This project is based on a DS1621 I2C Thermometer with Thermostat and an AT90S2313. If you have problems find them you can replace the first with DS1631 and the second with ATTINY2313. The temperature range is 0-125C. The thermostat range is 10-40C and you can set it with up or down buttons. With on/off switch you can turn on and off the load control. When is set to off the device acts as simple thermometer. You can use the ISP header JP1 to program the AVR on board.
DS1621 Thermometer with Thermostat - [Link]