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.. _2.1.4_potentiometer_nodejs:

2.1.4 Potentionmeter
=====================


.. 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
------------

The ADC function can be used to convert analog signals to digital
signals, and in this experiment, ADC0834 is used to get the function
involving ADC. Here, we implement this process by using potentiometer.
Potentiometer changes the physical quantity -- voltage, which is
converted by the ADC function.

Components
----------

.. image:: ../img/list_2.1.4_potentiometer.png



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

.. image:: ../img/image311.png


.. image:: ../img/image312.png


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

**Step 1:** Build the circuit.

.. image:: ../img/image180.png


.. note::
    Please place the chip by referring to the corresponding position
    depicted in the picture. Note that the grooves on the chip should be on
    the left when it is placed.

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

.. raw:: html

   <run></run>

.. code-block::

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

**Step 3:** Run the code.

.. raw:: html

   <run></run>

.. code-block::

    sudo node potentionmeter.js

After the code runs, rotate the knob on the potentiometer, the intensity
of LED will change accordingly.

**Code**

.. code-block:: js

    const Gpio = require('pigpio').Gpio;
    const ADC0834 = require('./adc0834.js').ADC0834;

    const adc = new ADC0834(17, 18, 27);
    const led = new Gpio(22, {mode: Gpio.OUTPUT});

    setInterval(() => {
        adc.read(0).then((value) => {
            console.log(`Current analogVal: ${value}\n`);
            led.pwmWrite(value);
        }, (error)=>{
            console.log("Error: " + error);
        });
    }, 100);

**Code Explanation**

.. code-block:: js

    const Gpio = require('pigpio').Gpio;

Import the ``pigpio`` module.

.. code-block:: js

    const ADC0834 = require('./adc0834.js').ADC0834;

We import an ``ADC0834`` constructor to use the adc0834 module.


.. code-block:: js

   const adc = new ADC0834(17, 18, 27);


Instantiate an ADC0834 object, the three parameters are its three pins.

This is a promise object, you may need to understand the concept from the following link.

* `Promise <https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise>`_


.. code-block:: js

    setInterval(() => {
        adc.read(0).then((value) => {
            console.log(`Current analogVal: ${value}\n`);
            led.pwmWrite(value);
        }, (error)=>{
            console.log("Error: " + error);
        });
    }, 100);

The value of ADC0834 channel 0 (channel 0 is connected to the potentiometer) is read every 100ms, and the value will be stored in ``value``.

Print ``value`` and use it to control the brightness of the LED, now you can see that the brightness of the LED changes with the value of the potentiometer.





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

.. image:: ../img/image181.jpeg