I haven’t posted about soil moisture sensors in ages, but I’ve completed a number of iterations and I thought it’d be fun to look over the evolution of the hardware. My goal has been to build a small, low power, inexpensive device, that I can place in indoor and outdoor plants to collect soil moisture, light, and temperature readings. I shared some early information on sensors more than a year ago and will have more to share, but this post will focus on the wireless sensor relay.
This device collects readings from one or more sensors at an interval (15 minutes), then broadcasts the readings to a receiver that uploads data to a store where I can crunch numbers, trigger alerts, and generate graphs.
Hobby-friendly PCB shops typically have a 2-week turn around. I’m new to this and wanted to prototype at my own pace, so I made my first boards on a CNC router. These are “channel isolation” boards, where an an outline is etched around the conductive channels on a copper clad board.
Ultimately, this enabled me to knock out a few boards in a weekend, but it required a lot of experimentation, tweaking, and router bits to get usable results. Producing a single board required a lot of effort to calibrate the CNC for two routing passes (for 2 sided boards) and drilling. A minor leveling or alignment issue would result in a useless board. Once I found a reasonable design, I moved over to a PCB shop, trading long turn-around times for less overall effort and more consistent results.
I experimented with a handful of different radios, but mostly focused on the TI CC2500 and Nordic NRF24L01. Both are available as ~12mm X 20mm modules with a trace antenna. The best price I found for the CC2500 at volume was about $2.00. The NRF24L01 was about twice that. The CC2500 was very inexpensive and has very low idle-time power consumption. But it required a lot of work to configure properly and handle errors. In my experience, it worked very poorly in the presence of noise from other CC2500s. The NRF24L01 worked out of the box, had better range, and was more resilient to interference. Ultimately, I tired of debugging the CC2500 and elected for the pricier NRF24L01.
My latest iteration is a 1.45″ square board, with screw terminals for any combination of 3 temperature, light, and moisture sensors. It uses the Nordic NRF24L01 2.4ghz radio with trace antenna, which gives it enough range to work anywhere inside or immediately outside my house. It runs on an Atmel AtTiny24 microcontroller. The sensor readings are taken from the AtTiny’s on board ADC (Analog-to-Digital Converter). The whole unit is powered by a 3.3V battery. Sensing and reporting every 15 minutes, the battery should last 2-3 years.
Medium: Acrylic with neodymium magnet (discs and rods). Laser cut.
So what does one do with these flavor spheres? Are they strictly novelty? I sure hope not because they’re pretty cool. From my first encounter, I knew I wanted to do something with Spherification, but I wanted it to be useful.
My first idea was to make an ice cream with alcohol. Here’s the idea: Normally, mixing alcohol and ice cream will yield a cold soup, since alcohol has a very low freezing point. Can spherification provide a barrier around alcohol that will enable it to be mixed with ice cream? I set out to find out.
The inspiration for this ice cream is a cherry cordial: dark chocolate, almond liqueur, and cherry.
Custard ice cream base, slightly tweaked from Alton Brown:
Candied Cherries from David Lebovitz:
Almond liqueur spheres from, erm, Kyle Scholz:
Making ice cream isn’t for the impatient. It’ll take at least 2 days.
Day 1 Steps:
Day 2 Steps:
Day 3: Ready to eat.
Alternative serving suggestion: Use spheres as a topping.
Spherification is a “Molecular Gastronomy” technique for making small edible spheres out of just about anything. Since my first flavor sphere experience I’ve wanted to learn more and make my own.
I had the Willpowder kit for basic spherification. The instructions sound straightforward:
But it wasn’t that easy. I chose to start with alcohol and I had a hard time blending the sodium alginate into amaretto. They just didn’t want to mix and my repeated attempts to blend them resulted in lots of air bubbles in the mixture. Worse, when I dropped the amaretto/alginate mixture into my calcium bath it splattered on the surface and spread out into a thin film. Fail.
I had read that others had more luck using “Reverse Spherification” with alcohol. I gave this a try but I could only produce large amorphous non-spheres by dunking a spoon of the mixture in the bath. Dropping from any height had the same issue as above. Also note, reverse spherification is done with calcium lactate gluconate and NOT calcium chloride. I tried. You don’t want the taste of calcium chloride in your spheres!
After some trial and error, I came across an article in Make Magazine that saved the day. The key was to first blend a mixture of sodium alginate and water to create a stable suspension. Here’s a short recipe:
I’ve found that the contents of the spheres leach out if they’re left to sit for some time. You want to use them as soon as you can to preserve color and flavor.
* Some people insist on using distilled water for the bath, since impurities in tap water might prevent the alginate and calcium from bonding. I took this suggestion and haven’t yet experimented with tap water.
Some other things I learned while spherificating:
Now to find something useful to do with these spheres…
I grow plants. For a time, I’ve wanted a low-cost sensor that can live in my plants and broadcast information about temperature, light, water, and drainage that I can compare to ideal growing conditions. I’ve set out to build such a device. This post focuses exclusively on the moisture sensor component.
Commercial grade soil moisture sensors are available, but they are cost-prohibitive for placing in dozens of plants, rather large, and sometimes have very high power requirements for a small device. I’ll need to make this component myself.
I have a handful of designs in mind for the sensor. A couple of other hobbyist projects use a variation on the gypsum block sensor:
I’ve elected for a different design because plaster is quick to absorb moisture and slow to dry. As a result, gypsum block sensors may provide a less granular measure and can inaccurately represent the wetness of the surrounding soil (perhaps I should prove this assertion?).
The designs I’m considering generally share a common component: The sensor is a simple design involving a pair of concentric electrodes, sand as a neutral moisture medium, and a plaster disk to filter out salts or impurities that may cause errors in measurement. These parts are assembled inside a 1/2″ plastic tube cap.
This is a resistive sensor that works when an external device applies a voltage across the electrodes. The medium between the electrodes (in this case, sand) acts as a resistor. As the moisture in the medium varies, the voltage carried across the electrodes varies. This voltage can be measured to determine how wet the medium is.
At the start of the test, I arranged the sensors in a pot of sand, then fully saturated the sand with water. The test ran for about three days, sampling (excessively) once every 30 seconds. Below is a plot of the measure taken by the four devices at 15-minute granularity.
While the measurements from the four devices are relatively consistent, there’s room for improvement in both precision (note the poor measurement granularity and flapping) and consistency across devices (I seem to have one “wet” sensor and one “dry” sensor). A few adjustments should offer an improvement.