HG22, as you are aware I was monitoring soil temperatures using DS18B20s and an Arduino. It worked really well as long as my power source held up. I had a blue-tooth connection to an old phone so I could check the temperatures on the fly rather than having to remove the SD card and transfer the data onto the PC. The blue-tooth was a big battery drain especially if they disconnected for some reason as it constantly tried to reconnect.
I am now playing with a 'solar heating system' in my small greenhouse but have been too busy rebuilding my fruit cage and other real gardening chores, to give it the time. Hopefully get back onto it when spring planting subsides.
Basically it is a black painted radiator feeding a dark green oil drum. The water is pumped during daylight hours for 1 minute every 15 minutes. My problem at the moment is the startup current for the pump motor keeps burning out relays.
The plan is eventually to have one radiator painted black feeding the oil drum, the pump will only run when the radiator temperature reaches a certain threshold above the oil drum temperature, thus saving battery power by only pumping when there is heat to be gained. A second radiator will be my propagator bench with a transparent "sun cloche" over it. At night when the temperature drops to a predetermined level another pump will run the water through the bench radiator to heat the cloche. I could also set it to pump the water to cool the bench radiator during the daytime if the temperature starts to rise too high on a very sunny day.
I had thought of keeping the drum on the sunny side of the greenhouse to add to the solar heating effect but am now thinking that using a plastic drum buried in the ground will insulate the drum well, may provide a little geo-thermal heating effect and also free up the floor space in the greenhouse.