This repository contains the firmware for a home-made soil thermometer that my father and I designed and built. A soil thermometer is a long rod inserted into the ground that measures soil temperature at various depths. If you enter the measurements into a Fourier soil model, and assume that the soil is uniform, you can extrapolate soil temperature to any depth, calibrated by just one free constant. Conversely, if you already know the temperature at various soil depths, you can derive the free constant experimentally, which gives you a pretty accurate model for soil temperature as a function of depth and seasonal variation.
Our thermometer is just over 120 cm long and contains seven temperature sensors spaced 20 cm apart. These sensors are steel-jacketed DS18B20 one-wire sensors that are powered up and sampled every 15 minutes by an ESP8266 microcontroller on a NodeMCU board. The microcontroller checks the sensors for errors, optionally consolidates multiple measurements, and transfers the data over WiFi to a web server. (Currently a Raspberry Pi on the house network which runs some custom PHP scripts to store the data and display a graph.) The Fourier model fitting is not done by this code or the web server, but externally in an Excel spreadsheet.
The code contains provisions for a powersaving mode which is currently not activated. In this mode, the ESP8266 polls the sensors every 15 minutes, but only sends the data once an hour. The code can also average these measurements to get a single representative temperature for the whole hour. These power saving features became obsolete once my father installed a solar charging system.
The code is written in C against the native non-RTOS API for the ESP8266,
specifically esp-open-sdk. (I
started off writing the code in NodeMCU-flavoured Lua, but things quickly got
weird, flaky, and short on memory.) Necessarily, a number of parameters had to
be hardcoded, such as the sensor addresses, the web server's IP address, and
the wifi station and password. (These are inside a header file not included in
this repository, called bin/secrets.h.)
The code is probably not very general, and would have to be customized for any other set of sensors. However, parts of it might be useful in other projects, such as the OneWire driver and the DS18B20 driver code. The whole system is event-driven by means of the Event API (posting event messages from various submodules and catching them in the main loop), which I found to be an interesting and effective design pattern for ESP8266 applications.
All code in this repository is licensed under the GNU General Public License v3.