Electronic Thermometer

This thermometer is capable of measuring temperatures from - 30 to +120 °F. A diode-connected 2N3904 transistor form
Electronic Thermometer - schematic

s a voltage divider with Rl, The transistor is used as the temperature sensor and, for best results, it should be connected to the rest of the circuit with twisted wire, as shown. As temperature increases, the voltage drop across the transistor changes by approximately -1.166 mV/°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. Resistors R5 and R6 are used to calibrate the current. At a temperature of about -30°F, the current through R4 (formed by connecting a 910- and a 1600- resistor in parallel) should equal the current through R5 and R6. A temperature of - 30°F will result in a meter reading of 0 mA, while a temperature of 120 °F will result in a meter reading of 1 mA. Divide the scale between those points into equal segments and mark the divisions with the appropriate corresponding temperatures. If you divide it into 150 equal segments, for instance, each division will equal one degree. Calibration is completed by placing the sensor in an environment with a known temperature, such as in an ice-point bath. The freezing point of water is approximately 32°F. Verify that the temperature is indeed 32°F using another thermometer that is known to be accurate. Then, simply place the sensor in the bath and adjust R6 until you get the correct meter reading.

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