Raspberry PI 4 Tachometer doesn't display data

[rooster37]

I've made a QT GUI to display RPM data from hall sensors. The Python program that I embedded in my program works with print statements. But the Main Window displays a zero that does not change. Can anyone help get the data to display. The following is the code I used:

import sys
#import spidev
import time, math
import os
import RPi.GPIO as GPIO

from PyQt5.QtCore import QTime, QTimer
from PyQt5.QtWidgets import QApplication, QMainWindow, QLCDNumber
from tach import Ui_MainWindow

gpio_pin_number = 12   # GPIO Pin used by sensor
wheel_diameter_in = 10   # wheel diameter in inches
adjustment = 1   # adjustment for gear ratio or number of magnets 
seconds = 1   # time to wait between printing values 
rpm = 0
mph = 0
elapsed_time = 0   # amount of time a full revolution takes
number_interrupts = 0   # counts interrupts (triggered by sensor)
previous_number_interrupts = 0
start_timer = time.time()



class tach(QMainWindow):
    def __init__(self, parent=None):
        super(tach, self).__init__(parent)

        self.ui = Ui_MainWindow()
        self.ui.setupUi(self)

        timer = QTimer(self)        
        
        timer.timeout.connect(self.showRPM)
        timer.start(1000)
        
        
        self.showRPM()

    def showRPM(self):
        
        def init_GPIO(): # initialize GPIO
            GPIO.setmode(GPIO.BCM)
            GPIO.setwarnings(False)
            GPIO.setup(gpio_pin_number,GPIO.IN,GPIO.PUD_UP)

        def calculate_elapsed_time(channel): 
            global number_interrupts, start_timer, elapsed_time, signal
            number_interrupts+=1    # increase by 1 whenever an interrupt occurs
            elapsed_time = time.time() - start_timer   # time each rotation takes
            start_timer = time.time()   # Set start_time to current time

        def calculate_speed(wheel_diameter):
            global number_interrupts, elapsed_time, rpm, mph
            if elapsed_time !=0:   # avoid DivisionByZero error
                rpm = (1/elapsed_time * 60) * adjustment
            wheel_circumf_in = math.pi * wheel_diameter_in   # wheel circumference in inches
            mph = (rpm * wheel_circumf_in) / 1056
       
        def init_interrupt():
            GPIO.add_event_detect(gpio_pin_number, GPIO.FALLING, callback = calculate_elapsed_time, bouncetime = 20)

            
            
           
        
        self.ui.RPMlcdNumber.display(rpm)
    
    
    
if __name__ == '__main__':
    
   
    import sys
    
    app = QApplication(sys.argv)
    
    tach = tach()
    tach.show()
    sys.exit(app.exec_())

[JonB]

@rooster37
Hello and welcome.

Firstly please take the trouble to enclose blocks of code like yours with a line of ``` before and after it, which is what the Code (</>) toolbar button above where you type in does. This is especially important in the case of Python code, where leading whitespace is vital.

But the Main Window displays a zero that does not change.

timer = QTimer(self)

You assign the QTimer to a local variable. That gets destroyed immediately after self.showRPM(), and hence there is no longer any timer running. Making it e.g. self.timer = QTimer(self) should fix.

[rooster37]

@JonB I changed timer = QTimer(self) to self.timer = QTimer(self) in def __init__(self, parent=None): and It still does not display the RPM. It displays a steady 0. I am a newbie when it come to python and QT.

[jsulm]

@rooster37 I don't understand your "def showRPM(self)" - does it really contain other functions like "def init_GPIO()"? If so you do not call them! Instead you always set "rpm" which never changes (because "def calculate_speed(wheel_diameter)" is never called)...

[JonB]

@rooster37
I you put a print("Called showRPM") as the first statement in the body of def showRPM(self) (i.e. by the looks of it just above self.ui.RPMlcdNumber.display(rpm)) you will know how many times it is called, and therefore whether your QTimer is working.

For the rest of it please answer @jsulm's questions.

[rooster37]

QTimer is working. The problem is that I didn't call the function.