As the thermometer is switched on only when necessary , and as a consequence it consumes not much energy, it was decided to use independent power source. The result is a fairly compact device. Below is a diagram of a thermometer. 
Based on the diagram of the thermometer taken a pretty popular PIC16F84 microcontroller, which is conducting a survey of digital DS18B20 temperature sensors, processes and displays the result on LCD.
In the scheme used a 10-digit 7-segment display MT10T7-7T with integrated controller production company "melt". The chip DS18B20 is a digital thermometer with I/o, measurement error, which is ±0.5°C. For the exchange of data between the microcontroller and the DS18B20 uses the 1-wire bus with a serial interface. Sensor that measures the temperature inside the room, located inside the housing, and a sensor that measures the temperature on the street, placed in a sealed plastic case. To the reading device was not influenced by the temperature of the walls of the house, the sensor it is desirable to make the rod from the wall. And even if the side of the house where the sensor is located, is Sunny, then it must be protected from direct sunlight.
To download the printed picture, a schematic diagram, firmware, source code.
