Python - How to make a voltmeter - the software

Voltmeter

The Python program running on the computer or Raspberry Pie follows.

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#!/bin/python3

# Lets import the requred libraries
import serial               # Needed to read from the serial port
from tkinter import *       # sudo apt-get install python3-tk, needed to create the GUI
from tkinter import messagebox # Needed to display a message bos with an error message

# Constants
SERIAL_PORT = '/dev/ttyUSB0' # This is the USB port we're using
SERIAL_RATE = 4800  # This is the baudrate from the PIC

# Variables
global R1 # Resistor; GND to PIC input
global R2 # Resistor; Input power to PIC input
global vout # Voltage fromthe voltage divider and into the PIC
global vin  # Voltage calculated from the Vout, Voltage in to the voltage divider

# Make the main window
window = Tk()                   # This creates a window
window.title("DVM")             # Set the title of the window
window.geometry('270x250')      # Set the size of the window

# Functions
# This function ends the program
def quit():
    window.quit() 

# This function read voltage and do the calculations. If the USB port is not found,
# an error message is displayed
def read_volt():
    R1 = 100600 # Measured with multimeter to get a bit more accurate. Std value = 100000
    R2 = 9950 # Measured with multimeter to get a bit more accurate. Std value = 10000

    try:
        ser = serial.Serial(SERIAL_PORT, SERIAL_RATE)   # Assign
        reading = ser.readline()                        # Read the AD value from the PIC

        vout = (int(reading)* 5.0) / 1024.0             # Calculate AD back to voltage
        vin = vout / (R2/(R1+R2))                       # Calculate original voltage
        new_volt = ('{0:0.2f}'.format(vin))             # Format the new_voltage to two decimals

        lbl_volt.config(text=new_volt)                  # Setting the text on the label
        lbl_volt.after(600, read_volt)                  #

    except:
        print ('Not found')
        messagebox.showerror('Not found','''\
        Cannot find USB to TTL
        Hit OK to exit.''')
        sys.exit()

# Make and place labels
lbl_header = Label(window, text="Digital voltmeter", font=("Helvetica", 26))
lbl_header.place(x=130, y=30, anchor="center")

lbl_volt = Label(window, text="0.00", font=("DSEG7 Classic", 48))
lbl_volt.place(x=120, y=120, anchor="center")

# Make and place buttons
btn_quit = Button(window, text="Quit", command=quit)
btn_quit.place(relx=0.5, y=200, anchor="center")

#This is needed to "autorun" this function, remove () and it will no "autorun"
btn_start = Button(window, text="Start", command=read_volt()) 
#btn_start.place(relx=0.5, y=200, anchor=W)

# Run the mainloop
window.mainloop()


# ASCIL drawing of the voltage divider

#   | <- input voltage
#   |
#   |
#   --
#   ||  <- R2
#   -- 
#   |
#   +-  <- To PIC
#   |
#   --
#   || <- R1
#   --
#   |
#   |
#   | <- to GND

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