Archive for the 'Physical Computing' category

Simple Arduino Motor Control

May 29 2013 Published by under Physical Computing

WP_20130529_001

One thing you might want to do with an Arduino or even a Raspberry Pi is control a motor. Of course, the GPIO pins on this board are really just designed to send and receive informational signals, not drive the load of something that’s going to draw any significant current like a motor. An LED is fine, but much beyond that and you really need to look at a different solution.

I simply wanted to control a motor from my Arduino. I have a little hobby motor. I measured that at 3 volts it draws 150 mA (milliAmps). The Arduino’s GPIO pins are rated at around 40 mA. So that wasn’t going to cut it. I know there are various motor controller shields that you can hook up to your Arduino for this purpose, but I didn’t think I needed to go that route. Those boards offer a lot of functionality and are great if you have a specific project in mind and need all that control without going through the hassle of figuring out how to do it. But they are also the very definition of a “black box”. You put some input into them and get some input out of them. There’s all kinds of components and chips and traces on the boards, so you don’t really have any idea what’s going on between the input and output.

150 mA - too high for Arduino's GPIO

150 mA – too high for Arduino’s GPIO

For me, it was far more important to know exactly what was going on at this stage. So I grabbed a breadboard, some components and wire, and got busy.

All you really need is a transistor, a resistor and a separate power supply for the motor. I’m not going to go into a deep explanation of transistors, because there are about 73 million deep discussions about them on the net. Basically, it’s an electronic switch with three leads called the emitter, base, and collector. You put the emitter/collector combo between the external power and your load, in this case a motor. By default, it acts as an open switch, so the motor does not run. But when you supply a small current between the base and emitter, this “closes the switch”, allowing the current to flow from collector to emitter, and your motor runs!

The important thing is that it only takes a very small control signal at the base to allow a much larger current to flow through the emitter/collector. So you can use the GPIO output signals to control the transistor, and use a separate power supply to control the motor itself without taxing your delicate little Arduino.

WP_20130529_002

The transistor I used is a PN2222 series. These are very common, general use transistors. You can get 10 of them for under $2. I checked the data sheet for this transistor and found it can handle a maximum of around 50-60 volts on the emitter/collector. (There are a few different varieties of these, so I’m just reporting averages.) So my 3 volt motor should be fine. It can also handle a peak load of 1 amp – again fine for my 150 mA motor. Motors do draw a lot more current when the first start, but this would allow me more than 6x the running current as a peak. The base/emitter voltage is around 5-6 volts max. The Arduino outputs are 5 volts, which is cutting it close, so I threw in a 1000 ohm resistor. This results in just a few milliamps being drawn from the GPIO pin, through the base of the transistor and back out the emitter.

motorcontrol

motorcontrol_bb

Then I hooked up the motor to a separate 3 volt power supply, passing it through the emitter/collector. With this setup, when the GPIO pin I choose is HIGH, the motor will run. When it’s LOW, the motor will stop. Despite the motor drawing at least 150 mA, the Arduino’s GPIO pins will only be serving up 2-3 mA about 4 mA by actual measurement – way below the 40 mA limit.

For software, I set up a simple sketch as follows:

[php lang=”c”]void setup() {
pinMode(7, OUTPUT);
}

void loop() {
digitalWrite(7, HIGH);
delay(1000);
digitalWrite(7, LOW);
delay(1000);
}[/php]

Obviously, I’m using GPIO pin 7 as an output. This will set it HIGH for one second, then low for one second, switching the motor on and off for that amount of time. And it works! And as far as I can tell, everything is within safe ranges.

So there you go. Of course, this isn’t going to give you all the features of a dedicated motor control shield, such as variable speed with pulse with modulation, multiple motors, forward/reverse, etc. But now you know exactly what’s going on and can build on it if you want. Or at least have some concept on what’s going on inside that fancy shield.

