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2.2.1 Photoresistor
===================


.. 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.

Components
----------

.. image:: img/list_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
-----------------

.. image:: img/image321.png


.. image:: img/image322.png


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

**Step 1:** Build the circuit.

.. image:: img/image198.png
    :width: 800

**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/

**Step 3:** Compile the code.

.. raw:: html

   <run></run>

.. code-block::

    gcc 2.2.1_Photoresistor.c -lwiringPi

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

.. raw:: html

   <run></run>

.. code-block::

    sudo ./a.out

The code run, the brightness of LED will vary depending on the intensity
of light that the photoresistor senses.

.. 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_pi5`.

**Code**

.. code-block:: c

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

    typedef unsigned char uchar;
    typedef unsigned int uint;

    #define     ADC_CS    0
    #define     ADC_CLK   1
    #define     ADC_DIO   2
    #define     LedPin    3

    uchar get_ADC_Result(uint channel)
    {
        uchar i;
        uchar dat1=0, dat2=0;
        int sel = channel > 1 & 1;
        int odd = channel & 1;

        digitalWrite(ADC_CLK, 1);
        delayMicroseconds(2);
        digitalWrite(ADC_CLK, 0);
        delayMicroseconds(2);

        pinMode(ADC_DIO, OUTPUT);
        digitalWrite(ADC_CS, 0);
        // Start bit
        digitalWrite(ADC_CLK,0);
        digitalWrite(ADC_DIO,1);    delayMicroseconds(2);
        digitalWrite(ADC_CLK,1);    delayMicroseconds(2);
        //Single End mode
        digitalWrite(ADC_CLK,0);
        digitalWrite(ADC_DIO,1);    delayMicroseconds(2);
        digitalWrite(ADC_CLK,1);    delayMicroseconds(2);
        // ODD
        digitalWrite(ADC_CLK,0);
        digitalWrite(ADC_DIO,odd);  delayMicroseconds(2);
        digitalWrite(ADC_CLK,1);    delayMicroseconds(2);
        //Select
        digitalWrite(ADC_CLK,0);
        digitalWrite(ADC_DIO,sel);    delayMicroseconds(2);
        digitalWrite(ADC_CLK,1);

        digitalWrite(ADC_DIO,1);    delayMicroseconds(2);
        digitalWrite(ADC_CLK,0);
        digitalWrite(ADC_DIO,1);    delayMicroseconds(2);

        for(i=0;i<8;i++)
        {
            digitalWrite(ADC_CLK,1);    delayMicroseconds(2);
            digitalWrite(ADC_CLK,0);    delayMicroseconds(2);

            pinMode(ADC_DIO, INPUT);
            dat1=dat1<<1 | digitalRead(ADC_DIO);
        }

        for(i=0;i<8;i++)
        {
            dat2 = dat2 | ((uchar)(digitalRead(ADC_DIO))<<i);
            digitalWrite(ADC_CLK,1);    delayMicroseconds(2);
            digitalWrite(ADC_CLK,0);    delayMicroseconds(2);
        }

        digitalWrite(ADC_CS,1);
        pinMode(ADC_DIO, OUTPUT);
        return(dat1==dat2) ? dat1 : 0;
    }

    int main(void)
    {
        uchar analogVal;
        if(wiringPiSetup() == -1){ //when initialize wiring failed,print messageto screen
            printf("setup wiringPi failed !");
            return 1;
        }
        softPwmCreate(LedPin,  0, 100);
        pinMode(ADC_CS,  OUTPUT);
        pinMode(ADC_CLK, OUTPUT);

        while(1){
            analogVal = get_ADC_Result(0);
            printf("Current analogVal : %d\n", analogVal);
            delay(100);
            softPwmWrite(LedPin, analogVal);
            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 **2.1.4
Potentiometer.c** for details.