Measuring a thermistor’s output

A thermistor is a resistor whose resistance ($R_t$) depends on temperature as shown in Figure 1. You will be using a Vishay NTCLE100E3103HB0 thermistor](http://www.farnell.com/datasheets/2792960.pdf), an NTC type with a negative temperature coefficient, i.e., its resistance decreases with temperature. Its nominal resistance at ${25}^{\circ }\mathrm{C}$ is $R_{25}=10\mathrm{k}\mathrm{\Omega }$.

The thermistor’s datasheet provides the following formula for converting its resistance to temperature in Kelvin:

$$\frac{1}{T(R_t)}=A_1+B_1\ln \frac{R_t}{R_{25}}+C_1{(\ln \frac{R_t}{R_{25}})}^2+D_1{(\ln \frac{R_t}{R_{25}})}^3$$

The constants for this thermistor type (in units of $K^{-1}$) are $A_1=3.354016\times {10}^{-3}$, $B_1=2.884193\times {10}^{-4}$, $C_1=4.118032\times {10}^{-6}$, $D_1=1.786790\times {10}^{-7}$.

Resistance versus temperature for a negative temperature coefficient (NTC) thermistor. $R_{25}$ is the nominal resistance of the thermistor at 25 C.

In order to automatically record the thermistor’s resistance, it is convenient to produce a voltage that depends on its resistance. Do this using a voltage divider as shown in Figure 2. Use a measured 10k resistor for $R_{ref}$, and $V_{in}=+5\mathrm{VDC}$.

• The optimal value for the reference resistor is one that makes the reference voltage ($V_{ref}$) most sensitive to changes in the thermister resistance around ${25}^{\circ }\mathrm{C}$, i.e. you want to maximize $\frac{d{V}_{ref}}{d{R}_t}$ around ${25}^{\circ }\mathrm{C}$. This turns out to be when $R_{ref}\sim R_{25}$.

Divider circuit to convert the thermistor’s resistance ($R_{th}$) to a measureable voltage.

Arduino Microcontroller

The voltage produced by the thermistor divider circuit is a complicated function of the temperature, so it would be convenient to directly convert the thermistor’s output into a temperature. A digital device called a microcontroller can automate this measurement and calculation. Microcontrollers are extremely versatile devices, and are handy for many tasks where complicated algorithms and nonlinear operations need to be implemented.

Voltage source

A device’s power supply typically provides only one voltage, but it is often the case that that different components within the device require different voltages. For example, the op-amp used later in this exercise may use $\pm 15$V, but we only want a maximum of 5 V input into the Arduino. Another common issue is that the input voltage is not known exactly a priori, but we need to supply a precise voltage to a circuit.

In this task you will use an LM7805 voltage regulator IC to generate a precise and stable voltage.

Wire up a 7805 regulator on your breadboard as follows:

• Connect your positive (Channel 2) DC power supply to the input pin of the 7805 (pin 1) The pin labels are listed in the IC’s datasheet.
• Connect the power supply Channel 2 COMMON output to the ground pin of the 7805 (pin 2).
• As with op-amps, we want to short any high frequency noise to ground, so connect both pins 1 & 3 to ground with capacitors.

Circuit for testing the LM7805 voltage regulator. Choose capacitors between 0.1 and 1 µF.

Except for removing the 1K resistor, do not disassemble this circuit. You will use it later.

Arduino Analog Input

The analog inputs of the Arduino are connected to a 10-bit analog-to-digital converter with a voltage range from 0 to 5 V.

WARNING: Applying voltages outside the range of 0 to +5 V to the Arduino analog input pins may destroy the Arduino’s Analog-to-Digital Converter (ADC).

Op-amp follower

The DC input impedance of the Arduino ADC is 100M, but it can temporarily be much, much lower as its internal capacitors charge and could even be below $R_t$, so if the ADC is fed directly by the thermistor divider, $V_{out}$ could sag leading to inaccurate temperature measurements. Therefore we will construct an op-amp follower to sit between the divider and the microcontroller. The follower presents a large input impedance to the divider, and a small output impedance to the microcontroller.

Wire up an op amp follower as shown in the figure below. An op amp follower is just a non-inverting amplifer (see Lab 4) with $R_2=0$ and $R_1=\mathrm{\infty }$, hence a gain of $V_{out}/V_{in}=1$. The Zener is added to provide some protection against over-voltages on the Arduino analog input pins.

