Gardening in the modern age means making things more complicated and arduous, with electrons, bits, and bytes. Behold: the garduino. My brother got me an arduino microcontroller board for Christmas, which to me was a solution looking for a problem. I finally found the problem: fresh herbs are expensive at the grocery store. But apparently not as expensive as adding a bunch of sensors and electronics to your garden.
Combining microcontrollers and gardening is a really popular idea. I think that’s because gardens have very simple inputs and outputs that are easy to wrap your head around. I guess people (myself included) see a notoriously simple and relaxed hobby and can’t help but feel compelled to overcomplicate it. But just about anyone can connect the dots between "Garden needs water" and "I am not a responsible human being who is capable of remembering to water a garden every day" and realize, "Hey, I can use technology to overcome my personal shortcomings," and more than that, "I can bend technology to my will and that will feel good to my ego and my sense of self-worth." After all, no one’s hobby is to buy an irrigation controller off the shelf of a hardware store. Thanks for watching, and let me know what you think.
A few technical details below... If there's anything I didn't address, feel free to shoot me a question.
Moisture sensors that measure the resistance or conductivity across the soil matrix between two contacts are essentially junk. First of all, resistance is not a very good indicator of moisture content, because it is highly dependent on a number of factors which might vary from garden to garden including soil pH, dissolved solids in the water, and temperature. Second, most of them are of poor quality with contacts that easily corrode. For the most part you'd be lucky to get one to last through an entire season. Capacitive sensors are generally more accurate because they are just measuring the change in dialetric properties of the soil which is less sensitive to other environmental factors. They also don't require any exposed conductive surfaces which means they can last a bit longer in the harsh environment of your backyard. My soil moisture sensor (and soil temperature sensor) came from Vegetronix.
The arudino’s analog inputs read voltage, so to use a resistive sensor (like the photoresistor I used to measure sunlight), you have to set up a voltage divider. This is just a really simple circuit which divides the voltage drop between your sensor and a known resistor. You know the current is the same for both, so you can calculate the resistance of your sensor using ohm’s law. The only problem here is that a photoresistor’s relationship to illuminance is log-log that is to say it spans several orders of magnitude. So if you use a big resistor (5k - 10k ohm) in your voltage divider, your sensor will be sensitive to low light levels, but you won’t be able to tell the difference between a sunny day and an overcast one. Since this thing’s going outside, I used a 100 ohm resistor, which should hopefully give me good differentiation between levels of brightness in the daylight.