“Growing plants is fun and watering and caring for them is not really a hassle. Microcontroller applications to monitor their health are all over the internet and the inspiration for their design comes from plant’s static nature and the ease of monitoring something that doesn’t run around and sweat. I am relatively new to plant growth and guides on the internet seemed to be written by well meaning but not engineer types. A friend who I asked “how much do I water them…” replied the only way is to heft the plant and if it feels light you water it. He is very good at “growing”. Sticking your finger into the soil does not really help much. Most of the Instructables utilize a cheap soil moisture probe which is prone to a variety of failures—the most blatant of which are inaccuracy and corrosion.
Reviewing the literature reveals that dirt can be up to 40% water and measuring this requires fairly expensive instruments. The cheaper probes rely on water conductance which will vary with dissolved salts and other factors. Above is a graph I did of a container of dirt weighed over 2 weeks followed by oven heating to 300 to remove all non attached water. Forty percent of the total soil is water and over ten hot days of direct sun it lost 75% of this water at a relatively linear rate. So what is the correct level of moisture? Depends on a variety of factors but when building this machine a good clue is to carefully water your plant to the level that you think is right and set it on the machine which carefully measures its weight and then within a set limit adds water when needed. The design can be modified for hanging plant baskets and pressurized water systems.
The machine had to run on solar energy, be autonomous with its own water supply, monitor its water supply by notifications to the web, sleep when not in use to minimize power and remember the base weight and how many waterings and other data in between sleep cycles. The new ESP32 seemed a good candidate for the brain.”