Of course, it goes without saying to be VERY careful when wiring up a circuit like this. You are hooking up an external power source to the same board that your Arduino is hooked up to. A wrong move and you could send electrons flooding into where they have no business being. That’s another advantage to using shields, which buffer the Arduino’s connections and add a layer of protection. But don’t worry too much. A little smoke won’t kill you. 🙂

WP_20130529_004

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Pull-up and Pull-down Resistors

May 22 2013 Published by under Electronics, Physical Computing

Working a lot with Raspberry Pi and Arduino stuff lately. The concept of pull-up and pull-down resistors came up quickly and confused me a little at first. So I thought I’d do a little demo of how they work and why they are needed. Doing this helped to clarify it for me, so maybe it’ll help you too.

Common scenario. You want to set up something that reads an input of a GPIO pin. Say, you want to know when the user is pressing a switch. You set up a simple circuit like so:

circuit_1

And here that is wired up:

WP_20130523_002

When you press the button, pin 25 goes high. No press, no high, must be low, right? Well, let’s see…

We’ll set up a simple Python script to read the status of pin 25:

[php lang=”Python”]#! /usr/bin/python

import RPi.GPIO as GPIO
import time

PIN = 25

GPIO.setmode(GPIO.BCM)
GPIO.setup(PIN, GPIO.IN)

while True:
if GPIO.input(PIN) == GPIO.HIGH:
print(“pin is high”)
time.sleep(.1)
else:
print(“pin is low”)
time.sleep(.1)[/php]

When I run this, I get 20-30 lines saying pin 25 is high, then 20-30 saying it’s low, back and forth, without touching the button. Sure enough when I press the button, it stays high, but apparently the definition of “low” is not “not high”.

After too many years in the highly binary world of software programming, some of us delving into hardware may be surprised to learn that a lot of electronics operate on “tri-state logic” as opposed to simple binary. Here, an input can be high, low, or floating. In this particular circuit, high means connected to 3.3 volts, low means connected to ground, and floating means… neither or somewhere in between. Actually, due to the sensitive nature of tiny electronics, system components can pick up various signals that create slight fluctuations in what it is reading. It’s rarely if ever going to pick up exactly 0.000 volts just because it’s not connected to 3.3 volts.

So we need to force the issue and say, “Yo! You are LOW!” My first thought on how to do this would probably have been to do something like this:

circuit_2

When the switch is in one position, pin 25 is connected directly to ground, sending it solidly low. When the switch is changed, it connects 25 to 3.3 volts, making it high. This will work just fine, but it’s a bit overengineered as it turns out.

How about if we go simpler and just connect pin 25 to ground and leave it that way, like so:

(Note: DO NOT ACTUALLY BUILD THIS CIRCUIT!)

circuit_3

Now, when the switch is open, pin 25 is undoubtedly low. But we have a problem that when you do hit the switch, you now have a short circuit directly between 3.3 volts and ground. Not good. Again, do not do this. The solution? Throw a resistor in there:

circuit_4

And in the real world:

WP_20130523_003

Now, when the switch is open, pin 25 is connected to ground through the 10K resistor. Even with the resistor though, it’s enough of a connection to make it solidly low. It’s not going to fluctuate.

Then we hit the switch, connecting the lower part of the circuit to the 3.3 volts. Because we have a fairly high resistance there, most of the current is going to go to pin 25, sending it solidly high. Some of it is also going to go to ground via R1. But Ohm’s Law tells us that 3.3 volts divided by 10,000 ohms will let about 0.33 milliamps in. That’s not going to break the bank.

So in this circuit, R1 is called a pull-down resistor because in its default state it pulls the signal down to 0 volts or a low state.

Now let’s swap things around a bit:

circuit_5

Now pin 25 is hard-wired to 3.3 volts through R1, making its default state high. However when the button is pressed, pin 25 will be directly connected to ground, sending it low. Because the resistor pulls pin 25 up to 3.3 volts and a high state, it’s now known as a pull-up resistor.

So, which one do you use? It depends on whether you want your default signal to be high or low.

Hope that helps.

