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\chapter {Interfacing a Thermistor}
\thispagestyle{empty}
\label{thermistor}
\newcommand{\LocTHERMfig}{\Origin/user-code/thermistor/figures}
\newcommand{\LocTHERMscicode}{\Origin/user-code/thermistor/scilab}
\newcommand{\LocTHERMscibrief}{Origin/user-code/thermistor/scilab}
\newcommand{\LocTHERMardcode}{\Origin/user-code/thermistor/arduino}
\newcommand{\LocTHERMardbrief}{Origin/user-code/thermistor/arduino}
A thermistor is basically a temperature dependent/sensitive resistor. It falls under the category of semiconductor sensors. Depending on the temperature in the vicinity of the thermistor, its resistance changes. Thermistors are available in two types, NTC and PTC. NTC stands for Negative Temperature Coefficient and PTC means Positive Temperature Coefficient. In NTC thermistors the resistance decreases with increase in temperature and vice versa. Whereas, for PTC the resistance increases with an increase in temperature and vice versa. The temperature ranges over $-55^{\circ}$celsius to $+125^{\circ}$celsius
Thermistors are available in shapes of bead, rod, flakes and discs. Due to their compact size and low cost, they have a wide application where rough temperature sensing is required. They however suffer from noise and need noise compensation. In this chapter we shall interface a thermistor with the \arduino\ board.
\section{Preliminaries}
A typical thermistor and its symbolic repersentation are shown in \ref{fig:therm} and \ref{fig:thermsym} respectively. The thermistor is available on the shield provided with the kit. It is a bead type thermistor. The thermistor is connected to the analog pin 4 of the \arduino\ board.
The connections for this experiment are shown in \ref{fig:therm-conn}. However, the user doesn't need to connect any wire or component explicitly.
\begin{figure}
\centering
\subfloat[Pictorial representation of a thermistor]{
\includegraphics[width=\smfig]{\LocTHERMfig/NTC-bead.jpg}
\label{fig:therm}} \hfill
\subfloat[Symbolic representation of a thermistor]{
\includegraphics[width=\smfig]{\LocTHERMfig/therm-sym.png}
\label{fig:thermsym}}
\caption{Thermistor}
\end{figure}
\begin{figure}
\centering
\includegraphics[width=\smfig]{\LocTHERMfig/THERMISTOR-Diagram.jpg}
\caption{Thermistor connection diagram}
\label{fig:therm-conn}
\end{figure}
\section{Reading thermistor from Arduino IDE}
In this section we will learn how to read values from the thermistor connected at pin 4 of the \arduino\ board. We shall also see how to drive a buzzer depending upon the thermistor values.
\begin{enumerate}
\item A simple code to read the values from thermistor is given in \ardref{ard:therm-read}. The arduino IDE based command for the analog read functionality is given by.
\lstinputlisting[firstline=9,lastline=9] {\LocTHERMardcode/therm-read/therm-read.ino}
where {\tt A4} represents the analog pin 4 to be read. The read value is stored in variable {\tt value} and is displayed using \lstinputlisting[firstline=10,lastline=10] {\LocTHERMardcode/therm-read/therm-read.ino}
The command on next line
\lstinputlisting[firstline=11,lastline=11] {\LocTHERMardcode/therm-read/therm-read.ino}
is used to put a delay of 500 milliseconds. This is to avoid very fast display of the read values. The entire reading and display operation is carried out 40 times.
The values can be observed over the serial monitor. The numbers displayed range from 0 to 1023. The thermistor used has a resistance of 10k at room temperature. It is connected in a voltage divider form using another fixed 10k resistor. A voltage of 5 volts is applied accross the series combination of the thermistor and the fixed 10k resistor. Voltage accross the fixed resistor is sensed and is given to the ADC. Hence at room temperature, both the resistors offer 10k resistance resulting in dividing the 5V equally. Therefore, at room temperature you may get the output of ADC around 500. If a heating or cooling source is available, one can observe the increase or desrease in the ADC output. Although the thermistor is of NTC type, the ADC output increases with increase in temperature. This is because the voltage accross the fixed resistor is sensed.
\item In this experiment, we will turn On and Off the buzzer depending on the temperature sensed by the thermistor. The program for this is available at \ardref{ard:therm-buzzer}. We shall use the ADC output to carry this out. The buzzer is connected on pin 3 which is a digital output pin. The ADC output value is displayed on the serial monitor. At the same time it is compared with value 550. Temperature of the thermistor is raised using body temperature by just holding it for a while. As soon as the ADC output exceeds 550, the buzzer is given a digital high signal, turning it On. A delay of half a second is introduced before the next value is read. This loop is executed 100 times.
\end{enumerate}
\begin{exercise}
Carry out the following exercise:
\begin{enumerate}
\item Put the thermistor in the vicinity of an Ice bowl. Take care not to wet the shield while doing so. Note down the ADC output value for 0$^{\circ}$celsius.
