Introduction: Twittering Laser Tripwire With Webcam Capture

This instructable will show you how to construct a laser tripwire that can twitter and grab an image from a webcam, as well as execute any command you can put in a bash script.
This instructable is actually quite simple and is even suitable as a beginner arduino project. It requires a GNU/linux (or possibly Mac) operating system with the arduino IDE and Processing IDE working properly. This project could also be implemented in Windows if you created a more complex processing application.

Hardware Requirements
Arduino Board
Laser pointer/pen
Light Dependant Resistor
100Kohm Resistor


Software Requirements
Arduino IDE
Processing IDE
Linux OS (I'm using Arch Linux for this project)
Webcam (part of xawtv)

Other Stuff:
Solder and Soldering Iron
Hot Glue Gun
Straw or Ballpoint Pen

Step 1: Connecting the LDR

I needed the LDR and the laser to be in a position where they would not move around so I connected my LDR and resistor directly to the Arduino without a breadboard, but you can do it however you like, below are the photos for soldering and bending them into shape I also included a simple diagram.

I cut the end off of a ballpoint pen so that I could slip it over the LDR to prevent ambient light from affecting the reading, you could also use a straw.

Step 2: Connecting the Laser Pointer

You can find these really cheap at some retail stores, I know that mine was under $3.

Unscrew the end and remove the batteries.

Use a hacksaw or sharp seraded knife to cut off 1 to 1/2 inch of the laser pointer, so that you can access the spring. You may not need to do this depending of what kind of laser pointer you found.

Connect and solder a wire to the spring ( I bent mine out to make it easier ) then rough up a small part of the outside of the laserpointer with a knife/sandpaper/pliers ect so that you can solder it to the outer casing. Then tape the button that activates the laser.

On my laser pointer the spring is + and the casing is - yours might be reversed. The best way to check is to take note of what position the batteries go in, the coin cells will have + marked on one side, if that side touches the spring then the spring is +.

Connect your + wire (Casing) to digital pin 13 on your arduino and you - wire (spring) to GND. 

Now upload the arduino sketch and ensure that you are getting readings from the LDR and that the laser is on.

To accomplish this you must uncomment this line:

and comment out this one:

If all is well you can position your Laser and LDR in a  fixed position, bounce the laser to the LDR with a mirror to see that its registering. When the laser is detected it should print a "1".  You may need to adjust the threshold values depending on your LDR.

Step 3: Everything in Place

Now you are going to need to make sure that your LDR and laser are not going to be moving around while you're testing the rest of the software.

I used a hot glue gun and used a small amount of hot glue on each edge of my arduino board to secure it to a plastic cassette case, (a piece of wood and some screws would be ideal but I didn't have any) The hot glue should be removed easy and if it doesn't it's in an unimportant location.

I then hot glued my laserpointer onto the platic cassette case and rested it against the arduino's digital headers. The LDR doesn't move around so I didn't have to worry about it moving. Your set up may vary but you get the idea.

You'll then want to make sure the whole thing won't move around, I used sticky tack on the bottom of the cassette case and put a jar of change on top of the USB cable so that it would not move around while testing.

You'll also want to affix your mirror to something, it's up to you to figure this one out I used sticky tack to adhere the mirror to an old and heavy Kodak brownie camera and set it on a stool with a few books on top while testing everything out. Later I just affixed the mirror to my camera tripod.

Step 4: Processing

Load the Processing sketch and leave this line commented out:

Make sure that your laser is pointed at the LDR and run the sketch. The screen area should be green and when you block the laser it should turn red, then after 10 seconds it should turn back to green.

The 10 second hold is to ensure that you're not twittering the same message multiple times, you can change this value by modifying the "int threshold=10000" variable.

Now that everything is working it's time to get your scripts in place

put twitter.trip into:

Change these to your twitter username and password

open up a terminal and type:
chmod a+x /home/your_username/twitter.trip

then run it

go to your twitter page and you should see "laser tripped on..." as your last tweet.

Now time to get the webcam working, you can skip this step if you don't want to use a webcam or if you know how to grab a frame from a webcam in linux via script.

First you want to make sure that the program called "webcam" is installed, it's a part of xawtv.

Arch Linux: pacman -S xawtv
Fedora: yum install xawtv
Debian/Ubuntu: apt-get install xawtv

Then configure .webcamrc however you need, mine is below.
The best part about webcam is that it has ftp support so that the image can be uploaded to a webserver, if you don;t want FTP comment that part out.

.webcamrc in /home/your_username/

device = /dev/video0
text="%Y-%m-%d %H:%M:%S"
#infofile = filename
fg_red = 0
fg_green = 0
fg_blue = 0
fg_red = 255
fg_green = 255
fg_blue = 255
width = 320
height = 240
delay = 0
wait = 0
input = pac207
#norm = pal
rotate = 0
top = 0
left = 0
bottom = -1
right = -1
quality = 75
trigger = 0
once = 1
archive = /home/action-owl/webcam/%Y-%m-%d--%H:%M:%S.jpg

host =
user = username
pass = password
dir  = public_html/img/webcam
file = webcam.jpg
tmp  = uploading.jpg
passive = 1
debug = 1
auto = 0
local = 0
ssh = 0

Step 5: Using Open() in Processing

The "open()" function works a little differently depending on your environment so you may need to find out what works for you, the method below should work.


If you are using Nautilus or Thunar File Managers(XFCE and Gnome):
Navigate to twitter.trip and right-click it.
Select "Open with other application"
select "Use a custom command"
and type in "bash"

Processing is supposed to send executables to the shell to be run but after trying many methods this was the one that worked. This is also why the sketch has a .trip extension.

Step 6: Now Run It.

Now everything should be ready to go!

Run the processing sketch and check your twitter and webcam archive location for the result.

Something is up with the file downloads.
So here's the code, again.

# twitter.trip
trip=$( date +'%A %b %d, %l:%M%p' )
stat="laser tripped: "$trip
result=`curl -u $user:$pass -d status="$stat" $url`
#save webcam pic
date +'%A %b %d, %l:%M%p' > /home/username/someplace

// processing code
import processing.serial.*;
Serial myPort;
char inBuffer;
int wait, now, timeout = 10000;
boolean hold = false;

void setup() {
  myPort = new Serial(this, Serial.list()[0], 9600);

void draw() {

  while (myPort.available() > 0)
    inBuffer = myPort.readChar();  

      if ( !hold )
        wait = millis();
        hold = true;

  now = millis();

  if (now > (wait + timeout))
    hold = false;


// arduino code

int ledPin =  13;
int analogPin = 0;
int ldrVal = 0;
int threshold = 500;

void setup()
  pinMode(ledPin, OUTPUT);
  digitalWrite(ledPin, HIGH);

void loop()                    
  ldrVal = analogRead(analogPin);

  if ( ldrVal > threshold )