Potentialmeter AnalogRead from Arduino to graph in Qt

[rosietesmen]

I already know how to connect Qt and Arduino, But I don't know how to make it plot in Qt or display number in Qt. I have tried the qt code from this example: https://www.youtube.com/watch?v=AX-HhBXBzGg#t=511.742042, changed the temp sensor to potentiometer, but it didn't work. I used LCDNumber to display, but I don't know what codes should I write in LCDNumber to read the values from Arduino. Hope you guys could give me some suggestion or example. Thanks!

[jazzycamel]

@rosietesmen

How are you getting the data from the Arduino? Are you using a library or code you have written yourself? Show us your (Qt/Arduino) code and we can be more help.

If you're using Firmata then data for pins configured as inputs is constantly being streamed to the host PC. Any good client (like mine QtFirmata :) ) will parse this data and store it for you. You then periodically check this data (via a timer for example) and update your plot.

As to plots, there are a number of very good libraries for plotting with Qt which you should definitely look at (QCustomPlot, Qwt). I've put a very basic example of drawing a custom plot (that I actually use for teaching the basics of drawing, not plotting!) on my GitHub in case it helps anyone: QtPlot.

[Macive Xiong]

Hey guys, thank you so much for your reply. I've been trying so hard on this project but I stuck on here.

 void MainWindow::updateP(QString command)
    {
        if(arduino->isReadable()){
            arduino->readAll();
        }else{
            qDebug() << "Couldn't write to serial!";
        }
        
        QBrush redBrush(Qt::red);
        QBrush blueBrush(Qt::blue);
        QPen blackpen(Qt::black);
        blackpen.setWidth(1);
        ellipse = scene->addEllipse(x, 100, 10, 10,blackpen,redBrush);
    }

    void MainWindow::readSerial()
    {
        MainWindow::updateP(ui->graphicsView->scene());
    }

I am trying to use the updateP for updating my potentialmeter's signal. I don't know what's the next step and how can I modify my code to the right one. The signal from potentialmeter will replace the value on X. How can I set the X = signal read of Potentialmeter?

[jazzycamel]

@Macive-Xiong

First point is that arduino->readAll() will return any data that has been received so you a) need to save it to a variable (QByteArray data=arduino->readAll(); and b) need to process/parse it to retrieve the value.

Second point is that you seem to be passing a QGraphicsScene object to your updateP function which accepts a QString argument.

Thirdly, you seem to be calling your updateP function as if it was static, why? Is it defined this way?

Finally, you say you are stuck: what with? Specifics about your application and your current issue help us give you useful answers.

As a footnote, the code above should look something like the following if it is to compile:

void MainWindow::updateP(){
  if(arduino->isReadable()) QByteArray data=arduino->readAll();
  else {
    qDebug() << "Couldn't read from serial";
    return;
  }

  // Parse data here and then create/draw your ellipse
}

void MainWindow::readSerial(){
  this->updateP();
}

[Macive Xiong]

@jazzycamel , thank you so much for your reply!! It's really helpful.
I was stuck cause I am not sure how can I get the value of potentialmeter into the ellipse parameter. Is that right if I write this way:

void MainWindow::updateP()
{
    if(arduino->isReadable()){
        QByteArray X_position = arduino->readAll();
    }else{
        qDebug() << "Couldn't wrtie to serial!";
        return;
    }
    //set scene
    QBrush redBrush(Qt::red);
    QPen blackpen(Qt::black);
    blackpen.setWidth(1);

    float x = X_position;
    ui->graphicsView->scene()->addEllipse(x , 100, 10, 10, blackpen, redBrush);

}

void MainWindow::readSeiral()
{
    this->updateP();
}

And I would like to define the data = x, but it still come out errors.
The error message : [release/moc_mainwindow.cpp] Error 1
Is there any suggestion for my code?

thank you so much.

[jazzycamel]

@Macive-Xiong

Assuming the QByteArray has come from a QSerialPort and contains only one float value represented as a string (you really should check all of these assumptions!) then getting a float value is relatively simple. The following example shows doing just this and then plotting an ellipse of diameter 20px at position x:

widget.h

#ifndef WIDGET_H
#define WIDGET_H

#include <QWidget>

class QGraphicsView;
class QGraphicsScene;

class Widget : public QWidget
{
    Q_OBJECT

public:
    Widget(QWidget *parent = 0);
    ~Widget();

private:
    QGraphicsView *view;
    QGraphicsScene *scene;
};

#endif // WIDGET_H

widget.cpp

#include <QDebug>
#include <QVBoxLayout>
#include <QGraphicsView>
#include <QGraphicsScene>

#include "widget.h"

Widget::Widget(QWidget *parent)
    : QWidget(parent)
{
    this->resize(1024,768);

    QVBoxLayout *l=new QVBoxLayout(this);
    scene=new QGraphicsScene(this);
    scene->setSceneRect(0,0,1024,768);
    view=new QGraphicsView(scene, this);
    l->addWidget(view);

    QByteArray data("255.0");
    bool ok=false;
    float x=data.toFloat(&ok);
    if(!ok){
        qDebug() << "Could not convert" << data << "to float!";
        return;
    }
    qDebug() << "x:" << x;
    scene->addEllipse(x, 768/2., 20, 20);
}

Widget::~Widget(){}

[Macive Xiong]

@jazzycamel

Hey, thanks for your help. I've tried the information you gave and I still got this error message:

:-1: error: [release/moc_mainwindow.cpp] Error 1

Here's my code:

void MainWindow::updateP()
{
    const QByteArray XP = arduino->readAll();
    bool ok = false;
    float x = XP.toFloat(&ok);
    if(!ok){
        qDebug() << "Could not convert" << XP << "to float!";
        return;
    }
    //set scene
    QBrush redBrush(Qt::red);
    QPen blackpen(Qt::black);
    blackpen.setWidth(1);

    qDebug() << "x:" << x;
    scene->addEllipse(x , 100, 10, 10, blackpen, redBrush);

}

void MainWindow::readSeiral()
{
    this->updateP();
}

Also, I added the signal and slots code:

QObject::connect(arduino, SIGNAL(readyRead()), this, SLOT(readSerial()));

Any suggestion? Thank you so much.

[jazzycamel]

@Macive-Xiong
If you're using QtCreator, could you go to the 'Compile Output' tab and give a bit more information on the error message? I don't see anything immediately obviously wrong with the code, but this is only a snippet after all and the issue might be elsewhere in your application.

[Macive Xiong]

This post is deleted!

[Macive Xiong]

Thanks for all you guys help. The code works fine and I have finished that.
My final codes:

void MainWindow::updateP()
{
    const QByteArray data = arduino->readAll();
    float x = data.toFloat();
    qDebug() << "couldn't convert" << data << "to float!";


    //set scene
    QBrush redBrush(Qt::red);
    QPen blackpen(Qt::black);
    blackpen.setWidth(1);

    qDebug() << "x:" << x;
    ellipse = scene->addEllipse( x , 100, 10, 10, blackpen, redBrush);
    return;

}

void MainWindow::readSerial()
{
    this->updateP();
}

I appreciate all your help:)

Please feel free if you have some questions on my project:)

[Macive Xiong]

This post is deleted!