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

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



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/

**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:`C code is not working?`.

**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.

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

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

.. raw:: html

   <run></run>

.. code-block::

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

**Step 3:** Run the executable file.

.. raw:: html

   <run></run>

.. code-block::

    sudo python3 2.2.1_Photoresistor.py

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

**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``. 
    
.. raw:: html

    <run></run>

.. code-block:: python

    import RPi.GPIO as GPIO
    import ADC0834
    import time

    LedPin = 22

    def setup():
        global led_val
        # Set the GPIO modes to BCM Numbering
        GPIO.setmode(GPIO.BCM)
        # Set all LedPin's mode to output and initial level to High(3.3v)
        GPIO.setup(LedPin, GPIO.OUT, initial=GPIO.HIGH)
        ADC0834.setup()
        # Set led as pwm channel and frequece to 2KHz
        led_val = GPIO.PWM(LedPin, 2000)

        # Set all begin with value 0
        led_val.start(0)

    def destroy():
        # Stop all pwm channel
        led_val.stop()
        # Release resource
        GPIO.cleanup()

    def loop():
        while True:
            analogVal = ADC0834.getResult()
            print ('analog value = %d' % analogVal)
            led_val.ChangeDutyCycle(analogVal*100/255)
            time.sleep(0.2)

    if __name__ == '__main__':
        setup()
        try:
            loop()
        except KeyboardInterrupt: # When 'Ctrl+C' is pressed, the program destroy() will be executed.
            destroy()

**Code Explanation**

.. code-block:: python

    def loop():
        while True:
            analogVal = ADC0834.getResult()
            print ('analog value = %d' % analogVal)
            led_val.ChangeDutyCycle(analogVal*100/255)
            time.sleep(0.2)

Read the analog value of CH0 of ADC0834. By default, the function
getResult() is used to read the value of CH0, so if you want to read
other channels, please input 1, 2, or 3 into () of the function
getResult(). Next, what you need is to print the value via the print
function. Because the changing element is the duty cycle of LedPin, the
computational formula, analogVal*100/255 is needed to convert analogVal
into percentage. Finally, ChangeDutyCycle() is called to write the
percentage into LedPin.

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

.. image:: img/image199.jpeg