13 responses so far

Raspberry Pi Mail Checker

May 21 2013 Published by under Physical Computing, Uncategorized

WP_20130521_001

I was looking for some Raspberry Pi project to do that not only used external hardware (even if that means only an LED for now), but also reached out into the net to deal with some kind of real time data. I ran into this tutorial on adafruit about checking your gmail with a Pi and lighting up an LED based on whether or not you have new mail. This was along the lines of what I wanted to do, but had two drawbacks. One, I don’t use gmail anymore, and two, it uses a python library called feedparser, which is actually an RSS parser. It only works because apparently you can access your gmail as an RSS feed or something. I wanted to do the same thing, but with any email service that supports, say, IMAP4.

A bit of digging around and I found that Python has an imaplib library that can directly access any IMAP4 based 12mail account. This is included by default in the Python distro that comes with Raspbian. So no further setup there. It took a bit of fiddling around with the docs, both of the library itself and the IMAP4 specs, but I figured out the basic sequence.

The library docs are here: http://pymotw.com/2/imaplib/

And the IMAP4 spec is here: http://tools.ietf.org/html/rfc3501.html

First, you create an IMAP object with one of the following lines, depending whether you need SSL or not. My server does need it.

[php lang=”python”]imap = imaplib.IMAP4(server, port)[/php]

or

[php lang=”python”]imap = imaplib.IMAP4_SSL(server, port)[/php]

Then, you log in:

[php lang=”python”]imap.login(user, password)[/php]

You then need to go into select mode to select IMAP folders and messages.

[php lang=”python”]imap.select()[/php]

Then you can do a search. By default, you’ll be searching in your inbox, but there are methods to change folders as well. Search criteria are pretty powerful, but I just wanted to see how many unread messages I have. This does that:

[php lang=”python”]type, data = imap.search(None, “UNSEEN”)[/php]

The first parameter is the encoding. Using None will return messages with any encoding. Second param is the search criteria. See the IMAP4 spec linked above for a comprehensive list on what you can search for.

This will return a type, data tuple of strings. The type will be “OK” or “NO” depending on success of the call. Note, even if it returns no unread messages, you’ll still get “OK” here, with an empty string in the data.

The data will be an list of strings. Actually, it seems to generally be an list containing a single string. This string is a space delimited list of numbers. These numbers are ids of the messages returned by the search. It’ll be something like this: “1 2 3 4 5″.

You can split this into a list of individual ids like so:

[php lang=”python”]messages = data[0].split()[/php]

And the length of this list tells you how many messages the search returned. Zero means no new mail. One or more and you have new mail! Fire up an LED!

Here’s the full program I wrote:

[php lang=”python”]#! /usr/bin/python

import imaplib, time
import RPi.GPIO as GPIO

GREEN_PIN = 25
RED_PIN = 24

class MailChecker:
def __init__(self, server, port):
GPIO.setmode(GPIO.BCM)
GPIO.setup(GREEN_PIN, GPIO.OUT)
GPIO.setup(RED_PIN, GPIO.OUT)
GPIO.setwarnings(False)

try:
self.m = imaplib.IMAP4_SSL(server, port)
except:
self.do_error(“Unable to contact server”)

def do_error(self, error):
# maybe flash another error LED
print(error)
exit()

def log_in(self, user, password):
try:
self.m.login(user, password)
except:
self.do_error(“Unable to log in”)

def check_mail(self):
type, data = 0, 0
try:
self.m.select()
type, data = self.m.search(None, “UNSEEN”)
except:
self.do_error(“Unable to check messages”)

if type == “NO”:
self.do_error(“Problem checking messages”)

self.report(data)

def start_checking(self, interval):
while True:
self.check_mail()
time.sleep(interval)

def report(self, data):
message_count = len(data[0].split())
if message_count > 0:
print(“You’ve got %i new messages” %
message_count)
GPIO.output(RED_PIN, GPIO.LOW)
for i in range(1, 100):
GPIO.output(GREEN_PIN, GPIO.LOW)
time.sleep(.2)
GPIO.output(GREEN_PIN, GPIO.HIGH)
time.sleep(.2)
else:
print(“No new mail right now”)
GPIO.output(GREEN_PIN, GPIO.LOW)
GPIO.output(RED_PIN, GPIO.HIGH)

if __name__ == “__main__”:
user = “your_user_name”
password = “your_password”
server = “your_email_server_url”
port = 993
# or whatever port you need to use

checker = MailChecker(server, port)
checker.log_in(user, password)
checker.start_checking(60)[/php]