\end{enumerate}
\end{exercise}
\section {Reading thermistor from scilab scripts}
In this section we will explain a few scilab scripts to read values from thermistor. The {\tt cmd\_analog\_in} command will be used to read from thermistor connected to an analog input pin. The experiments will be carried out using scilab.
\begin{enumerate}
\item \sciref{sci:therm-read} is used to read values from thermistor. First the serial port is opened using the command {\tt open\_serial} and passing the correct port number to it. The command {\tt cmd\_analog\_in} is used to read from the analog pin. The pin number is passed to this command as an argument. The read value is stored in some variable. The value is then displayed on the scilab console. A sleep of 500 millisecond is executed using the {\tt sleep} command and then the reading process is repeated 20 times by putting it in a {\tt for} loop. After the loop is finished the serial port is closed using the {\tt close\_serial} command.
\item In this experiment we will use scilab script to turn on and off a buzzer using the thermistor values. The changes in the thermistor resistance is sensed as a voltage change between 0 to 5V. The ADC maps the thermistor voltage readings in to values ranging from 0 to 1023. This means 0 for 0 volts and 1023 for 5 volts. At room temperature the thermistor offers 10K resistance. The circuit arrangement makes the output voltage half of 5 volts i.e. around 2.5 volts. Hence the output is around 500. In this experiment we compare the ADC ouput value with 550 and as soon as the value exceeds 550 the buzzer is turned on. See \sciref{sci:therm-buzzer} for the complete code. We use {\tt if} loop to achieve this functionality. Command {\tt cmd\_digital\_out} is used to turn on and off the buzzer. Correct port number on which the buzzer is connected is passed to this command as an argument. The entire process is repeated 500 times.
\begin{exercise}
Carry out the exercise below:
Convert the ADC output readings to degree celsius. There are two ways to do so.
\begin{enumerate}
\item In the first method,
\begin{align}
\frac{1}{T}=A+B*ln(R)+C*(ln(R))^3
\label{therm-abc}
\end{align}
equation \ref{therm-abc} can be used if the value of A, B, C and R are known. The temperature T is in kelvin and thermistor resistance R is in ohms. The values of A, B and C can be found out by measuring thermistor resistance against three known values of temperatures. Once a set of three values of T and R are known it will result in three equations with three uknowns. The values of A, B, C can be found out by solving the three equations simultaneously. Once the values of A, B, C are known, the same equation can be used to directly convert resistance to kelvin. It can be then converted to celsius. This method is preffered when the temperature coefficient of thermistor is not known or is known very approximately. This method is bit cumbersome but can give accurate temperature conversion.
\item In the second method,
\begin{align}
\frac{1}{T}=\frac{1}{T_0}+\frac{1}{\beta}*ln\left(\frac{R}{R_0}\right)
\label{therm-beta}
\end{align}
equation \ref{therm-beta} can be used if the value of $\beta$ i.e. the Tempreature Coefficient of Resistance of the thermistor used is known. $R$ is the resistance of thermistor at temperature $T$ in kelvin. $R_0$ is the resistance of thermistor at room temperature $T_0$ in kelvin.
\end{enumerate}
\end{exercise}
\end{enumerate}
\section{Reading thermistor from Xcos}
In this section we will carry out the same experiments discussed in the previous sections but through Xcos. One should go through \secref{sec:xcos-start} before continuing.
\begin{enumerate}
\item The xcos diagram for performing the simple thermistor read operation is as shown in \figref{fig:therm-read}. The location of the xcos file is mentioned in the caption of the figure.
\begin{figure}
\centering
\includegraphics[width=\smfig]{\LocTHERMfig/therm-read-xcos.png}
\caption[Xcos diagram to read thermistor values]{Xcos diagram to read thermistor values.
This is what one sees when
{\tt \LocTHERMscibrief/therm-read-xcos.zcos} is invoked.}
\label{fig:therm-read}
\end{figure}
The parameters of the blocks can be changed by right clicking on the block and choosing {\tt Block Parameters}. One can also double click on the block. The values for each block is tabulated in \tabref{tab:therm-read}. All other parameters are to
be left unchanged.
\begin{table}
\centering
\caption{Xcos parameters to read thermistor}
\label{tab:therm-read}
\begin{tabular}{llc} \hline
Name of the block & Parameter name & Value \\ \hline
ARDUINO\_SETUP & Identifier of Arduino Card & 1 \\
& Serial com port number & {\tt your port number} \\ \hline
TIME\_SAMPLE & Duration of acquisition(s) & 100 \\
& Sampling period(s) & 0.1 \\ \hline
ANALOG\_READ\_SB & Analog Pin & 4 \\
& Arduino card number & 1 \\ \hline
CSCOPE & Ymin & 200 \\
& Ymax & 600 \\
& Refresh period & 100 \\ \hline
CLOCK\_c & Period & 0.1 \\
& Initialisation Time & 0 \\ \hline
\end{tabular}
\end{table}
\begin{figure}
\centering
\includegraphics[width=\smfig]{\LocTHERMfig/therm-read.png}
\caption[Output of Xcos diagram to read thermistor values]{Output of Xcos diagram to read thermistor values.
