thermometer

3 thermometer A common or garden diode like the 1N4148 is in principle an excellent sensor for a reasonably accurate electronic thermometer because the voltage drop across the diode de- creases by 2 mV for every degree Centigrade rise in temperature. As can be seen in figure 1, a constant reference voltage is applied to the non inverting input of the op-amp. The current flowing through the resistor, and therefore through the diode, is also held at a constant level. Variations in the output voltage of the op-amp can occur only as a result of a change in the voltage drop across the diode and this in turn can only be caused by tempera- ture variations. The output voltage is therefore directly proportional to the temperature of the diode. In the complete circuit diagram shown in figure 2, the op-amp is A2 and the diode is Dl. The reference voltage is derived from IC1 via voltage divider R3/Pl/R4. The output volt- age of A2 is amplified by op-amp A3. The non-inverting input of A3 is al...