This creates the IMAP object and logs in. Then the start_checking method sets up an infinite loop that searches once every minute. If it finds there are unread messages, it blinks a green LED a bunch of times then leaves it on. No new mail, red LED. The rest is basic GPIO code.

You can change the behavior however you want, maybe remove all the non-error print lines. Then move it to your Pi, do a chmod +x on it to make it executable and run it as root:

[php lang=”python”]sudo ./checkmail.py &[/php]

The trailing & character will fork the process and allow you to close the terminal session while keeping the program running – especially useful if you’re sshing into your Pi.

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The Ridiculous Raspberry Pi Winamp Remote – Part II

May 14 2013 Published by under Physical Computing, Technology

Recap

As stated in my previous post, the plan was to hit a button that’s connected to my Raspberry Pi, which will trigger Winamp running on my pc laptop to play/pause. This without the Pi being physically connected to the pc – all via wifi and my local home network.

The hardware setup

Raspberry Pi with USB wifi, a small breadboard, 100k ohm resistor, a pushbutton and some hookup wire.

GPIO pin 25 goes to one side of the switch, 3.3 volts to the other. You hit the switch, pin 25 gets juice and goes HIGH. The 100k ohm resistor is also hooked to pin 25 on one side and ground on the other, connecting 25 to ground when the switch is not connected, ensuring it is in a LOW state. For those of you in the know, this is a basic pull down resistor. If you’re not familiar with the concept, as I was not last week, this is a good explanation: http://www.resistorguide.com/pull-up-resistor_pull-down-resistor/

The Python

The Raspberry Pi is running a simple program written in Python:

[php lang=”Python”]import urllib
import RPi.GPIO as GPIO
import time

BTN = 25
GPIO.setmode(GPIO.BCM)
GPIO.setup(BTN, GPIO.IN)

while True:
val = GPIO.input(BTN)
if val == GPIO.HIGH:
urllib.urlopen(“http://192.168.1.14:9000”)
time.sleep(1)[/php]

This just sets up pin 25 as an input and starts an endless loop which reads the value of pin 25. If it’s high, it pings a url and sleeps for a second. The sleep is so you don’t get multiple calls at once. There are more robust ways of handling this, but this was quick and dirty and works as long as I’m the only user and I know not to hold the button down too long.

So what is this url?

The Node

On my laptop where Winamp is playing, I started a simple node server. When a client connects to the server, it calls a function called playPause. This executes an external program called MediaControllerConsole.exe via child_process.

[php lang=”javascript”]function playPause() {
var path = “MediaControllerConsole.exe”;

var exec = require(‘child_process’).exec;
var child = exec(path, function( error, stdout, stderr)
{
if ( error != null ) {
console.log(“err: ” + stderr);
// error handling & exit
}
});
}

var net = require(“net”);
var server = net.createServer();
server.on(“connection”, function(client) {
playPause();
client.end();
});

server.listen(9000);[/php]

So what is MediaControllerConsole.exe?

The C#

I had the connection from the Pi to my laptop. Now I needed Node to control Winamp. There may be some way to do this more directly, but I found that I could write a really simple C# console application that called SendKeys.Send() to trigger key presses on the machine. I looked in Winamp’s options and saw that CTRL-ALT-HOME is a global keyboard shortcut to Winamp’s pause function. It turns out that this actually acts as a play/pause toggle, which is even more perfect for my use case. Here’s the C# app I wrote:

[php lang=”c”]using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace MediaControllerConsole
{
class Program
{
static void Main(string[] args)
{
System.Windows.Forms.SendKeys.SendWait(“^%{HOME}”);
}
}
}[/php]

Since this was a console app and not a Windows Forms app, I had to add system.windows.forms.dll as a reference to the Visual Studio project and call SendWait instead of Send (which has more hooks into the forms stuff). The string “^%{HOME}” means that while holding the CTRL key (^) and the ALT key (%), press the HOME key.

