Jesper have built this thermostat to control electric heating element. For this he used Dallas thermal sensor DS1621 which can be read via I2C interface. He used AVR 2313 series MCU for reading sensor data and displaying it on 2 seven segment LED indicators.
Controlling relay toggle point is made easy – Just two buttons UP and DOWN. Pressing both buttons at same time allows to set hysteresis and LED refresh duty cycle. Nice thing that device has power supply converter built int that is ready to be plugged anywhere in you need to control temperature. Circuit and source code is available for download.