python + Arduino + Tk: Toggling an LED

Whoa! This is so old I don’t even know where to start!

• It’s using Python 2, so if it works at all it probably won’t for much longer, and Tkinter is something completely different under Python 3
  (grrreat planning there, Python guys …)
• pyfirmata is likely ancient history too.

Phil sent me a note last week asking how to turn an LED on or off using Python talking through Firmata to an Arduino. This was harder than it looked.

It turns out the hard part is getting the value from the Tkinter Checkbutton itself. It seems that some widgets don’t return values directly, so you must read the widget’s value with a get() method. This appears to work:

#!/usr/bin/python
# turn an LED on/off with a Tk Checkbutton - scruss 2012/11/13
# Connection:
# - small LED connected from D3, through a resistor, to GND

import pyfirmata
from Tkinter import *

# Create a new board, specifying serial port
# board = pyfirmata.Arduino('/dev/ttyACM0') # Raspberry Pi
board = pyfirmata.Arduino('/dev/tty.usbmodem411') # Mac

root = Tk()
var = BooleanVar()

# set up pins
pin3 = board.get_pin('d:3:o') # D3 On/Off Output (LED)

def set_led():  # set LED on/off
    ledval = var.get()
    print "Toggled", ledval
    pin3.write(ledval)

# now set up GUI
b = Checkbutton(root, text = "LED", command = set_led,
                variable = var)
b.pack(anchor = CENTER)

root.mainloop()

This is explained quite well here: Tkinter Checkbutton doesn’t change my variable – Stack Overflow. I also learnt a couple of things about my previous programs:

You don’t really need to set up an Iterator unless you’re reading analogue inputs
My “clever” cleanup-on-exit code actually made the script hang on Mac OS.