Monday, July 15, 2013

Controlling two DC motors with Raspberry Pi and the L293D dual H-bridge motor driver

In order to move our Raspberry Pi powered rover, we will need at least two DC motors to power a left and right set of wheels. The motors will be used to move the rover forward and reverse, as well as rotate left and right.

To accomplish this, we figured out how to modify the DC motor tutorial on  to go from controlling one DC motor to two independent DC motors. Controlling two DC motors was accomplished with one L293D dual H-bridge motor driver chip, additional wiring configuration and code modifications.

Let's first look at the L293D dual H-bridge motor driver and GPIO wiring:

In the above figure, all connections in blue and purple are used identically in the original tutorial which demonstrates how to control one DC motor.

Used to control direction of motor 1:
  • GPIO 4
  • GPIO 17
Pulse with modulation control (PWM):
  • GPIO 18
DC motor connections:
  • M1+
  • M1-
Power source connections:
  • Battery+
  • 5v Pi
  • GND
The annotations in green are what was added in order to get the second DC motor to work with the shared L293D dual H-bridge motor driver.

Used to control direction of motor 1:
  • GPIO 23
  • GPIO 24
Pulse with modulation control (PWM): 
  • GPIO 18

DC motor connections:
  • M2+
  • M2-
Ground (not sure if necessary):
  • GND

At this time, we are using the PWM kernel module included in Occidentalis v0.2. This module is used to controll the rotational speed of the motors, which should both rotate at the same speed. Having both motors sharing GPIO 18 seems to work fine. The drawback of using the PWM module is that there is only one, and we cannot use it for both DC motors and servos. If we find that the default maximum speed of the two motors is fine, we may later free up GPIO 18 in order to add a servo.

Our large breadboard was getting messy from other projects, and this one requires a good amount of wiring, so I cleaned out everything from previous projects before starting this one.

I place the L293D dual H-bridge motor driver on a separate half-size bread board in order to keep all motor related wiring isolated on one board and left the GPIO breakout on the large breadboard (since we will be connecting more stuff to it later on).

Lets now look at the code. In order to have efficient code for the motors, I created a Motor class, so we can instantiate two motors with one class definition.

Motor class:

  1. class Motor(object):
  2.         def __init__(self, in1_pin, in2_pin):
  3.                 self.in1_pin = in1_pin
  4.                 self.in2_pin = in2_pin
  6.                 GPIO.setup(self.in1_pin, GPIO.OUT)
  7.                 GPIO.setup(self.in2_pin, GPIO.OUT)
  9.         def clockwise(self):
  10.                 GPIO.output(self.in1_pin, True)    
  11.                 GPIO.output(self.in2_pin, False)
  12.         def counter_clockwise(self):
  13.                 GPIO.output(self.in1_pin, False)
  14.                 GPIO.output(self.in2_pin, True)
  16.         def stop(self):
  17.                 GPIO.output(self.in1_pin, False)    
  18.                 GPIO.output(self.in2_pin, False)

In the Adafruit tutorial, there is a helper function called set(), which I left as is outside of the class. I may incorporate it into a class later on, but I have not yet decided where it would best fit.

set() function:

  1. def set(property, value):
  2.     try:
  3.         f = open("/sys/class/rpi-pwm/pwm0/" + property, 'w')
  4.         f.write(value)
  5.         f.close()      
  6.     except:
  7.         print("Error writing to: " + property + " value: " + value)

Below is the main code that will pull it all together:

  1. import RPi.GPIO as GPIO
  2. GPIO.setmode(GPIO.BCM)
  3. left_in1_pin = 4
  4. left_in2_pin = 17
  5. right_in1_pin = 23
  6. right_in2_pin = 24
  7. class Motor(object):
  8.         def __init__(self, in1_pin, in2_pin):
  9.                 self.in1_pin = in1_pin
  10.                 self.in2_pin = in2_pin
  12.                 GPIO.setup(self.in1_pin, GPIO.OUT)
  13.                 GPIO.setup(self.in2_pin, GPIO.OUT)
  15.         def clockwise(self):
  16.                 GPIO.output(self.in1_pin, True)    
  17.                 GPIO.output(self.in2_pin, False)
  18.         def counter_clockwise(self):
  19.                 GPIO.output(self.in1_pin, False)
  20.                 GPIO.output(self.in2_pin, True)
  22.         def stop(self):
  23.                 GPIO.output(self.in1_pin, False)    
  24.                 GPIO.output(self.in2_pin, False)
  26. def set(property, value):
  27.     try:
  28.         f = open("/sys/class/rpi-pwm/pwm0/" + property, 'w')
  29.         f.write(value)
  30.         f.close()      
  31.     except:
  32.         print("Error writing to: " + property + " value: " + value)
  34. try:
  35.         set("delayed", "0")
  36.         set("frequency", "500")
  37.         set("active", "1")
  38.         left_motor = Motor(left_in1_pin, left_in2_pin)
  39.         right_motor = Motor(right_in1_pin, right_in2_pin)
  41.         direction = None
  43.         while True:    
  44.                 cmd = raw_input("Command, f/r/o/p/s 0..9, E.g. f5 :")
  46.                 # if enter was pressed with no value, just stick with the current value
  47.                 if len(cmd) > 0:
  48.                         direction = cmd[0]
  49.                 if direction == "f":
  50.                         left_motor.clockwise()
  51.                         right_motor.clockwise()
  52.                 elif direction == "r":
  53.                         left_motor.counter_clockwise()
  54.                         right_motor.counter_clockwise()
  55.                 elif direction == "o"# opposite1
  56.                         left_motor.counter_clockwise()
  57.                         right_motor.clockwise()
  58.                 elif direction == "p":
  59.                         left_motor.clockwise()
  60.                         right_motor.counter_clockwise()        
  61.                 else:
  62.                         left_motor.stop()
  63.                         right_motor.stop()
  65.                 # only need to adjust speed if we want to      
  66.                 if len(cmd) > 1:
  67.                         speed = int(cmd[1]) * 11
  68.                         set("duty", str(speed))
  70. except KeyboardInterrupt:
  71.         left_motor.stop()
  72.         right_motor.stop()
  73.         print "\nstopped"

download the code here

to run:
$ sudo python 

Command, f/r/o/p/s 0..9, E.g. f5 :f

f == forward
r == reverse
o == opposite directions
p == opposite directions
s == stop

CTRL-C gracefully stops the motors.

Here it is in action!

Coming soon. Web GUI driven DC motors...

Tuesday, July 9, 2013

Sidetracked: Using the light sensor on the Raspberry Pi - Talking Alarm Clock

One thing that came with my RasPi kit was the light sensor. Getting it to work was easy enough using this great tutorial on adafruit

After playing with the sensor with the example script, and figuring out the rc range of values that indicate that the light is on or off in my home office, I added some logic to print "THE LIGHT IS ON" and "THE LIGHT IS OFF".

OK that's good to know, but what else can I do with that information? Hmmmm. I know!

I decided to make a simple "alarm clock" in python which takes a time as an input on the command line, poll the current time every second, and trigger an alarm once the set time is reached. An alarm requires sound, and since we are building a robot, I thought I would play around with some text-to-speech software for the Raspberry Pi and use that as the voice to wake someone up (and we will use it for other things robot-related down the road)

After playing with a couple text to speech apps, I found that the best one for the Pi is espeak. One of the many things I like about espeak is its tun-ability (pitch/dialect/speed/etc..). One problem I found with running espeak on the Pi is that the words would slow down towards the end of long sentences (like TRON getting derezzed in the grid). I'm not sure if that is due to a CPU/memory resource issue with the Pi and sound driver, but I found a workaround for the slowdown by using an option in espeak which allows output to a wav file. The wav file sounds fine and has no lag, so my talking alarm clock will generate a wav file on the fly (with a random name) and play it with little or no noticeable delay.

I used the built in aplay  for playing the wav files. Aplay is part of the alsa sound driver tools on the Pi.

Where does the light sensor come into all of this? Oh yeah. The alarm clock won't shut up until you get up and turn on the light. Once the light sensor detects that the light is on, the alarm clock will deactivate. This will be great for getting my son out of bed when the school year starts back up in the fall!

Since I am delving into object oriented development, I made the following classes:

  • LightSensor
  • Voice
  • AlarmClock
The AlarmClock has-a voice and has-a light sensor and does all of the time keeping and triggering of the alarm.

An AlarmClock object can be enabled by calling the function AlarmClock.enable_alarm_<st|int>()

Usage: <executable> HH:MM
  • HH:     00-23"
  • MM:    00-59"
Here is an example of the LightSensor class:

class LightSensor(object):

        def __init__(self):
                self.rc_pin = LIGHT_SENSOR

        def rc_time(self):
                reading = 0
                GPIO.setup(self.rc_pin, GPIO.OUT)
                GPIO.output(self.rc_pin, GPIO.LOW)
                GPIO.setup(self.rc_pin, GPIO.IN)
                # This takes about 1 millisecond per loop cycle
                while (GPIO.input(self.rc_pin) == GPIO.LOW):
                        reading += 1

                return reading

        def light_is_on(self):
                reading = self.rc_time()
                if reading > 400: #value when light is off
                        return False
                        return True

light_is_on() returns True if the light is on, False if it is off.

