pyFirmata's write() function

Question

I'm working along the book "Python Programming for Arduino" written by Pratik Desai (annoyingly smart guy).

I got stuck in exercise where student is learning to implement a slider that changes intensity of LED that is connected to a pin. I marked where the code isn't working as it should.

Code is:

import tkinter
from pyfirmata import ArduinoMega
from time import sleep

port = '/dev/ttyACM0'
board = ArduinoMega(port)
sleep(5)
lenPin = board.get_pin('d:11:o')

top = tkinter.Tk()
top.title('Specify time using Entry')
top.minsize(300, 30)
timePeriodEntry = tkinter.Entry(top, bd=5, width=25)
brightnessScale = tkinter.Scale(top, from_=0, to=100, 
orient=tkinter.HORIZONTAL)
brightnessScale.grid(column=2, row=2)
tkinter.Label(top, text='Time (seconds)').grid(column=1, row=1)
tkinter.Label(top, text='Brightness (%)').grid(column=1, row=2)

def onStartPress():
    time_period = timePeriodEntry.get()
    time_period = float(time_period)
    ledBrightness = brightnessScale.get()
    ledBrightness = float(ledBrightness)
    startButton.config(state=tkinter.DISABLED)
    lenPin.write(ledBrightness / 100.0) # this part of code ain't working
    sleep(time_period)
    lenPin.write(0)
    startButton.config(state=tkinter.ACTIVE)


timePeriodEntry.grid(column=2, row=1)
timePeriodEntry.focus_set()
startButton = tkinter.Button(top, text='Lit Up', command=onStartPress)
startButton.grid(column=1, row=3)
exitButton = tkinter.Button(top, text='Exit', command=top.quit)
exitButton.grid(column=2, row=3)

top.mainloop()

According to the book this code should work. I did some basic checks such as printing out the variable ledBrightness to see if it gets the correct value and it is getting the correct value. Problem is when I run the program it isn't working. The LED won't come on at all. It works only when I replace the variable with 1 (True) which turns the LED on or 0 (False) which turns it back off, however without any option to adjust intensity.

What I'm doing wrong here? If the write() function can accept only 1 or 0 how come that this book says you can customize the input?

Answer 1

From the documentation:

write(value)

Output a voltage from the pin

Parameters: value – Uses value as a boolean if the pin is in output mode, or expects a float from 0 to 1 if the pin is in PWM mode. If the pin is in SERVO the value should be in degrees.

get_pin(pin_def)

Returns the activated pin given by the pin definition. May raise an InvalidPinDefError or a PinAlreadyTakenError.

Parameters: pin_def – Pin definition as described below, but without the arduino name. So for example a:1:i.

‘a’ analog pin Pin number ‘i’ for input ‘d’ digital pin Pin number ‘o’ for output ‘p’ for pwm (Pulse-width modulation)

All seperated by :.

You need to define the pin as PWM not output.

lenPin = board.get_pin('d:11:p')

Then lenPin.write(value) doesn't just accept 0 and 1, but any float number between 0 and 1.