Design a Temperature Indicator

This circuit is a temperature indicator circuit or differential instrumentation amplifier using a transducer bridge. This circuit is calibrate in degrees Celsius or Fahrenheit. In the circuit used buffer in  and  points for exact voltage of  and  points. Because gain is always 1 of buffer circuit. Then the output voltage of buffers is input voltage of differential amplifier. 

The differential amplifier is difference voltage of  and  points using 741 Op Amp. When temperature is increased then resistance  is also decreased and output voltage  is decreases and when temperature is decreased then resistance  is also increased and output voltage  is increases.

The temperature indicator is a circuit that indicates of temperature in degrees Celsius or Fahrenheit. The temperature is inversely proportional to the resistance or transducer.

  The Fig.01 simplified a temperature indicator circuit using a transducer bridge. A restive transducer whose resistance changes as a function of some physical energy or temperature is connected in one arm of the bridge with a small circle around it and is denoted by , where  is the resistance of the transducer and  is the change in resistance.

Fig.01 - Temperature indicator

In the circuit used as the transducer in the bridge circuit is a thermistor and replaced output voltmeter to temperature indicating meter. Then temperature indicating meter is calibrate in degrees Celsius or Fahrenheit. 

The bridge can be balanced at a desired reference condition, for instance 25⁰C. As the temperature varies from its reference value, the resistance of the thermistor changes and the bridge become unbalanced. This unbalance bridge in turn produces the meter movement.