Practicum 4B

The goal of this practicum is to design, build, test, and manufacture a circuit which can convert temperatures into voltages.  Unlike previous practicums, this circuit is going to be designed entirely by you as an introduction to the process of electrical design. You will first test your circuit on a breadboard; then, you will solder a similar circuit onto your robot’s PCB. 

In a previous practicum, you measured ambient temperature using a Linear Active Thermistor integrated circuit sensor (MCP9700). The sensor produces a linear mapping between temperature and voltage. The MCP9700 has three terminals: one for power, one for ground and one for the output voltage.

In this practicum, you will be introduced to a different temperature sensor: a thermistor. The thermistor has only two terminals, plus and minus, because it acts like a resistor with a resistance value that depends on the temperature. The change in resistance with temperature in a thermistor is non-linear.

If you have already connected your thermistor to your umbilical then use that thermistor for the experiments today.  Otherwise follow the instructions here to attach your umbilical to the thermistor. You only need to do section 3. Make sure you are soldering the thermistor onto the same side of the umbilical that you soldered the motor. The thermistor will go into the water with the robot and motor.

You will be using this 10kOhm thermistor (NXFT15XH103) for today. Note that this is a shortened version of the original datasheet with just what you need for today's practicum. On page 1, under item 1, there is an equation describing the relationship between the thermistor’s resistance and temperature. To use this equation for today's practicum, you will need to plug in the B value, which  is a constant that varies based on temperature range and the thermistor. You should use the B-value for 25-80°C for this lab. This equation is not a perfect fit, but it is good enough for our purposes. (Note that T0 refers to 25 C or 298 K, and be wary of your units when using these equations).

Using those coefficients you can create a calibration curve to relate Temperature to Resistance, which you can use in this practicum to compare against your experimentally created calibration curve. 

With temperature (°C) on the x-axis and resistance (kΩ) on the y-axis, plot the function relating the two. Plot temperature over the full range we’ll use in this lab: about 0°C to 70°C.  Remember that the equation above is given in degrees Kelvin.

Begin by placing the thermistor in the ice bath along with a thermometer. Record the resistance of the thermistor and the temperature on the thermometer.

Next, place the both the thermistor and thermometer in the room temperature bath. Record the resistance and the corresponding temperature.

Finally, place the thermistor and thermometer in the hot water bath. Record both the resistance and temperature.

Use the equation you found in Step 4.1 to convert each of the resistances to temperatures. 

Superimpose each resistance vs. temperature point you measured onto the graph from Section 4.1 to compare your measured data to what you would expect based on the formula from the datasheet. Add this plot to your submission sheet (Question 1) 

Unfortunately, most digital to analog converters cannot directly measure resistance; therefore, you must design a circuit that can convert the changing resistance into an easily measurable voltage.

In this section, you will need to design a voltage divider circuit comprised of your thermistor and a resistor of your choice to meet the following constraints:

• The supply voltage, Vin, will be 5V. Ground will be at the other end. 

• The difference between the voltage output when you put your thermistor in the hot bath (~60C) and when you put your thermistor in the ice bath (~0C) must be greater than 2V.

• The output voltage must increase when the temperature increases.

This task can be broken into three steps: 

1.  Finding an expression for Vout as a function of the resistor values and input voltage.

2.  Deciding whether R1 or R2 is the thermistor.

3.  Deciding the value of the other resistor.  

Pursue those steps in that order to complete your design.  Deciding the value of the other resistor is most easily accomplished by considering the maximum and minimum values of resistance that you expect to see.

Next you will use software to check if your design is correct.  Setup an equation for your voltage divider.

Set up the equations describing your thermistor’s behavior and your circuits behavior in software of your choice (Excel works great; Matlab and Python also work well for the initiated). Check your design by plotting its predicted Vout for a variety of input temperatures; i.e. use your program to calculate the voltage output of your circuit for temperatures between 0 degrees Celsius and 60 degrees Celsius.  Plot the result on a graph with voltage on the y-axis and temperature on the x-axis. Adjust the y-axis so that it runs from 0-5 Volts. Check that you meet all of the design constraints.  This graph will be helpful for your homework, so feel free to save your spreadsheet.

Now it is time to test your designed circuit on a breadboard. If there is not a resistor for your ideal value, use an existing resistor that is close in value or, if nothing is particularly close, ask a proctor or instructor for your desired resistor value.

Construct your voltage divider circuit from Figure 5.1 on a breadboard and power it with 5V.  Use a multimeter to measure both the input voltage and the resistance of the resistor you selected. Since your equation depends on the exact values of the input voltage and resistance, it is important that they are measured beforehand.

Using a multimeter, measure the voltage Vout at the output node as you place the thermistor in each of the three baths from Section 4. For each bath, convert the output voltage into temperature to verify that your conversions are correct.

Take a picture of your breadboard thermistor circuit and add it to the submission sheet (Question 2)

Test your soldered circuit by measuring the voltage at the exposed pin labeled TEMP and use AGND as the ground for your measurement.  If TEMP is not working, you may have an issue with your op-amp, and you can debug that by testing just the thermistor without the op-amp using the T_SENSE test point.  Put your thermistor into different baths and make sure you measure the same results as with your breadboard circuit. Plot your measured data (voltage vs temperature). Save these data points since you will use them for your homework.

Add the plot to your submission sheet (Question 3)

Take a photo of your complete board and add it to your submission sheet (Question 4)

1.    Murata Thermistor Datasheet: https://drive.google.com/open?id=1leiB2mRnPxhZ7hLnxq3vnmnNOU1aAgqp

2.    Voltage Divider Image: https://upload.wikimedia.org/wikipedia/commons/thumb/2/21/Resistive_divider2.svg/220px-Resistive_divider2.svg.png