PHY 405: Electronics Lab
Lab # 3
Lab # 3: Analog meets digital
In this lab you will learn how to use a microcontroller to build a thermometer that
logs the temperature to a computer. You will also learn how to communicate with the
oscilloscope through a serial port, using Python programs.
Background preparation
• Thermistors: Wikipedia.
• Serial communication: Wikipedia.
• Analog to digital converters (ADC): AOE.
Task 1: Measuring a thermistor’s output
In this task, you will build a circuit to measure the resistance of a thermistor, a resistor
whose resistance depends on temperature. The thermistor1 is an NTC type (negative
temperature coefficient, i.e., its resistance decreases with temperature). Its nominal
resistance at 25 C is R25 = 10 kΩ.
6
Temperature, T−273.15 [C]
20
40
60
0
80
5
R/R25
4
3
2
1
0
260
280
300
320
Temperature, T [K]
340
360
Figure 1: Resistance versus temperature for a negative temperature coefficient (NTC) thermistor. The
quantity R25 is the nominal resistance of the thermistor at 25 C.
1 The thermistor you will use is a Vishay # NTCLE100E3103HB0.
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PHY 405: Electronics Lab
Lab # 3
The thermistor’s datasheet provides the following formula for converting its resistance to temperature (in Kelvin):
2
3
R
1
R
R
+ D1 ln
.
= A1 + B1 ln
+ C1 ln
T (R)
R25
R25
R25
(1)
The constants for this thermistor type (in units of K −1 ) are A1 = 3.354016 ×
10−3 , B1 = 2.884193 × 10−4 , C1 = 4.118032 × 10−6 , D1 = 1.786790 × 10−7 .
a) Measure the resistance of the thermistor, Rth , using a multimeter. Record the
room temperature, Troom , and verify that Rth is consistent with the curve shown
in Figure 1.
b) In order to automatically record the thermistor’s resistance, it is convenient to
produce a voltage that depends on its resistance. You will do this using a voltage
divider.
Wire up a resistive divider circuit as shown in Figure 2. Use a value of Rref between
1-5 k, and Vin of +5 V. Measure and record the value of Rref before you put it in.
Measure the output voltage of the divider when you touch the thermistor’s head
with your fingers. Estimate your hand’s temperature using the calibration curve in
Figure 1, and the calculated division ratio of the divider.
Vin
Vout
Rth
Rref
Figure 2: Divider circuit to convert the thermistor’s resistance to a measurable voltage.
(30 mins)
Task 2: Converting the thermistor’s output to a temperature
As you observed in the previous task, the voltage produced by the thermistor divider
circuit is a complicated function of the temperature. Wouldn’t it be convenient to
somehow directly convert the thermistor’s output into a temperature? In this task,
you will use a digital device called a microcontroller (uC) to automate this calculation.
Microcontrollers are extremely versatile devices, and are handy for many tasks where
complicated algorithms and nonlinear operations need to be implemented.
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PHY 405: Electronics Lab
Lab # 3
a) Plug in your uC board into any available USB port on the computer. (You may
disconnect the “LabDAQ” instrument if necessary.) Start up the Arduino program
on the desktop, and configure the software to communicate with an Uno board.
Make sure that the Board is chosen to be “Arduino/Genuino Uno”, and the Port is
chosen correctly for the uC that you plugged in.
To upload a program (called a “sketch”) to the uC board, you will need to first
compile it (indicated by the Verify button, shaped like a check mark), and upload
it (using the button shaped like a right arrow). Try the simple Blink example first,
and modify the code so that you get familiar with the programming interface.
Next, complete the AnalogReadSerial example,2 using a potentiometer to generate a variable voltage.
Warning: Never send in voltages outside the range of 0 to +5 V into the analog
input pins of the uC. If you significantly exceed this range, you may destroy the
microcontroller’s ADC.
Feel free to try out any of the other examples.
b) Connect the output of the thermistor’s divider circuit to one of the uC’s analog
inputs. Write a uC program to convert the output of the thermistor circuit into
temperature, and print out the temperature measurements on the serial port. A
basic program to get you started (arduino thermistor.pde) is available on the
course webpage.
Use the uC and the thermistor to measure the root-mean-squared (rms) temperature fluctuations in the room over a span of one hour, and report this value. Estimate
the contribution of the uC’s internal circuitry to the observed fluctuations, and figure out the true extent of the room’s temperature variations. (Hint: what happens
if there is just a resistor, rather than a thermistor, connected to the uC’s input?)
In your report, describe how you estimated the true value of the rms temperature
fluctuations.
(1 hr 20 mins)
Task 3: Communicating with the oscilloscope
The front-end of the TDS 210 oscilloscope is also just a specialized ADC. In this task, you
will use a Python program to communicate with the scope, and read out waveforms
from it. This allows you to download data from the scope for further analysis.
a) Connect the scope to the computer through a USB (Universal Serial Bus) port.
You may have to plug the scope’s USB cord into one of the front panel USB sockets
on the desktop computer. Watch for the serial port number when the USB device
driver installs (the serial port address will be something like “COM4”). Download
the program called scope serial.py from the course webpage, and open it in a
Notepad window. Edit the address listed in the program to match the correct COM
2 The Blink and AnalogReadSerial examples can be found under File
3
→ Examples.
PHY 405: Electronics Lab
Lab # 3
port address for the scope, and save the program on the desktop. Run the program
using python.exe. If you have configured the COM port address correctly (and
turned on the scope!), you should see a window listing the scope’s serial number.
Read through scope serial.py and try to understand what the various functions
do.3 Feel free to modify and play with the Python program.
b) Set up the function generator to output a 1.213 kHz triangle wave, and connect
its output to the scope. Using scope serial.py, acquire the waveform from
the scope and save it into a text file for further analysis. In your report, plot
the waveform and its power spectrum. A basic program to calculate the power
spectrum of a waveform is available on the course webpage.
(40 mins)
3 The TDS 210 programming manual posted on the course webpage might help you make sense of the
commands.
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