Pictured below is a thermostat we recently made to maintain the temperature in a plant propagator fitted with 12V heat pads.
The propagator is located in a potting shed with two lead acid batteries charged a 40W PV solar panel with a solar charge controller. Our thermostat includes a waterproof stainless steel encased DS18B20 digital temperature sensor which is to be buried in the moist compost in the propagator. When the measured temperature falls below 17 degrees Celcius, the heating pads are turned on via the on board 10A rated relay, and then remain on until the temperature of the compost reaches 23 degrees Celcius.
Normally for these types of simple thermostat we use a PICAXE microcontroller, but we chose instead to use an Arduino Pro Mini as this meant that we did not have to deal with processing negative temperatures programmatically. (PICAXE only has integer mathematics, so cannot easily deal with fractions of numbers or negative numbers. If the propagator is not used for a while during the winter, below zero temperature could be measured as it is located outdoors.)
If you need any kind of bespoke thermostatic controller, email email@example.com with details of your requirements.
(See here: http://www.reuk.co.uk/wordpress/programmable-automatic-plant-propagator-thermostat/ for a user programmable version of this device for which different temperature ranges can be set.)