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.. _2.2.2_py:

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

.. note::

   .. image:: ../img/mcp3008_and_adc0834.jpg
      :width: 25%
      :align: left
    

   Depending on your kit version, please identify whether you have **ADC0834** or **MCP3008** and proceed with the matching section.

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.

Required Components
------------------------------

In this project, we need the following components. 

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

It's definitely convenient to buy a whole kit, here's the link: 

.. list-table::
    :widths: 20 20 20
    :header-rows: 1

    *   - Name	
        - ITEMS IN THIS KIT
        - LINK
    *   - Raphael Kit
        - 337
        - |link_Raphael_kit|

You can also buy them separately from the links below.

.. list-table::
    :widths: 30 20
    :header-rows: 1

    *   - COMPONENT INTRODUCTION
        - PURCHASE LINK

    *   - :ref:`cpn_gpio_board`
        - |link_gpio_board_buy|
    *   - :ref:`cpn_breadboard`
        - |link_breadboard_buy|
    *   - :ref:`cpn_wires`
        - |link_wires_buy|
    *   - :ref:`cpn_resistor`
        - |link_resistor_buy|
    *   - :ref:`cpn_thermistor`
        - |link_thermistor_buy|
    *   - :ref:`cpn_adc0834`
        - \-

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 ~/raphael-kit/python/

**Step 3:** Run the executable file

.. raw:: html

   <run></run>

.. code-block:: 

    sudo python3 2.2.2_Thermistor.py

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

**Code**

.. note::

    You can **Modify/Reset/Copy/Run/Stop** the code below. But before that, you need to go to  source code path like ``raphael-kit/python``. After modifying the code, you can run it directly to see the effect.


.. raw:: html

    <run></run>

.. code-block:: python

    #!/usr/bin/env python3
    # -*- coding: utf-8 -*-

    import RPi.GPIO as GPIO
    import ADC0834
    import time
    import math

    def init():
        ADC0834.setup()

    def loop():
        while True:
            analogVal = ADC0834.getResult()
            Vr = 5 * float(analogVal) / 255
            Rt = 10000 * Vr / (5 - Vr)
            temp = 1/(((math.log(Rt / 10000)) / 3950) + (1 / (273.15+25)))
            Cel = temp - 273.15
            Fah = Cel * 1.8 + 32
            print ('Celsius: %.2f °C  Fahrenheit: %.2f ℉' % (Cel, Fah))
            time.sleep(0.2)

    if __name__ == '__main__':
        init()
        try:
            loop()
        except KeyboardInterrupt:
            ADC0834.destroy()

**Code Explanation**

.. code-block:: python

    import math

There is a numerics library which declares a set of functions to compute
common mathematical operations and transformations.

.. code-block:: python

    analogVal = ADC0834.getResult()

This function is used to read the value of the thermistor.

.. code-block:: python

    Vr = 5 * float(analogVal) / 255
    Rt = 10000 * Vr / (5 - Vr)
    temp = 1/(((math.log(Rt / 10000)) / 3950) + (1 / (273.15+25)))
    Cel = temp - 273.15
    Fah = Cel * 1.8 + 32
    print ('Celsius: %.2f °C  Fahrenheit: %.2f ℉' % (Cel, Fah))

These calculations convert the thermistor values into centigrade degree
and Fahrenheit degree.

.. code-block:: python

    Vr = 5 * float(analogVal) / 255
    Rt = 10000 * Vr / (5 - Vr)

These two lines of codes are calculating the voltage distribution with
the read value analog so as to get Rt (resistance of thermistor).

.. code-block:: python

    temp = 1/(((math.log(Rt / 10000)) / 3950) + (1 / (273.15+25)))

This code refers to plugging Rt into the formula
**T\ K\ =1/(ln(R\ T/R\ N)/B+1/T\ N)** to get Kelvin temperature.

.. code-block:: python

    temp = temp - 273.15

Convert Kelvin temperature into centigrade degree.

.. code-block:: python

    Fah = Cel * 1.8 + 32

Convert the centigrade degree into Fahrenheit degree.

.. code-block:: python

    print ('Celsius: %.2f °C  Fahrenheit: %.2f ℉' % (Cel, Fah))

Print centigrade degree, Fahrenheit degree and their units on the
display.

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

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