An inexpensive electronic thermometer is capable of measuring temperatures over a range of from -30 degree F to +120 degree F. A diode-connected 2N3904 transistor used as the temperature sensor forms a voltage divider with R1. As temperature increases, the voltage drop across the transistor changes by approximately -1.166 millivolts-per degree F.

The Circuit Diagram of Electronic Thermometer

As a result, the current at pin 3 of IC1, a 741 op amp with a gain of 5, decreases as the temperature measured by the sensor increases. A second 741 op amp, IC2 is configured as an inverting amplifier. Resistors R5 and R6 calibrate the circuit. Calibration is also straightforward. When properly done, a temperature of 30 degree F will result in a meter reading of 0 milliamps, while a temperature of 120 degree F will result in a meter reading of 1 milliamp.

Divide the scale between those points into equal segments and mark the divisions with the appropriate corresponding temperatures. The calibration is completed by placing the sensor in an environment with a known temperature, such as an ice-point bath. Place the sensor in the bath and adjust R6 until you get the correct meter reading.