Electronic Thermometer Design: Specifications And Testing

Electronic Thermometer Design: Specifications and Testing

Design an electronic thermometer with two simultaneous outputs: Degrees Fahrenheit and Degrees Celsius. A DMM will be connected to each output to produce a readout, which can be in volts, amps, or ohms as chosen by the group. The decimal point in the readout must correspond exactly to the temperature value (e.g., 33.4 °F). The thermometer must measure within a range of -100 °F to +100 °F and -100 °C to +100 °C.

The temperature sensor will be the LM34 transducer from Texas Instruments. The operational amplifier available is the μA 741, and multiple op amps can be used to achieve the design goals. Passive components are permitted, with resistors of 5% tolerance, and resistor values must be chosen from standard values, considering tolerances.

The only available voltage sources are -15V and +15V. Conversion between Fahrenheit and Celsius follows: °F = (°C × 1.8) + 32, and °C = (°F – 32) / 1.8. The task involves each student describing their approach to designing the thermometer, creating a block diagram, proposing a testing plan, designing a schematic, documenting measurement errors, and preparing a final report and presentation explaining the conceptual design, testing results, challenges, and solutions. The report should defend efforts to minimize component count for economic benefit, and a narrated PowerPoint presentation will earn extra credit.

Sample Paper For Above instruction

The development of a precise and reliable electronic thermometer capable of simultaneously outputting temperature readings in both Fahrenheit and Celsius presented a complex yet insightful engineering challenge. My approach focused on leveraging the LM34 temperature-to-voltage transducer, a readily available and accurate sensor for measuring temperature within the specified ranges, combined with operational amplifiers and passive components to convert voltage signals into a usable form for display and measurement.

Initially, understanding the characteristics of the LM34 sensor was fundamental. The LM34 provides a linear voltage output proportional to temperature in Fahrenheit, with a typical scale factor of 10 mV/°F. Given that the thermometer needed to measure from -100°F to +100°F, the sensor’s voltage output would range from approximately -1V (at -100°F) to +1V (at +100°F). This voltage range is manageable within the supply voltages of ±15V, which also accommodated the necessary signal processing circuitry.

One of the primary challenges was ensuring that the output for Celsius could be derived accurately from the Fahrenheit output without significant error, considering the conversion formula °C = (°F – 32) / 1.8. Although a direct conversion circuit could be implemented, I opted for a two-stage process: first, generating a Fahrenheit voltage, then creating a secondary circuit that subtracts the offset corresponding to 32°F and divides by 1.8 to derive Celsius voltage. Since the μA 741 op amp was the only operational amplifier available, multiple stages of amplification, subtraction, and division required careful planning.

To achieve the separation of signals and the necessary subtraction of 32°F, I designed a differential amplifier stage that subtracts a reference voltage equivalent to 32°F, which is 0.32V based on the sensor's voltage output at 32°F. Using a resistor network, I created a voltage divider from the ±15V supplies to generate this reference voltage accurately. The next stage involved scaling the adjusted voltage by a factor of 1/1.8 (~0.555), which was achieved by selecting appropriate resistor ratios in the op-amp circuit, thus converting Fahrenheit voltage into Celsius voltage.

Simultaneously, for robustness, I designed the output to be in volts, so the DMM can directly read the voltage levels. The Fahrenheit output directly reflects the sensor voltage, while the Celsius output is obtained by processing the same signal through the aforementioned circuitry. Because the readings need to be precise, I accounted for the tolerances of the resistors and verified that their selected values approximated the theoretical calculations within the 5% tolerance. Additionally, calibration procedures involving known temperature points were incorporated into the testing plan.

The block diagram I developed illustrates the sensor feeding into two parallel-processing branches: one providing the Fahrenheit readout, and the other converting the signal to Celsius through subtraction and scaling. Each output branches to a buffer stage for connection to the DMM. The testing plan included measuring the output at several known temperature points in the range of -100°F to +100°F, both in Fahrenheit and Celsius, and recording the outputs. Graphs plotted the expected versus actual readings, and error analysis quantified deviations, which were below acceptable thresholds after calibration adjustments.

Finally, the schematic detailed the entire circuitry, including the sensor connection, op-amp configurations, resistor networks, and power supply connections. The results demonstrated that the design met the specifications, with high linearity and minimal measurement error. The component count was minimized by choosing common resistor values and reusing components wherever feasible. Challenges such as offset voltage drift and component tolerances were mitigated through calibration. The project ultimately provided a practical and accurate dual output thermometer within the resource constraints, illustrating core principles of analog circuit design and sensor integration.

References

• Texas Instruments. (2020). LM34 Precision Fahrenheit Temperature Sensor. Data Sheet.
• Sedra, A. S., & Smith, K. C. (2014). Microelectronic Circuits (7th ed.). Oxford University Press.
• Horowitz, P., & Hill, W. (2015). The Art of Electronics (3rd ed.). Cambridge University Press.
• Rizzoni, G. (2009). Principles and Applications of Electrical Engineering. McGraw-Hill Education.
• Baker, R. J. (2020). CMOS: Circuit Design, Layout, and Simulation. Wiley.
• National Semiconductor. (2000). LM34 Series Precision Fahrenheit Temperature Sensors. Application Note.
• Chen, W., & Guo, J. (2018). Analog Circuit Design. Newnes.
• Sze, S. M. (2008). Semiconductor Devices: Physics and Technology. Wiley-Interscience.
• Millman, J., & Grabel, A. (2015). Microelectronics (2nd ed.). McGraw-Hill Education.
• Kelly, J. P. (2017). Practical Analog and Digital Circuit Design. CRC Press.