Simple Low Cost Digital Thermometer Circuit

The circuit diagram shows the simplest form of a digital thermometer which can measure temperatures up to 150°C with an accuracy of ±1°C. This digital thermometer circuit is of low cost because the reading portion of this equipment is done by a 1V full scale deflection digital voltmeter.

Circuit Diagram of  : Right Click >> View Image to view full size

The operational amplifier IC 741 provides a constant flow of current through the base-emitter junction of npn transistor BC108 (T1). The temperature to be measured is appeared as base emitter junction voltage of this transistor. So again the transistor used this way makes a low-cost sensor.The small variation in voltage across the base-emitter junction is amplified by second operational amplifier (IC4). The we can connect the output of the IC4 to the voltmeter leads. Preset VR1 is used to set the zero-reading on the meter. Also VR2 is used to set the range of temperature measurement.

The power supply to the two op amps given via 3-terminal positive voltage regulator IC 7805 (IC1) and negative low-dropout regulator IC 7660 (IC2). The entire circuit works off a 9V battery.

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