Final recap

Pressing the button connected to the Pi sends pin 25 to HIGH which triggers Python to call across the wifi network to the Node server which starts a child process running the C# program which sends keystrokes into the system which Winamp is listening for, causing it to play or pause. All so I don’t have to reach over and press two keys.

Ridiculous! But it works!

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The Ridiculous Raspberry Pi Winamp Remote – Part I

May 14 2013 Published by under Physical Computing, Technology

Back story

I wanted a media center. I priced out cases, power supplies, hard drives, cheap motherboards, etc. and figured I could build one for a couple hundred bucks or so. Then I heard about people using Raspberry Pi’s as media centers. Did a bit of research and got me a Pi and set up OpenElec on it. Plugged in an old 500 GB USB drive. I had a media center for about a quarter of what I was going to pay.

Thing is, getting the Pi set up was so much fun, I decided to buy another one just to play with. A week later I bought an Arduino Uno. I even pulled my barely touched Beagle Board out of cold storage and plugged it in. I’ve been having a blast the last couple of weeks playing with all these new toys. Bought a bunch of electronic parts, breadboard, hookup wire, gathered up some other junk I had stashed here and there.

Pi, duino, beagle, semi-organized tronics junk

Back back story

When I was 11 or 12 years old, one of my class mates brought in this Radio Shack 75-in-1 Electronics kit and did a little presentation on how to hook up various circuits. I was amazed. I begged and pleaded my mom to get me one for Christmas. I actually remember the exact thought I had at the time – that with that kit, I would never, ever be bored again. I got my kit, and while the never, ever part was questionable, I certainly spent many, many, many hours playing with electronics over the next few years. I outgrew the kit and bought all sorts of other build-it-yourself stuff. I learned how to solder. I could tell you the difference between pnp and npn transistors and precisely how they worked on a molecular level.

75in1

Where I grew up was a vocational school, Blue Hills Regional, that offered an electronics program. It was perfect timing. I signed up to go there for high school. Then disaster struck, in the form of my own bad judgment. My stepfather was an electrician. In some misguided attempt to win favor with him, I switched from the electronics program to the electrical program and wound up becoming an electrician myself. I learned soon after graduation how much I hated being an electrician, stopped doing it, and it wasn’t until about 15 years later that I discovered Flash and programming and a similar joy to what I had found doing electronics.

Back to the present

So getting involved with the Raspberry Pi and Arduino and Beagle Board, I found myself playing with LEDs and resistors for the first time in decades. It’s been great rediscovering all this stuff I used to know back when I was just a kid. So I’ve become a master at making LEDs blink, but I wanted to do something that could be considered practical on some small level.

The problem

Great projects are often the result of solving some problem. My problem is controlling the media on my pc. These days I’m doing work on my Mac, but usually have some podcast or music playing on Winamp on my pc laptop. If I want to pause or restart, I either have to move the mouse over there (I’m using Synergy) and click on the play button. Or reach over and hit the function key and the media play/pause key. It’s a horrible problem, I know. Not sure how I’ve managed to go on this long. I try to be strong.

But my thought was to have a single button sitting on the desk. I push the button, the media toggles. A bizarre plan started to form. Have a button hooked to the Pi send a message across the network to toggle the music state. I have achieved this. And I’m sitting in the living room clicking this button relentlessly, starting and stopping the music playing in my home office. My wife has just retreated upstairs, pretending not to be annoyed.

The solution

Crap, this post is already too damn long. Stay tuned for Part II in which our hero details how he accomplished this nearly impossible feat in the most convoluted way possible, using only Python, Node.js and C#.

Click for Part II of this amazing saga.

One response so far