After a somewhat unsuccessful and rather expensive attempt at warming a greenhouse, electrical engineer Dr Craig Hollabaugh rigged up a system that keeps the winter chill away by warming the roots of his plants. The WarmDirt system has already helped his plants survive the coldest of Colorado's cold months, and is now getting ready to provide warmth to seedlings during the expected April freeze. This past season's survivors were all flowers but next winter, the setup will be used for growing veggies.

Dr Hollabaugh began the project by constructing a coffin-sized wooden box and lining the bottom with the kind of electric heat cable that prevents pipes from freezing. He covered the heat source with four inches (101.6 mm) of sand and then placed some plant pots on top. At first, the heat was either switched on or off courtesy of an AC timer, but a desire for more control and much more data led to the creation of the WarmDirt system.

Inside the box, Dr Hollabaugh installed some probes to measure the temperature of the heated sand and the soil inside the pots, the air both inside and outside the box, as well as humidity and ambient light. A prototyping PCB control board sporting an Atmel ATMEGA328P microcontroller was mounted in a waterproof NEMA-4 outdoor electrical enclosure. The system architecture for control and monitoring is a mix of Arduino, XBee, MQTT with Mosquitto, Python, replicated MySQL and jQuery. The (almost) real-time system information is accessed wirelessly via a browser.

All the source code, board design, layout, and so on are available via Dr Hollabaugh's github project page under Creative Commons BY-SA 3.0 (link at the end).

"The WarmDirt board was designed for temperature control but can be used for general purpose motor (stepper or dual servo at 5+ amps) control with encoder support, AC power control, analog voltage sensing, wireless communications and Arduino bootloader," Dr Hollabaugh told us. "It's quite a little board for a variety of projects."

A DC motor was connected to a suitably-branded, 3D-printed spool to the side of the unit that was to take care of the automatic lifting and lowering of the transparent coffin lid. Unfortunately, the stepper motor chosen couldn't quite manage to keep the lid open, so a linear actuator was recently added to raise the lid when the outside air temperature rises above 43 degrees Fahrenheit (6°C) and lower it when it falls below 41.5 degrees Fahrenheit (5.2°C). A switch has been included to indicate when the lid is closed.

The PID controller calculation has been tweaked to ensure that the potted soil temperature never falls below 47 degrees Fahrenheit (8.3°C), and the WarmDirt system uses a triac-based solid state relay for AC power-switching to allow for tighter control of the temperature of the heating element.

The WarmDirt system is still a work in progress with a few niggles to iron out but as it stands, it works very well on its own. Dr Hollabaugh just needs to visit the box from time to time for watering.

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