In July, 2012, I visited with a friend of mine who was living in San Jose, California. He ended up showing me a small Kumquat tree he had picked up, and planned to grow in a large pot.
I became rather attached to the idea of a potted fruit/citrus tree of some sort. Unfortunately, even the dwarf varieties of many fruit/citrus trees still grow to be rather quite large, so that somewhat limits the options. However, one stuck out as a good contender for size, the pacific northwest climate, disease/pest resistance and aesthetics. The Meyer Lemon.
From my reading, the Meyer Lemon is now known as the “improved” variety due to increased disease resistance. This in addition to a workable size of 4-5′ tall when mature (if pruned) and a tolerance for the climate in the pacific northwest made this an easy choice. Plus who doesn’t like some lemon with some tea or a coke?
The flip side to this story is that I have a terrible history taking care of plants. About the only plant I can reliably keep alive is “lucky bamboo” and that’s because I can fill up the reservoir with water, and it will be good for the next week or two. This lead me to pick up a soil moisture meter at my local home improvement store. The most basic one they had also had a light sensor and a pH meter, which both were somewhat interesting. In any case, the meter became a useful tool in determining what the tree needed, but not in making me into an attentive caretaker.
As such, I needed a device that could live in the pot (water/bug/whatnot proof), and somehow get the necessary information back to me, without me having to go and check on it (the issue in the first place). I did a little searching and found the Botanicals Kit on SparkFun, but that’s obviously not waterproof, and requires power and ethernet cords to the pot. Quite a hassle when it’s outside… Looking over the available schematics, I decided I’d build my own. It would need to run for a reasonable period of time on batteries, and would report the data back to my computer wirelessly.
Working with an ATmega328 as the brains of the system, a couple ZigBee Series 2 wireless radios, and some NiMH rechargable AA batteries, I had the basis of the system to be. The sensors (soil moisture and light intensity) were easy to implement. A couple of stainless steel rods connected with a junk-box transistor and some resistors formed the soil moisture meter, and a junkbox solar cell formed the light intensity meter. The harder part was going to be getting the system to run for a reasonable period of time on a few AA batteries.
I ended up spending a few afternoons fiddling with the sleep and power saving modes available with the ATmega328, and managed to get sleep power draw below 1mA. (This number depends A LOT on voltage. At 5v it draws approximately 0.8mA, however, at 4v the consumption is an order of magnitude less at 0.07mA. Also note I’m not using any of the deep sleep modes. I had to keep the main clock running to be able to wake from a timer. Deeper sleep modes offer a lot more power savings.)
However, the power draw while active is much larger, bringing the ATmega up to full operations, taking measurements and powering up the Xbee radio and transmitting data. At 5v this adds up to about 140mA. Though I suspect it’s a lot lower at closer to 4v.
Unfortunately, this makes it rather hard to guesstimate how long the batteries will last, but if the system continues to draw power at a 5v rate (ignoring that the voltage will get lower as the batteries run down), it should last about two weeks on a charge. However, as the batteries get lower, the system will end up using far less power, so how long it will actually last is anyone’s guess. That is until the batteries run low and I go and charge them. Then we’ll know exactly how long they last.