A simple op amp and silicon diode are the heart of the temperature-to-voltage converter that will permit you to use an ordinary voltmeter-either analog or digital-to measure temperature. User adjustments make it possible for a reading of either 10 mV or 100 mV to represent 1 °F or C. Temperature sensor D1 is a 1N4148 silicon diode. It has a temper

ature coefficient of -2 mV/ °C. U1, a 741 op amp, is connected as a differential amplifier. A voltage divider consisting of R3 and Zener diode D2 provides a 6. 2 V reference voltage. D2 is shunted by potentiometer R4, so that the offset can be adjusted to align the output voltage with either the Celsius or Fahrenheit scale, as desired. Gain control R2 is adjusted so the output of the op amp is in the scale or voltage range of the meter being used. R4, the offset adjust control, is then adjusted so the output voltage represents either degrees F or C. The thermometer adapter can be calibrated by adjusting R4 while the probe sensor is at a known temperature.

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