This digital thermometer circuit diagram uses a common 1N4148 diode as the temperature sensor. The temperature coefficient of the diode, -2 mV / 0C is exploited for this application to create an accurate electronic thermometer. To display the measured temperature, a digital multimeter is used and so we can measure temperature values from -9.990C up to +99.90C. Calibration of the digital thermometer To set the minimum level (00C), place the diode in a glass of water filled with crushed ice (check the temperature first with a normal thermometer) wait until the thermometer shows zero degrees centigrade. Set P1 so that the digital voltmeter will display 000 when the diode senses zero degree centigade. To set the maximum level (1000C), place the diode sensor into a boiling water and adjust P2 so that the digital meter exactly displays 99.9. source [link]