This is what one sees when
{\tt \LocTHERMscibrief/therm-read-xcos.zcos} is executed.}
\label{fig:therm-read-output}
\end{figure}
The thermistor readings can be varied by bringing a heating or cooling source in the vicinity of it. The graph as shown in \figref{fig:therm-read-output} will show the variations in the ADC output that is displayed.
\item In the second experiment, we will switch On and Off a buzzer depending on the thermistor readings (ADC output). The xcos diagram for this experiment is as shown in \figref{fig:therm-buzzer}.
\begin{figure}
\centering
\includegraphics[width=\smfig]{\LocTHERMfig/therm-buzzer-xcos.png}
% \includegraphics[width=\smfig]{\LocLDRfig/ldr-led-xcos.PNG}
\caption[Xcos diagram to read the value of thermistor, which is used to turn the buzzer on or off] {Xcos diagram to read the value of the thermistor, which is used to turn the buzzer on or off. This is what one sees when {\tt \LocTHERMscibrief/therm-buzzer-xcos.zcos} is invoked.}
\label{fig:therm-buzzer}
\end{figure}
The parameters of the blocks can be changed by right clicking on the block and choosing {\tt Block Parameters}. One can also double click on the block. The values for each block is tabulated in \tabref{tab:therm-read}. All other parameters are to
be left unchanged.
\begin{table}
\centering
\caption{Xcos parameters to read thermistor and switch the buzzer}
\label{tab:ldr-led}
\begin{tabular}{llc} \hline
Name of the block & Parameter name & Value \\ \hline
ARDUINO\_SETUP & Identifier of Arduino Card & 1 \\
& Serial com port number & {\tt your port number} \\ \hline
TIME\_SAMPLE & Duration of acquisition(s) & 100 \\
& Sampling period(s) & 0.1 \\ \hline
ANALOG\_READ\_SB & Analog pin & 4 \\
& Arduino card number & 1 \\ \hline
CMSCOPE & Ymin & 0 300 \\
& Ymax & 1 600 \\
& Refresh period & 100 100 \\ \hline
CLOCK\_c & Period & 0.1 \\
& Initialisation time & 0 \\ \hline
SWITCH2\_m & Datatype & 1 \\
& threshold & 550 \\
& pass first input if field & 0 \\
& use zero crossing & 1 \\ \hline
DIGITAL\_WRITE\_SB & Digital pin & 3 \\
& Arduino card number & 1 \\ \hline
\end{tabular}
\end{table}
The graph as shown in \figref{fig:therm-buzzer-output} will show the variations in the ADC output that is displayed and the corresponding switching of buzzer whenever the limits are crossed.
\begin{figure}
\centering
\includegraphics[width=\smfig]{\LocTHERMfig/therm-buzzer.png}
\caption[Output of Xcos diagram to switch buzzer through thermistor values]{Output of Xcos diagram to switch buzzer through thermistor values. This is what one sees when {\tt \LocTHERMscibrief/therm-buzzer-xcos.zcos} is executed.}
\label{fig:therm-buzzer-output}
\end{figure}
\end{enumerate}
\section{Arduino Code}
\label{sec:therm-arduino-code}
\addtocontents{ard}{\protect\addvspace{\codclr}}
\begin{ardcode}
\acaption{Read and display the thermistor values}
{Read and display the thermistor values. Available at
\LocTHERMardbrief/therm-read/therm-read.ino}
\label{ard:therm-read}
\lstinputlisting{\LocTHERMardcode/therm-read/therm-read.ino}
\end{ardcode}
\begin{ardcode}
\acaption{Turning the buzzer on and off using thermistor values}
{Turning the buzzer on and off using the thermistor values read by ADC. Available at
\LocTHERMardbrief/therm-buzzer/therm-buzzer.ino.}
\label{ard:therm-buzzer}
\lstinputlisting{\LocTHERMardcode/therm-buzzer/therm-buzzer.ino}
\end{ardcode}
\section{Scilab Code}
\label{sec:therm-scilab-code}
\addtocontents{cod}{\protect\addvspace{\codclr}}
\begin{scicode}
\ccaption{Read and display the thermistor values}
{Read and display the thermistor values. Available at
\LocTHERMscibrief/therm-read.sce}
\label{sci:therm-read}
\lstinputlisting{\LocTHERMscicode/therm-read.sce}
\end{scicode}
\begin{scicode}
\ccaption{Turning the buzzer on and off using thermistor values}
{Turning the buzzer on and off using the thermistor values read by ADC. Available at
\LocTHERMscibrief/therm-buzzer.sce.}
\label{sci:therm-buzzer}
\lstinputlisting{\LocTHERMscicode/therm-buzzer.sce}
\end{scicode}
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