My Voice class has two main methods that can be called by a program:
  • say(<string>) - say the string that is passed to it
  • say_cl(<command line string>) - say the output of the cl command passed to it. In my case, I used the classic UNIX program "fortune"
Code for everything would be messy to paste here, so you can get all of my alarm clock code on github:

The README file should tell you all you need to know to get it up and running!

Monday, July 1, 2013

Links for later

After posting the last article on reddit, a couple fine folks over there recommended using some other frameworks. I will definitely be taking a look at webiopi - link.

Just looking at some of the examples online, webiopi looks a bit easier than how I was approaching things.

I also found an example on how to use webiopi for doing almost exactly what I want to do!

At the same time, it's good to try and solve a problem with the tools and knowledge you have before doing things the easy way.

Sunday, June 30, 2013

Taking LEDs to the next level: web controlled GPIO ports

***UPDATE: github repository of code:

In my last post I covered the basics on how to get an LED to turn on with a very simple Python script.  From there I created an application that turns on an LED if an IP address is detected on the network.

What I really want to do is be able to create a web based interface that can be used to control the GPIO outputs over the network. Being that my knowledge of Python is limited, I decided to take a break from the pi and hone some Python skills via the excellent (and FREE!) course over at And now I am back!

The Author of Learn Python the Hard Way assumes you are a complete n00b, and walks you all the way up to the grand finally project of creating a web based Python application. Perfect! Just what I needed!

I started at chapter one and went all the way to the end. After learning just enough to be dangerous, I put together a little web based app that displays a button for the green LED, a button for blue LED and a button for the red LED. Once a user clicks one of the three buttons, the corresponding LED lights up on the bread board and its status along with the other two lights after each click. A user can open the application on their mobile device (iPhone in my case) and control the LEDs via the web browser.

Here is an example of how it works:

Using by mobile phone, point to running app's url (all lights off here):

Click the green button and the green LED is turned on, and the web application is updated to show that it is on:

Click the red button and the red LED is turned on, and the web application updates to show that it and the green LED are now on, and so on and so forth...

The code for making LEDs turn on and off is simple enough, but getting the web framework working was the main hurdle for me, along with getting the code to generate dynamic web content on the fly.

The framework I used was This  framework was chosen due to the fact that it was used in the tutorials mentioned above, and plus, if it was good enough for Aaron Swartz, it's good enough for me. As I get more experience I may play with other frameworks such as django, but seems tiny and simple enough for the pi.

Following the setup process here for installing the needed packages and setting up the directory structure, then working my way up to the chapter on getting user data from a web form (yes, I read every chapter and did every exercise in each one). I was able to get this thing going.

After many trials and errors, I was able to create a web application called ledctrl which does just what I want it to do.

The main code for my apps is located in the ledctrl/bin directory. The file is called, which does all of the work. I decided to start using more of an object oriented approach in my Python coding, since that is what I'm used to doing in other languages. After the code snipped, I'll explain what I was thinking when I wrote it:

First of all, let's look at the class I created called LightSwitch

  • LightSwitch is a class I defined which initializes the GPIO and the current LED color being passed to it in the __init__() function, since these don't need to be global operations.

  •  flip_switch() is a method in LightSwitch which flips the current LED opposite of what it is already set to. This is done by checking the current status:
    • led_status = GPIO.input(self.LED)
      • if light-status is GPIO.LOW, then the LED is off already, so flip it to on, otherwise flip it off. Simple.
  • get_light_statuses() is a method in LightSwitch which returns the current statuses of all three LEDs as a dictionary data structure. The values in this dictionary are needed for generating the correct buttons in the dynamically generated HTML.

All of these class methods are executed from the Index class, which instantiates a LightSwitch object.

Any time the Index class renders the HTML form in either GET or POST, it passes the dictionary of LED statuses to the HTML template.

I also put in a global DEBUG flag, to print stuff out while I'm testing my code. Very helpful, and you can leave all your print statements in the code and set the DEBUG flag to "False" when you are ready to "go-live" ;)

The HTML form which passes information back and forth between my main python code looks like this (sorry for the formatting due to the long lines of HTML code):

The HTML form gets the dictionary of light_statuses. If the dictionary is valid, it looks at each LED color in the dictionary, and if the status for the LED is off (0) or on (1) display the corresponding HTML code for that color. Maybe there is a better way to generate this code (I'm sure there is), but this is what I got to work!

