This really sounds like a cool project, and may be used in various projects where the temperature needs to be kept under control. The temperature controller is based on an AT90S2313(ATtiny2313) and a Dallas DS1621 Digital Thermometer. The temperature is displayed on a dual 7-segment display, and two buttons are used for setting parameters. A high current relay switches the heating element. The displays are common cathode displays. The segments of both displays are tied together and multiplexing is handled in software. Note that the decimal point segment also are used.
The following project its a basic thermometer that can measure data from 2 sensors, for example you place one sensor inside the house and another one outside. The schematic is plain simple DS1621 or DS1631 digital temperature sensor and a bunch of resistors and diodes. You can use smd parts or trough-hole, it doesn’t really matter but if you want to keep things small and cool, i suggest you use smd. Also on the pcb you will solder a DB9 connector so you can stick the small pcb directly into your COM port. The measurement is than showed on the windows taskbar.
DS1621 pc thermometer: [Link]