz. The solution was one I remember from years ago, which is basically to use one signal from a crystal on a working board to drive the crystal-less circuit on another. Think of it as a timing jumpstart, complete with jumper cables. To pull this off, I added a test point terminal to pin 5 (one of the two crystal pins) of the ATiny2313 on a USBTinyISP board and jumpered from that to pin 5 on the unresponsive ATiny2313. This was what it took to get it working again. From there, actually, it wasn't too hard to flash the ATiny2313 with the correct firmware. The key was to set the fuses first (who knows why), attach a 12 MHz signal to pin 5, and then flash the correct firmware. For some reason all the sites that described doing these things elided important details (such as what the fuse settings needed to be), but this site came closest to perfect clarity. It was also somewhat unclear about what firmware to use, and many sites kept referring me to an Adafruit page that had moved. Here is the firmware I ended up using, which is in a zip file on this server. For those who need to know the precise Avrdude commands I issued, here they are (remember, I was using a working USBTinyISP to flash a ATiny2313 to make a controller capable of running a second USBTinyISP):
avrdude -pattiny2313 -c usbtiny -b 19200 -U hfuse:w:0xdf:m -U lfuse:w:0xef:m
avrdude -pattiny2313 -c usbtiny -P /dev/ttyUSB0 -b 19200 -u -v -Uflash:w:main.hex
My day of electronic tinkering was not yet over, even after a long hot bath during which I'd been reading the latest copy of Make Magazine (it was an issue focused on DIY space projects and featured an interview of The Martian author Andy Weir by Mythbuster Adam Savage). Gretchen meekly came to me and told me she'd drowned her SandDisk Clip Zip MP3 player by accidentally tossing it into a half-full cup of stale coffee when returning from her brief dog walk this morning. I looked at it, and it looked pretty hopeless. But I cleaved it apart, unsoldered the battery, soaked it in water, and then dried it off with a heat gun. When it seemed dry, I plugged in some headphones and a USB cable (for power) to see if it had any life left. I heard some static and weird oscillations, suggesting some of the electronics was still intact. So I soaked it in alcohol and set it aside on top of the Japanese tea pot (a constant heat source) to dry overnight. My urge is always to fix things, no matter how broken they are. This is true even if the only thing I will really be able to achieve is an autopsy.
Not having much hope for the drowned MP3 player, I looked online for a replacement. The SanDisk Clip Zip is a very good MP3 player, so I hoped to find a simple replacement. What I discovered was that SanDisk no longer makes the Clip Zip. Instead they make something called the SanDisk Clip Sport, which supposedly has twice the battery life but cannot accept a RockBox open source firmware (people such as myself prefer to have open source firmware when buying MP3 players). Evidently RockBox enthusiasts have driven up prices of the remaining SanDisk Clip Zips, as the ones I found on eBay all seemed significantly more expensive than they had the last time I'd checked. But for this particular use, which is just to play music for Gretchen when she walks the dogs in the morning, a Rockbox firmware isn't necessary. So I ordered her a SanDisk Clip Sport. It cost $25 (with free shipping) on eBay and I hope it doesn't suck.
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