Build a Electronic Thermometer Circuit Diagram. A thermometer thermos , meaning "hot" and metron, "measure") is a device that measures 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. An inexpensive electronic thermometer is capable of measuring temperatures over a range of from -30°F to +120°F. A diode-connected 2N3904 transistor used as the temperature sensor forms a voltage divider with Rl. As temperature increases, the voltage drop across the transistor changes by approximately -1.166 millivolts-per°F. 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. Electronic Thermometer Circuit Diagra...