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.. _2.1.7_py_mcp3008:

2.1.7 Potentiometer(MCP3008)
=============================

.. 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
------------

The ADC function is used to convert analog signals into digital values. 
In this experiment, we use the MCP3008 ADC chip to perform this conversion. 
A potentiometer is used to generate a variable voltage, which changes the physical quantity. 
The MCP3008 then converts this analog voltage into a digital value that can be read and processed by the Raspberry Pi.

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

In this project, we need the following components. 

.. image:: ../img/list2_2.1.4_potentiometer.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_led`
        - |link_led_buy|
    *   - :ref:`cpn_potentiometer`
        - |link_potentiometer_buy|
    *   - :ref:`cpn_mcp3008`
        - \-


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

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

    *   - T-Board Name
        - physical
        - WiringPi
        - BCM

    *   - SPICE0
        - pin24
        - 10
        - 8
    *   - SPIMOSI
        - pin19
        - 12
        - 10
    *   - SPIMISO
        - pin21
        - 13
        - 9
    *   - SPISCLK
        - pin23
        - 14
        - 11
    *   - GPIO22
        - pin15
        - 3
        - 22


.. image:: ../img/schematic_2.1.7_potentiometer_mcp3008.png

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

**Step 1:** Build the circuit.

.. image:: ../img/july24_2.1.7_potentiometer_mcp3008.png


.. note::
    Please place the chip by referring to the corresponding position
    depicted in the picture. Note that the grooves on the chip should be on
    the left when it is placed.

**Step 2:** Set up the SPI interface and install the ``spidev`` library (see :ref:`spi_configuration` for detailed instructions). If you have already completed these steps, you can skip this.

**Step 3:** Open the code file

.. raw:: html

   <run></run>

.. code-block::

    cd ~/raphael-kit/python

**Step 4:** Run.

.. raw:: html

   <run></run>

.. code-block::

    sudo python3 2.1.7-2_Potentiometer.py

After the code runs, rotate the knob on the potentiometer, the intensity
of LED will change accordingly.

.. warning::

    If there is an error prompt  ``RuntimeError: Cannot determine SOC peripheral base address``, please refer to :ref:`faq_soc` 

**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

    import spidev
    import time
    import RPi.GPIO as GPIO

    # GPIO pin for PWM output
    PWM_PIN = 22

    # Setup GPIO
    GPIO.setmode(GPIO.BCM)
    GPIO.setup(PWM_PIN, GPIO.OUT)

    # Initialize PWM on GPIO22 at 1000Hz
    pwm = GPIO.PWM(PWM_PIN, 1000)
    pwm.start(0)  # Start with 0% duty cycle

    # Initialize SPI
    spi = spidev.SpiDev()
    spi.open(0, 0)  # Bus 0, CE0
    spi.max_speed_hz = 1000000

    def read_adc(channel):
        """
        Read analog value from MCP3008
        :param channel: ADC channel (0-7)
        :return: 10-bit integer (0-1023)
        """
        if channel < 0 or channel > 7:
            return -1
        adc = spi.xfer2([1, (8 + channel) << 4, 0])
        value = ((adc[1] & 3) << 8) | adc[2]
        return value

    def MAP(x, in_min, in_max, out_min, out_max):
        """
        Map a value from one range to another
        """
        return (x - in_min) * (out_max - out_min) / (in_max - in_min) + out_min

    try:
        while True:
            # Read analog value from CH0
            res = read_adc(0)
            print('res = %d' % res)

            # Convert to 0–100% duty cycle
            duty_cycle = MAP(res, 0, 1023, 0, 100)

            # Update PWM duty cycle
            pwm.ChangeDutyCycle(duty_cycle)

            time.sleep(0.2)

    except KeyboardInterrupt:
        pass

    finally:
        pwm.stop()
        GPIO.cleanup()
        spi.close()



**Code Explanation**

#. ``RPi.GPIO`` is used to generate PWM signals to control an LED. ``spidev`` is used for SPI communication with the MCP3008. ``time`` is used to add delays in the loop.

   .. code-block:: python

       #!/usr/bin/env python3

       import spidev
       import time
       import RPi.GPIO as GPIO

#. Configure GPIO pin 22 for PWM output using ``RPi.GPIO``. Set up SPI communication with the MCP3008 (Bus 0, CE0) at 1 MHz.

   .. code-block:: python

       PWM_PIN = 22

       GPIO.setmode(GPIO.BCM)
       GPIO.setup(PWM_PIN, GPIO.OUT)

       pwm = GPIO.PWM(PWM_PIN, 1000)  # 1kHz frequency
       pwm.start(0)  # Start with 0% duty cycle

       spi = spidev.SpiDev()
       spi.open(0, 0)
       spi.max_speed_hz = 1000000

#. This function reads analog data from the MCP3008 on the specified channel (0–7) using the SPI protocol. The result is a 10-bit integer ranging from 0 to 1023.

   .. code-block:: python

       def read_adc(channel):
           if channel < 0 or channel > 7:
               return -1
           adc = spi.xfer2([1, (8 + channel) << 4, 0])
           value = ((adc[1] & 3) << 8) | adc[2]
           return value

#. This function maps a value from one numerical range to another. It’s used to scale ADC values to PWM duty cycle percentages.

   .. code-block:: python

       def MAP(x, in_min, in_max, out_min, out_max):
           return (x - in_min) * (out_max - out_min) / (in_max - in_min) + out_min

#. In the main loop, the program continuously reads analog data from channel 0 of the MCP3008, maps the value to a PWM range (0–100), and sets the LED brightness accordingly. The loop updates every 0.2 seconds. If interrupted (e.g., Ctrl+C), the program stops the PWM signal and cleans up the GPIO configuration.

   .. code-block:: python

       try:
           while True:
               res = read_adc(0)
               print('res = %d' % res)

               duty_cycle = MAP(res, 0, 1023, 0, 100)
               pwm.ChangeDutyCycle(duty_cycle)

               time.sleep(0.2)

       except KeyboardInterrupt:
           pass

       finally:
           pwm.stop()
           GPIO.cleanup()
           spi.close()