• Note that the zener is “backwards”, since it is reverse breakdown that is used to limit the voltage.
  • The forward orientation of a zener is identifed by a stripe at one end, corresponding to the end with the () in the figure.

Op-Amp Follower with Zener diode to limit output voltage to below 4.7 V.

Breadboard Layout

As you start building more complicated circuits, breadboard organization makes them easier to build correctly and debug when things go wrong. As an example of moderate organization, here is my breadboard at the end of today’s exercise.

Example of Prof. Bailey’s breadboard for today’s exercise R-10.

• Use consistent colour wires. For example, my convention for this lab is:
  • Black wires are Common.
  • Yellow wires are DC Power Supply Output 1.
  • Green wires are DC Power Supply Output 2 ($V_{supply}^{+}$).
  • Blue wires DC Power Supply Output 3 ($V_{supply}^{-}$).
  • White or Orange wires are internal connections.
  • Red wires are Input or Output
• Use the power rails columns for the $V_{supply}^{+}$ and $V_{supply}^{-}$.
• Create Common rows or columns to provide easy access.
  • In general it is best to only have a single Common connection for any circuit.
• I used some short white wires to hold the Arduino in place on the board.

Analog Input Test

To learn how to measure voltages using the Arduino, build this simple circuit feeding an Arduino Analog Input pin.

• Connect your LM7805 voltage regulator circuit output to a resistive divider with a 10K resistor in series with a 10K potentiometer.
  • A potentiometer (or “trimpot”) is a small 3-wire variable resistor. -If a voltage is is applied across the outer two wires, the middle wire provides an output voltage that can be adjusted by rotating the slot on top with your little screwdriver.
    • In this case, connect pin 1 of the trimpot to the 10K resistor and pin 2 to ground.
• Feed the output (from trimpot pin 1) into your zener-protected op-amp follower circuit.
• Feed the output of this op-amp circuit into pin A0 of your Arduino.
• Connect an Arduino ground (GND) pin to your circuit ground so the Arduino has a reference Common potential for the pin A0 voltage measurement.

Use the File $\to$ Examples $\to$ 01.Basics $\to$ AnalogReadSerial example to read the voltage from pin A0.

• To see the output from the running sketch, click Tools $\to$ Serial Monitor
  • It is best to change the delay in the sketch from 1 to >300 so the numbers don’t scroll too fast.
  • Make sure the Autoscroll box below the output screen is ticked.
• If pin A0 does not work, e.g. because it has been fried by applying to large a voltage, try another pin and change the sketch code accordingly.

By changing the potentiometer, you can feed various voltages into the Arduino. By measuring the input voltage with your multimeter, and comparing it to the values reported by the Arduino, measure the conversion factor between volts and the Arduino output.

Arduino Temperature Measurement

Replace your potentiometer with your thermistor.

• i.e. Take voltage across thermistor reference resistor and feed it into the op amp follower and then take that (zener protected) output in Arduino Analog Pin 0
• At this point, your circuit should look similar to Figure @ref(fig:Breadboard))

Modify an Arduino program that converts the output of the thermistor circuit into temperature and prints it out on the serial port. The basic program, arduino_thermistor.ino, is available on the course resources page.

• This sketch will not run correctly “out of the box”. You need to set various parameter values: V_divider, R_fixed, A_1, B_1, C_1, D_1, and possibly others.

Use Tools $\to$ Serial Plot to show the temperature as a function of time, while heating up the thermistor with your hand and then removing your hand to let it cool down.

• Note that the only way to stop an Arduino sketch is to replace it with another one, e.g Blink.
  • This also means that the Arduino keeps running whatever sketch was last loaded into it, even if the Arduino IDE is closes, as long as the Arduino has power.
• If you connect the thermister to longer wires, it is a more useful sensor that can be brought close to sources of heat or cold.
  • Note: The thermistor does not work in water, since the water conducts electricity between the thermistor’s metal leads. With Toronto tap water, this parallel resistance is very similar to the thermistor resistance.

Python Communications with Arduino

Although Serial Monitor and Serial Plot are very useful, it is generally better to to transmit data from the Arduino to our own computer where it can be stored and analyzed. We will (unsuprisingly) use Python for this.