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2.2.2 Thermistor
================

Introduction
------------

Just like photoresistor can sense light, thermistor is a temperature
sensitive electronic device that can be used for realizing functions of
temperature control, such as making a heat alarm.

Components
----------

.. image:: ../img/list_2.2.2_thermistor.png



Schematic Diagram
------------------

.. image:: ../img/image323.png


.. image:: ../img/image324.png


Experimental Procedures
-----------------------

**Step 1:** Build the circuit.

.. image:: ../img/image202.png

**Step 2:** Go to the folder of the code.

.. raw:: html

   <run></run>

.. code-block:: 

    cd ~/davinci-kit-for-raspberry-pi/nodejs/

**Step 3:** Run the code.

.. raw:: html

   <run></run>

.. code-block:: 

    sudo node thermistor.js

With the code run, the thermistor detects ambient temperature which will
be printed on the screen once it finishes the program calculation.

**Code**

.. code-block:: js

    const Gpio = require('pigpio').Gpio;
    const ADC0834 = require('./adc0834.js').ADC0834;

    exports.ADC0834 = ADC0834;

    const adc = new ADC0834(17, 18, 27);

    setInterval(() => {
      adc.read(0).then((value) => {
        var Vr = 5 * value / 255;
        var Rt = 10000 * Vr / (5 - Vr);
        var temp = 1 / ((Math.log(Rt/10000) / 3950)+(1 / (273.15 + 25)));
        var cel = (temp - 273.15).toFixed(2);
        var Fah = (cel * 1.8 + 32).toFixed(2);
        console.log(`Celsius: ${cel} C  Fahrenheit: ${Fah} F\n`);
      }, (error)=>{
        console.log("Error: " + error);
      });
    }, 1000);

**Code Explanation**

.. code-block:: js

    setInterval(() => {
      adc.read(0).then((value) => {
        var Vr = 5 * value / 255;
        var Rt = 10000 * Vr / (5 - Vr);
        var temp = 1 / ((Math.log(Rt/10000) / 3950)+(1 / (273.15 + 25)));
        var cel = (temp - 273.15).toFixed(2);
        var Fah = (cel * 1.8 + 32).toFixed(2);
        console.log(`Celsius: ${cel} C  Fahrenheit: ${Fah} F\n`);
      }, (error)=>{
        console.log("Error: " + error);
      });
    }, 1000);

We can read the ``value`` of the thermistor through the statement ``adc.read(0).then((value) => {...})``


.. code-block:: js

    var Vr = 5 * value / 255;
    var Rt = 10000 * Vr / (5 - Vr);
    var temp = 1 / ((Math.log(Rt/10000) / 3950)+(1 / (273.15 + 25)));
    var cel = (temp - 273.15).toFixed(2);
    var Fah = (cel * 1.8 + 32).toFixed(2);
    console.log(`Celsius: ${cel} C  Fahrenheit: ${Fah} F\n`);

These operations convert the thermistor value to a Celsius temperature value.

.. code-block:: js

    var Vr = 5 * value / 255;
    var Rt = 10000 * Vr / (5 - Vr);

These two lines of code are used to calculate the voltage distribution from the read values, resulting in Rt (resistance of the thermistor).

.. code-block:: js

    var temp = 1 / ((Math.log(Rt/10000) / 3950)+(1 / (273.15 + 25)));  

This code refers to substituting Rt into the formula **TK=1/(ln(RT/RN)/B+1/TN)** to get the temperature in Kelvin.

.. code-block:: js

    var cel = (temp - 273.15).toFixed(2);

This paragraph is to convert the Kelvin temperature to Celsius with two decimal places.

.. code-block:: js

    var Fah = (cel * 1.8 + 32).toFixed(2);

This paragraph converts Celsius to Fahrenheit with two decimal places.

.. code-block:: js

    console.log(`Celsius: ${cel} C  Fahrenheit: ${Fah} F\n`);

Print Celsius, Fahrenheit and their units on the terminal.  

Phenomenon Picture
------------------

.. image:: ../img/image203.jpeg