Well, now that we have working ‘betas’ in the field, it is time for me to get back to the workbench. It’s clear from the November test runs that power management is going to be my next priority. But I approach this problem differently from someone with more technical ability, who could simply dig into datasheets, rooting around for micro-controllers that are especially optimized for low power consumption.
One of my primary design goals is to make this thing easy for anyone (including me!) to build (plumbing parts, standard Arduino, AA batteries, etc) but the practical limitations of fieldwork are also affecting the design. I currently envision a service cycle where we dive to retrieve the units, bring them to the surface, swap the batteries and retrieve the data, and then immediately return them to the sample locations in the caves. I would love to come up with a system that does not break the watertight seal around the electronics, like the optical system of the HOBO Waterproof Shuttle, or the external screw contacts of the ReefNet Sensus; but it is going to be a quite a while before my Kung Fu is that strong. And it might be non trivial to put this kind of kind of transmission window in the current pvc housing.
More typically, one would download data from the units by simply connecting the motherboards to a USB port cable once they were on dry land. But it’s pretty common to get caught in downpour, so just taking your laptop into the field for this procedure can be risky. (In fact it’s not to much of a stretch to say that is how we kill many of our machines.) Now multiply this fiddle factor by the 5-10 units we might have deployed in a given cave and you see where potential bottlenecks could arise on the real world side of things.
So I am still planning to use SD cards as the data storage device, even though I know that card writing is overwhelmingly the biggest power user in the entire system. Somehow I need to buffer the data with a low power system, and only write to the cards once per day.
The sensor read cycles are only generating about 60 characters of data at the moment, so one strategy is to try buffering to the arduinos internal 1k of eeprom memory, which would give me about 15 sample cycles before I had to do a save. This approach has the benefit that it would need no extra hardware, which means no change whatever in the current physical build of the units if the internal eeprom is robust enough for all that traffic without becoming corrupted. Unfortunately, the routines for reading and writing data to EEPROM are kind of primitive, limited to a single byte at a time. And I will have to work on my chops before I am comfortable using pointer variables to directly access memory. But perhaps there are easier ways to write data to an eeprom?
If I add a larger external eeprom chip to the SPI lines, I would have 16-32k to play with; more than enough to do a whole days worth of reading before I have to fire up the SD card. I have also found several DS3231 real time clock shields that already include I2C eeprom memory, so if the libraries are out there, they might be pretty easy to implement and many of these chips support page writes to simplify the code…or perhaps not. And many of the “combo” shields I’ve come across seem to have rechargeable (LIR2032) cells on them, so I am concerned that there is the power hog of a charging circuit in there somewhere.
Alternatively, some RTC’s have 1-2K of internal eeprom themselves, so perhaps I could find an RTC based on those chips. I would not want to give up the accuracy of the DS3231 though.
And while I am sorting all that out the Sparkfun power management tutorial shows that there is still room for other power reductions that can be achieved through software alone. The problem of burden voltage on multimeters, means I will need to buy better equipment to even measure differences down in µA territory. I am still kicking myself for not getting my hands on one of the EEVblog uCurrents before he ran his spectacularly successful kickstarter, which of course means that I wont be able to get my hands on one for months while he deals with the resulting backlog.
And finally, it would not hurt to get a ‘real’ voltage divider in the units, so I know what Vcc actually is, rather than relying on the twitchy internal VCC reading trick to monitor the power supply.
I definitely have my work cut out for me…