A simple thermometer built with operational amplifiers and a normal or protective diode such as 1N4148, instead of the temperature sensor can be made using electronic diagram below.
A constant reference voltage is applied to the noninverting input of operational amplifier. Current passing through the diode, is also maintained at a constant level. Variations in operational amplifier output voltage can appear only as a result of a change in diode voltage drop and this, in turn, can be caused only by variations in temperature. The output voltage is directly proportional to the diode temperature.
Reference voltage is obtained from the voltage divider IC1 by R3 / P1 / r4. A2's output voltage is amplified by operational amplifier A3.
Noninverting input of A3 is maintained at a constant level (also obtained with R3 / P1 / r4) and the values of R6 and R8 were chosen so that 0 V to correspond to an ambient temperature of 0 degrees C.
In order to measure above and below zero without the use of a symmetrical supply is necessary to use a regulator IC1, A2 and A3 for generating a reference voltage steady enough. An additional amplifier, A1, together with R1 and R2, generates a voltage of 2.5 V relative to the negative power bar. These are then used as 2.5 V for the rest of the circuit board.
IC2's pin 11 is therefore at -2.5 V and pin 4 to 6.5 V, compared with this table. Therefore, power operational amplifiers is symmetric.
Power consumption of the circuit is about 5 mA.
If you need a continuous, you must use a single power source, for IC1 does not need to be stabilized. The circuit is calibrated by adjusting P1 IUI, to obtain 0 V at 0 ° C, and then P2, to get 0.999 V to 99.9 ° C.