The open source project allows you to control DC appliances based on the temperature of two thermistor inputs. It uses AVR ATmega48 as main processor. The controller has both green and white LEDs to indicate status. The project output is connected to N-Channel MOSFET, AOD444, for switching with current up to 4A. For higher current you can use relay.
To get you started, Jeffrey Nichols from ubertechnics has included a temperature comparison program for an automatic heat exchanger. The default program allows you to experiment with a wide variety of temperature control applications out of the box. Application for this project including temperature controlled switch, automatic heat exchangers, automotive switching (e.g. fan relays, solenoids, intercooler spray, methanol/water injection, and A2W intercooler pump control), heater control and peltier control. Download Schematic and firmware.