August 2011 Archives

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.