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.. _2.1.7_js_pi5_mcp3008:

2.1.7 Potentiometer(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
------------

The ADC function is used to convert analog signals into digital values. 
In this experiment, we use the MCP3008 ADC chip to perform this conversion. 
A potentiometer is used to generate a variable voltage, which changes the physical quantity. 
The MCP3008 then converts this analog voltage into a digital value that can be read and processed by the Raspberry Pi.

Required Components
------------------------------

In this project, we need the following components. 

.. image:: ../img/list2_2.1.4_potentiometer.png

It's definitely convenient to buy a whole kit, here's the link: 

.. list-table::
    :widths: 20 20 20
    :header-rows: 1

    *   - Name	
        - ITEMS IN THIS KIT
        - LINK
    *   - Raphael Kit
        - 337
        - |link_Raphael_kit|

You can also buy them separately from the links below.

.. list-table::
    :widths: 30 20
    :header-rows: 1

    *   - COMPONENT INTRODUCTION
        - PURCHASE LINK

    *   - :ref:`cpn_gpio_board`
        - |link_gpio_board_buy|
    *   - :ref:`cpn_breadboard`
        - |link_breadboard_buy|
    *   - :ref:`cpn_wires`
        - |link_wires_buy|
    *   - :ref:`cpn_resistor`
        - |link_resistor_buy|
    *   - :ref:`cpn_led`
        - |link_led_buy|
    *   - :ref:`cpn_potentiometer`
        - |link_potentiometer_buy|
    *   - :ref:`cpn_mcp3008`
        - \-

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_2.1.7_potentiometer_mcp3008.png


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

**Step 1:** Build the circuit.

.. image:: ../img/2.1.7_Potentiometer_bb.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:** Open the code file.

.. raw:: html

   <run></run>

.. code-block::

    cd ~/raphael-kit/nodejs/

**Step 3:** Run the code.

.. raw:: html

   <run></run>

.. code-block::

    sudo node potentionmeter-2.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 mcpadc = require('mcp-spi-adc');

    // Open MCP3008 channel 0 (analog input CH0)
    const adc = mcpadc.openMcp3008(0, { speedHz: 1000000 }, (err) => {
    if (err) {
        console.error("Failed to open ADC channel:", err);
        process.exit(1);
    }

    console.log("MCP3008 channel 0 opened successfully.");

    // Initialize LED on GPIO22 with PWM output mode
    const led = new Gpio(22, { mode: Gpio.OUTPUT });

    // Read ADC value every 100ms and update LED brightness
    setInterval(() => {
        adc.read((err, reading) => {
        if (err) {
            console.error("Error reading ADC:", err);
            return;
        }

        // Convert floating-point value (0.0鈥?.0) to PWM range (0鈥?55)
        const pwmVal = Math.round(reading.value * 255);

        console.log(`Current analogVal: ${pwmVal}`);

        // Update LED brightness using PWM
        led.pwmWrite(pwmVal);
        });
    }, 100);
    });


**Code Explanation**

.. code-block:: js

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

This line imports the ``pigpio`` module, which allows for precise PWM and GPIO control on the Raspberry Pi.

.. code-block:: js

    const mcpadc = require('mcp-spi-adc');

This line imports the ``mcp-spi-adc`` library, which enables communication with the MCP3008 ADC over the SPI interface.

.. code-block:: js

    const adc = mcpadc.openMcp3008(0, { speedHz: 1000000 }, (err) => {
        if (err) {
            console.error("Failed to open ADC channel:", err);
            process.exit(1);
        }
    });

Initializes MCP3008 analog input channel 0. Sets the SPI communication speed to 1 MHz. If initialization fails, logs an error and exits the program.

.. code-block:: js

    const led = new Gpio(22, { mode: Gpio.OUTPUT });

Creates a GPIO object for pin 22 on the Raspberry Pi. This pin is configured as an output and will be used to control the brightness of an LED via PWM.

.. code-block:: js

    setInterval(() => {
        adc.read((err, reading) => {
            if (err) {
                console.error("Error reading ADC:", err);
                return;
            }

            const pwmVal = Math.round(reading.value * 255);
            console.log(`Current analogVal: ${pwmVal}`);
            led.pwmWrite(pwmVal);
        });
    }, 100);

Every 100 milliseconds, this function reads the analog value from MCP3008 channel 0. The ADC returns a normalized floating-point number between 0.0 and 1.0. This value is scaled to the 0–255 range and written to GPIO22 using ``pwmWrite()`` to control the LED brightness. The PWM value is also printed to the console.

.. Phenomenon Picture
.. ------------------

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