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.. _measure_light_arduino:

Measure Light Intensity
========================
A photoresistor or photocell is a light-controlled variable resistor. The resistance of a photo resistor decreases with increasing incident light intensity; in other words, it exhibits photo conductivity.

Therefore, we can use a photoresistor to measure light intensity, and then show it through 5 LEDs.

Schematic
-----------

.. image:: img/Measure_Light_Intensity.png

Wiring
-------------------------

In this experiment, we will use 5 LEDs to show the light intensity. The higher the light intensity is, the more LEDs will light up, vice versa.

.. image:: img/wiring_measure_light.png

Code
----------
.. raw:: html

  <iframe src=https://create.arduino.cc/editor/sunfounder01/b0f8012d-69e5-443a-a1c1-9e690d568696/preview?embed style="height:510px;width:100%;margin:10px 0" frameborder=0></iframe>

How it works?
---------------
.. code-block:: arduino

  const int ledPins[] = {11, 12, 13, 14, 15};
  const int photocellPin = A0;  //photoresistor attach to A0
  int sensorValue = 0;        // value read from the sensor
  int Level = 0;           // sensor value converted into LED 'bars'


First of all, there are still various initialization definitions, setting pins and setting initial values ​​of variables.

In order to quickly set the input/output status and HIGH/LOW for the 5 LEDs in the following code, here we use the array ``ledPin[]`` to define the 5 LEDs connected to the corresponding pins of the Pico.

The element number of the array usually starts from 0. For example, ``ledPin[0]`` refers to GPIO11, and ``ledPin[4]`` refers to GPIO15.

* `array <https://www.arduino.cc/reference/en/language/variables/data-types/array/>`_

.. code-block:: arduino

  void setup()
  {
    Serial.begin(9600);  // start serial port at 9600 bps:
    for (int i = 0; i < 5; i++)
    {
      pinMode(ledPins[i], OUTPUT);// make all the LED pins outputs
    }
  }


In ``setup()``，using the ``for()`` statement set the 5 pins to OUTPUT. The variable ``i`` is added from 0 to 5, and the ``pinMode()`` function sets pin11 to pin15 to OUTPUT in turn.

.. code-block:: arduino

  sensorValue = analogRead(photocellPin); //read the value of A0
  Level = map(sensorValue, 0, 1023, 0, 5);  // map to the number of LEDs
  Serial.println(Level);
  delay(10);

In ``loop()``, read the analog value of the photocellPin(A0) and store to the variable sensorValue.

The ``map()`` function is used to map 0-1023 to 0-5. It means that the value range of the photoresistor (0-1023) is equally divided into 5 levels, 0-204.8 belongs to Level 0, 204.9-409.6 belongs to Level 1, and 819.2-1023 belong to Level 4.
If the value of variable ``sensorValue`` is 300 at this time, then ``Level`` is equal to 1.

* `map() <https://www.arduino.cc/reference/en/language/functions/math/map/>`_

.. code-block:: arduino

  for (int i = 0; i < 5; i++)
  {
    if (i <= Level ) //When i is smaller than Level, run the following code.
    {
      digitalWrite(ledPins[i], HIGH);     // turn on LEDs less than the level
    }
    else
    {
      digitalWrite(ledPins[i], LOW);     // turn off LEDs higher than level
    }
  }

Now we need to find a way to display the brightness level at this time with LEDs.

The ``for()`` statement is used here to perform loop detection in the ``ledPin[]`` array. If the element bit in the array is less than the value of ``Level``, the corresponding GPIO is set to high level, that is, the corresponding LED is lit.
If ``Level`` is equal to 1, turn on the LEDs on GPIO11 and GPIO12.