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.. _2.2.1_c_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:: img/list2_2.2.1_photoresistor.png

Principle
---------

A photoresistor or photocell is a light-controlled variable resistor.
The resistance of a photoresistor decreases with increasing incident
light intensity; in other words, it exhibits photo conductivity. A
photoresistor can be applied in light-sensitive detector circuits, and
light- and darkness-activated switching circuits.

.. image:: img/image196.png
    :width: 200
    :align: center



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_2.2.1_photoresistor_mcp3008.png


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

**Step 1:** Build the circuit.

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


For C Language Users
^^^^^^^^^^^^^^^^^^^^

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

.. raw:: html

   <run></run>

.. code-block:: 

    cd ~/davinci-kit-for-raspberry-pi/c/2.2.1-2/

**Step 3:** Compile the code.

.. raw:: html

   <run></run>

.. code-block:: 

    gcc 2.2.1_Photoresistor.c -o photoresistor -lwiringPi -lm

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

.. raw:: html

   <run></run>

.. code-block:: 

    ./photoresistor

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

.. note::

    If it does not work after running, or there is an error prompt: \"wiringPi.h: No such file or directory\", please refer to :ref:`install_wiringpi`.

**Code**

.. code-block:: c

    #include <wiringPi.h>
    #include <wiringPiSPI.h>
    #include <stdio.h>
    #include <softPwm.h>

    #define SPI_CHANNEL 0      // Use SPI channel 0 (CE0)
    #define SPI_SPEED   1000000 // 1 MHz SPI speed
    #define LedPin      3       // GPIO3 (WiringPi) for LED PWM

    // Read ADC value from MCP3008, channel 0~7
    int readMCP3008(int channel) {
        if (channel < 0 || channel > 7) return -1;

        unsigned char buffer[3];
        buffer[0] = 1;                          // Start bit
        buffer[1] = (8 + channel) << 4;         // SGL/DIF = 1, D2-D0 = channel
        buffer[2] = 0;

        wiringPiSPIDataRW(SPI_CHANNEL, buffer, 3);

        // Combine the result
        int result = ((buffer[1] & 3) << 8) | buffer[2];
        return result;
    }

    int main(void) {
        if (wiringPiSetup() == -1) {
            printf("wiringPi init failed!\n");
            return 1;
        }

        if (wiringPiSPISetup(SPI_CHANNEL, SPI_SPEED) == -1) {
            printf("SPI setup failed!\n");
            return 1;
        }

        softPwmCreate(LedPin, 0, 100); // Init software PWM

        while (1) {
            int analogVal = readMCP3008(0); // Read from CH0
            printf("ADC Value: %d\n", analogVal);

            // Scale 10-bit ADC value (0–1023) to PWM range (0–100)
            int pwmVal = analogVal * 100 / 1023;
            softPwmWrite(LedPin, pwmVal);

            delay(100);
        }

        return 0;
    }


**Code Explanation**

The codes here are the same as that in 2.1.4 Potentiometer. If you have
any other questions, please check the code explanation of :ref:`2.1.4_c_mcp3008` for details.


For Python Language Users
^^^^^^^^^^^^^^^^^^^^^^^^^



**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 ~/davinci-kit-for-raspberry-pi/python

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

.. raw:: html

   <run></run>

.. code-block::

    sudo python3 2.2.1-2_photoresistor.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 ``davinci-kit-for-raspberry-pi/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 RPi.GPIO as GPIO
    import spidev
    import time

    # GPIO pin for PWM LED
    PWM_PIN = 22

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

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

    # Initialize SPI (MCP3008 on Bus 0, CE0)
    spi = spidev.SpiDev()
    spi.open(0, 0)
    spi.max_speed_hz = 1000000  # 1 MHz

    # Function to read MCP3008 ADC value
    def read_adc(channel):
        """
        Read analog value from MCP3008 (channel 0–7)
        Returns: 10-bit value (0–1023)
        """
        if channel < 0 or channel > 7:
            return -1
        r = spi.xfer2([1, (8 + channel) << 4, 0])
        value = ((r[1] & 3) << 8) | r[2]
        return value

    # Main loop to read ADC and set PWM brightness
    try:
        while True:
            analogVal = read_adc(0)
            print(f"value = {analogVal}")

            # Scale ADC value (0–1023) to duty cycle (0–100)
            duty_cycle = analogVal * 100 / 1023
            pwm.ChangeDutyCycle(duty_cycle)

            time.sleep(0.2)

    except KeyboardInterrupt:
        pass

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

**Code Explanation**

#. Import necessary libraries:

   - ``RPi.GPIO`` to control GPIO pins and generate PWM signal.
   - ``spidev`` to interface with the MCP3008 ADC via SPI.
   - ``time`` to handle timing and delays.

   .. code-block:: python

       #!/usr/bin/env python3

       import RPi.GPIO as GPIO
       import spidev
       import time

#. Configure GPIO pin 22 as PWM output using BCM mode. Then, initialize software PWM at 1000 Hz with a starting duty cycle of 0%.

   .. code-block:: python

       # GPIO pin for PWM LED
       PWM_PIN = 22

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

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

#. Set up the SPI interface to communicate with MCP3008 on bus 0, chip enable 0 (CE0), and configure SPI speed to 1 MHz.

   .. code-block:: python

       # Initialize SPI (MCP3008 on Bus 0, CE0)
       spi = spidev.SpiDev()
       spi.open(0, 0)
       spi.max_speed_hz = 1000000  # 1 MHz

#. Define a function ``read_adc(channel)`` to read analog values from MCP3008. The function sends three bytes to the chip and reconstructs a 10-bit analog value (0–1023) from the response.

   .. code-block:: python

       # Function to read MCP3008 ADC value
       def read_adc(channel):
           """
           Read analog value from MCP3008 (channel 0–7)
           Returns: 10-bit value (0–1023)
           """
           if channel < 0 or channel > 7:
               return -1
           r = spi.xfer2([1, (8 + channel) << 4, 0])
           value = ((r[1] & 3) << 8) | r[2]
           return value

#. This is the main loop that:

   - Reads analog input from channel 0 of the MCP3008.
   - Converts the value to a PWM duty cycle (0–100%).
   - Adjusts the brightness of the LED using ``pwm.ChangeDutyCycle()``.
   - Repeats every 0.2 seconds.

   .. code-block:: python

       # Main loop to read ADC and set PWM brightness
       try:
           while True:
               analogVal = read_adc(0)
               print(f"value = {analogVal}")

               # Scale ADC value (0–1023) to duty cycle (0–100)
               duty_cycle = analogVal * 100 / 1023
               pwm.ChangeDutyCycle(duty_cycle)

               time.sleep(0.2)

#. When the user interrupts the program with Ctrl+C, the PWM and GPIO are properly cleaned up, and the SPI interface is closed.

   .. code-block:: python

       except KeyboardInterrupt:
           pass

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