In the kit Digital Thermometer, thermometer is device to measure the temperature or temperature gradient using a variety of different principles. A thermometer has two important elements: the temperature sensor) in which some physical change occurs with temperature, plus some means of converting this physical change into a numerical value (e.g. the scale on a mercury thermometer).
In this kit digital thermometer can measure temperatures up to 150C with an accuracy of +_1C. The temperature is read on a 1V full scale-deflection (FSD) digital voltmeter. In this Operational amplifier IC 741 provides a constant flow of current through the base-emitter junction of NPN transistor BC107 named as Q. In the BC107 the voltage across the base-emitter junction of the transistor is proportional to its temperature. The transistor used this way makes a low-cost sensor. You can use silicon diode instead of transistor. The small variation in voltage across the base-emitter junction is amplified by second operational amplifier, before the temperature is displayed on the meter. Preset is used to set the zero-reading on the meter and preset PR2 is used to set the range of temperature measurement. The IC7660 performs supply voltage conversions from positive to negative. The entire circuit works off a 5V.