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.. _2.2.1_py_pi5_mcp3008:

2.2.1 Photoresistor(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
------------

Photoresistor is a commonly used component of ambient light intensity in
life. It helps the controller to recognize day and night and realize
light control functions such as night lamp. This project is very similar
to potentiometer, and you might think it changing the voltage to sensing
light.

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

In this project, we need the following components. 

.. image:: ../python_pi5/img/list2_2.2.1_photoresistor.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_mcp3008`
        - \-
    *   - :ref:`cpn_photoresistor`
        - |link_photoresistor_buy|

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

.. .. image:: ../python_pi5/img/2.2.1_photoresistor_schematic_1.png

.. 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:: ../python_pi5/img/schematic_2.2.1_photoresistor_mcp3008.png


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

**Step 1:** Build the circuit.

.. image:: ../python_pi5/img/july24_2.2.1_photoresistor_mcp3008.png

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

.. raw:: html

   <run></run>

.. code-block::

    cd ~/raphael-kit/python-pi5

**Step 4:** Run the executable file.

.. raw:: html

   <run></run>

.. code-block::

    sudo python3 2.2.1-2_Photoresistor_zero.py

When the code is running, the brightness of the LED will change according to the light intensity sensed by the photoresistor.

.. 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-pi5``. 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
    from gpiozero import PWMLED

    # Initialize a PWM LED on GPIO pin 22
    led = PWMLED(22)

    # Initialize SPI communication (Bus 0, CE0 -> GPIO8)
    spi = spidev.SpiDev()
    spi.open(0, 0)  # Bus 0, CS0
    spi.max_speed_hz = 1000000  # 1 MHz

    # Function to read from MCP3008 channel (0–7)
    def read_adc(channel):
        """
        Read analog value from MCP3008 (0–1023)
        """
        if channel < 0 or channel > 7:
            return -1
        # MCP3008 protocol: Start bit, Single-ended mode, Channel (3 bits), filler
        r = spi.xfer2([1, (8 + channel) << 4, 0])
        value = ((r[1] & 3) << 8) | r[2]
        return value

    # Define a function for mapping values from one range to another
    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

    # Main loop for reading ADC value and controlling LED brightness
    def loop():
        while True:
            # Read analog value from channel 0 of MCP3008
            analogVal = read_adc(0)
            print('value = %d' % analogVal)

            # Map 0–1023 to PWM range 0.0–1.0
            led.value = analogVal / 1023.0

            # Wait for 0.2 seconds
            time.sleep(0.2)

    # Run the main loop and handle KeyboardInterrupt for graceful shutdown
    try:
        loop()
    except KeyboardInterrupt:
        led.value = 0  # Turn off LED before exiting


**Code Explanation**

#. This segment imports the ``PWMLED`` class from the ``gpiozero`` library for controlling PWM LEDs, ``spidev`` for SPI communication with MCP3008, and ``time`` for executing sleep/delay operations.

   .. code-block:: python

       #!/usr/bin/env python3
       import spidev
       import time
       from gpiozero import PWMLED

#. Initializes a PWM LED connected to GPIO pin 22, and sets up the SPI interface for MCP3008 (Bus 0, CE0). The SPI clock speed is set to 1 MHz.

   .. code-block:: python

       # Initialize a PWM LED on GPIO pin 22
       led = PWMLED(22)

       # Initialize SPI communication (Bus 0, CE0 -> GPIO8)
       spi = spidev.SpiDev()
       spi.open(0, 0)  # Bus 0, CS0
       spi.max_speed_hz = 1000000  # 1 MHz

#. Defines a function to read from a specific MCP3008 ADC channel. It sends a 3-byte command over SPI and extracts a 10-bit value (0–1023) from the response.

   .. code-block:: python

       # Function to read from MCP3008 channel (0–7)
       def read_adc(channel):
           """
           Read analog value from MCP3008 (0–1023)
           """
           if channel < 0 or channel > 7:
               return -1
           # MCP3008 protocol: Start bit, Single-ended mode, Channel (3 bits), filler
           r = spi.xfer2([1, (8 + channel) << 4, 0])
           value = ((r[1] & 3) << 8) | r[2]
           return value

#. Defines a helper function ``MAP()`` that maps a number from one range to another. This is useful for converting raw ADC values into a suitable PWM range.

   .. code-block:: python

       # Define a function for mapping values from one range to another
       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

#. This section implements a loop that repeatedly reads an analog value from channel 0 of the MCP3008, maps it to a PWM brightness value (0.0–1.0), and applies it to the LED. The loop pauses for 0.2 seconds between readings.

   .. code-block:: python

       # Main loop for reading ADC value and controlling LED brightness
       def loop():
           while True:
               # Read analog value from channel 0 of MCP3008
               analogVal = read_adc(0)
               print('value = %d' % analogVal)

               # Map 0–1023 to PWM range 0.0–1.0
               led.value = analogVal / 1023.0

               # Wait for 0.2 seconds
               time.sleep(0.2)

#. Executes the loop and gracefully handles ``KeyboardInterrupt``. When the user stops the program (Ctrl+C), the LED is turned off before exiting.

   .. code-block:: python

       # Run the main loop and handle KeyboardInterrupt for graceful shutdown
       try:
           loop()
       except KeyboardInterrupt:
           # Turn off LED before exiting
           led.value = 0