Electronic thermometer using op amp

ELECTRONIC II – Digital Thermometer Project / Temperature Sensor

Your are task with the design an electronic thermometer with the following specifications and

constrains:

1. The thermometer will have two simultaneous outputs: Degrees Fahrenheit and Degrees Celsius.

2. A DMM will be connected to each output to produce a readout. Your group engineering group can choose the output variable in Volts, Amps or Ohms.

3. Independently of the chosen variable, the decimal point in the readout must correspond to the decimal point in the temperature. For example a readout of 33.4 at the output for

degrees F, should indicate 33.4 °F. Same approach for degrees C.

4. The electronic thermometer must have a range of -100 °F to +100 °F and -100 °C to +100 °C

5. The sensor will be the LM34 Temperature-to-Voltage transducer manufactured by National Semiconductor (Note that National Semiconductor was recently acquired by

Texas Instruments).

6. The Operational Amplifier that is available to your group is the µA 741. 7. Your group can use as many Op Amps as it needs. However, because increasing the

number of components will reduce the economic benefit to your company, reducing the

number of components in your design will result on a bonus, that in this case will be in

form of extra credit.

8. You can use as many passive components as needed. Your group only has access to 5% tolerance resistors. Using the table below, if your calculations indicate a resistor of 2,345

Ω, you will have to choose to use either a 2,200Ω or a 2,400Ω resistor.

1. The only voltage sources available at your workbench are fixed sources of -15V and + 15 V.

2. The conversion factors between temperature systems are as follows:

°F = (°C x 1.8) + 32

°C = (°F – 32) / 1.8

5% Standard Values Decade multiples are available from 10 Ω through 22 MΩ

10 11 12 13 15 16 18 20 22 24 27 30

33 36 39 43 47 51 56 62 68 75 82 91

Tasks:

1. Each student will submit a paragraph indicating how you will approach the design of the

electronic thermometer.

2. Block Diagram

Students present a block diagram of the design. Students submit a plan for testing the

circuits in order to demonstrate that it meets the specifications.

3. Final Schematic

Students submit the electronic schematic of the design. Students present the results of their

testing procedures in tabular and graphic form. Students will also document the measurement

errors associated to the electronic thermometer, that is, the difference between the expected

ad experimental values for a given temperature.

4. Draft Report and presentation

Students submit a report to the instructor. The report should include at least: conceptual design,

results of the testing plan, challenges they faced, solutions to these problems, etc. If applicable,

defend the position that your group has used the minimum possible number of

components. The report will be submitted as a Power Point or a narrated PowerPoint. Groups

submitting a narrated Power Point will receive extra credit.

Instructor Notes:

 It had occurred to me that the LM35 is not available in multi-sim. Below is my suggestion:

You can try the LM34 comparison site (see link below) and see which one fits the temperature range. If

you cannot find one that meets the criteria, we will have to change the system spec, which commonly

occurs during product development. As for simulations, the ideal case is to use Multisim and use the real

parts (circuit models) provided by Multisim and run a whole set of functional simulations. If Multisim

does not have any parts that you want to use, you can try using alternative parts based on their parts

library. If you still cannot find anything close to it, you then only need to create a block diagram showing

the major chips that are being used in the system. No sim is required.

http://www.ti.com/product/LM34/compare

This is by no means creating headache for you. Consider this as part of the learning opportunity,

because in reality, this is what product design is about. Finding the right parts for your design. We do

have limitations on the software side due to the fact that Multisim is somewhat for academic purpose

only. On the other hand, we should try our best to get the most out of this exercise, from a learning

standpoint, even though the design, software resources are limited.

http://www.ti.com/product/LM34/compare