And here it is in action:

Monday, June 17, 2013

More fun with the pi!

Getting the hang of this!

Input buttons
Light sensors 
Serial to USB

All at the same time!

Tuesday, June 11, 2013

Getting LEDs to light up with Raspberry Pi's GPIO with Python

In order to get into some hardware related projects with the RASPI, I figured it would be a good idea to figure out how to do some very basic stuff with the GPIO.

The first project I think anyone does is make the LEDs light up. There are plenty of tutorials out there with bits of information scattered about, but I'll try to use my experiences to supply all the information (and references to information) here.

First of all, it is a good idea to know what each pin does on the Raspberry Pi GPIO and get your Pi all ready and set up to work with Python.

Here is a great tutorial via adafruit that gives you a quick and dirty overview the GPIO, steps to take and libraries to install in order to program it via Python.

Once you have that all set up, in order to make an LED light up programmatically, you need to have it wired properly between a resistor that is connected to one of the numbered GPIO pins (4, 8, 7, 17, 22, 23, 24 or 25),  and the ground pin (GND).

I wired my LEDs using the adafruit Pi Cobbler and a breadboard, but you could attach wires directly to the GPIO if so inclined.

Here is the example I followed for connecting LEDs to the Pi's GPIO.

And here is mine wired up to three LEDs:

In this example I have my LEDs wired up to GPIO ports:
  • GREEN_LED = 24
  • RED_LED = 23
  • BLUE_LED = 18
The resistors are the ones that came with my kit and are rated at 560 ohm 5%, and resistors rated between 330 ohm and 1000 ohm are fine.

Here is a handy resistor decoder I use now and then.

In the above picture, the black wire is contacted between the GND pin and the negative blue column on the bread board.

Here is a diagram (note the totally fake cobbler)

For the red LED, the resistor is connected to GPIO pin 23 and an unused row on the breadboard (row 18..chosen arbitrarily). The positive end of the LED (long wire) is also in row 18 and the negative end (short wire) is connected the the blue negative column where the ground wire is connected.

I did the same for the other two LEDs but with their respective GPIO ports. Green was further away from the GPIO pin, so I used the yellow and green jumper wires to make a similar circuit.

Once everything was connected, I wrote a simple Python program to cycle through the LEDs. Somehow it worked!

Here is the code, and then some 'splainin':

#!/usr/bin/env python

import RPi.GPIO as GPIO, time, os


RED_LED = 23



    while True:
        print "Red Light"
        GPIO.output(GREEN_LED, False)
        GPIO.output(RED_LED, True)
        GPIO.output(BLUE_LED, False)
        print "Blue Light"
        GPIO.output(RED_LED, False)
        GPIO.output(BLUE_LED, True)
        GPIO.output(GREEN_LED, False)
        print "Green Light"
        GPIO.output(BLUE_LED, False)
        GPIO.output(GREEN_LED, True)
        GPIO.output(RED_LED, False)
except KeyboardInterrupt:

Break it on down:


24 is the GPIO pin. I don't like to hard code values and plus it makes code more readable/maintainable when you do things this way.


Initialize the GPIO pin used for the green LED for output

GPIO.output(GREEN_LED, True)

Turn the LED on

GPIO.output(GREEN_LED, False)

Turn the LED off


Uninitialize all the GPIO pins

And there you have it!

Here is a little Python code that pings an IP address. Once an IP address pings back, an LED is turned on. In this example, when one of three iPhones is detected on my home network, an associated LED will light up. The light will turn back off if the IP address becomes unreachable.

#!/usr/bin/env python

# This program will detect specific IP address on the network and 
# Enable an LED if it is detected 

import RPi.GPIO as GPIO, time, os
from subprocess import call


host1 = "" # my iPhone
host2 = "" # wife's iphone
host3 = "" # an iPad

RED_LED = 23


def detect_IP(ip_add, color_led):
 ping_command = "ping -c 1 %s > /dev/null 2>&1" % ip_add
 if call(ping_command, shell=True) > 0:
  GPIO.output(color_led, False)
  GPIO.output(color_led, True)

    while True:
        detect_IP(host1, GREEN_LED)
        detect_IP(host2, RED_LED)
        detect_IP(host3, BLUE_LED)
except KeyboardInterrupt:

Your Pi is ready

My Raspi finally arrived yesterday (jeesh a little over a week to arrive from adafruit). My son and I instantly unpacked it.