The DS18B20 Digital Thermometer provides 9 to 12–bit centigrade temperature measurements and has an alarm function with nonvolatile user-programmable upper and lower trigger points. The DS18B20 communicates over a 1-Wire bus that by definition requires only one data line (and gound) for communication with a central microprocessor.

The OneWire protocol which in fact needs two wires (data and ground) is fantastic for many reasons: you can have a very long cable (250 feets), you can have multiple devices on this cable and with a simple code you can poll those devices to get temperature, voltage, switch position, wind speed, etc.