There are two ways of controlling a mains load: proportional and digital.
Digital uses a relay/triac to turn on/off the load. It"s called digital because there are only two states - fully on or fully off. It"s dead simple to do, uses only 2-3 components, it"s very robust, but it does stress the hell out of whatever you"re using as the switch.
Proportional control allows the amount of power applied to the load (heater, fan, light bulb, etc) to be varied almost infinitely. This gives you much better control, at the expense of a fairly complicated circuit, as other folks have mentioned - zero crossing to reduce "hash" or interference, and so on.
Regardless of which type you want to use, you need some kind of feedback to tell you when the temperature is correct/too high/too low, and that"s much more difficult to implement on a controller without ADC inputs. I use solid-state temperature sensors exclusively; thermocouples and the like are terribly susceptible to noise, current flow, humidity, and so on. The LM35 series of sensors are fantastic, but they do need an ADC to read the device"s output voltage properly. The alternative (given that, as far as we know, the GPIO support on the RasPi doesn"t include analogue inputs) is a simple op-amp circuit with a "set temp" knob. That will definitely work with the GPIO inputs, easy-peasy.
Gert"s board (again, as far as I know, hint hint ) would be ideal for use as a simple on/off type controller. The circuit needed to drive the relay (mechanical or solid-state) is absolutely trivial, reliable, and safe, so that"s probably the least-worst option.
I design and use proportional 240VAC controllers in my studio, safely and cleanly controlling loads up to 15kW.
As I"ve offered in another thread, I"d be more than happy to whip up some AC controllers for use with the RasPi. These would not only be safe, they"d comply with any electrical certification as well. They would be FAR cheaper than an over-the-counter solution, and more fun as well!
However, since there seems to be quite a bit of interest in using the RasPi in cars as well, it would be interesting to offer a couple of options. For example, what about a "universal" PCB, where one set of components would be added for mains control, and a slightly different set for control of, say, a 12v fridge, or courtesy light, etc?
I"m also thinking about a board with 2,3, or more controllable outputs, so a studio ( in my own case) could be controlled by a single "Pi box", with an appropriate GUI to manage the settings, and so on.
I do apologise if I"m standing on anyone"s toes here, these are just thoughts generated by this really interesting thread...
Questions, comments, and suggestions are most welcome!
Thanks for reading this far!