Public Lab Research note


I For One Welcome Our New IRCAM Overlords

by donblair | | 2,740 views | 0 comments |

Read more: publiclab.org/n/7036


A Raspberry Pi or other Linux-running portable computer can be used as a small sensing platform; typical USB computer peripherals like webcams can be plugged in directly and controlled via simple Python scripts. There are already many easy-to-follow tutorials online for accomplishing this, and the associated instructions are usually even easier to follow that typical Linux OS instructions: the first step is usually to grab a Raspberry Pi and download a 'fresh distribution' of the operating system, so everyone following the tutorial starts with precisely the same hardware and software. So, if you've any interest in playing around with Linux, it's a great way to start -- there's a huge, supportive community online, and the hardware is relatively cheap.

Below I'm posting some quick notes on setting up a Raspberry Pi with button control, so that arbitrary python scripts can be run on the device by pressing a button -- in this case, taking a webcam snapshot. Then, for fun, I added in servo control -- for a 'panning timelapse', perhaps.

Controlling a python script with a button on the Raspberry Pi

  1. Download Occidentalis image and burn it to an SD card
  2. Boot up the RPi with the SD card, and configure settings on pi as prompted -- especially "expand root" and "locale" and "timezone". This, and any of the software-update steps below, will require that the RPi is connected via ethernet.
  3. "sudo apt-get update"
  4. "sudo apt-get install python-dev"
  5. "sudo apt-get install python-rpi.gpio"
  6. "supo apt-get install fswebcam"
  7. Grab a webcam and plug it into the USB port
  8. For button action, wire up as per diagram here
  9. Demo python code for using a button here

Sample code for triggering a webcam with a button:

#!/usr/bin/env python

from time import sleep
import os
import RPi.GPIO as GPIO

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

while True:
    if ( GPIO.input(25)== False ):
            #os.system('mpg321 vader.mp3 &')
            #os.system('fswebcam test.jpeg')
            print "Switch connected"
            os.system('fswebcam test.jpeg')
    else:
            print "Switch disconnected"
    sleep(0.1);

Adding servo control

There's a great tutorial on adafruit on how to wire up the RPi to control a servo motor here. And there's info on multiple servos here.

Code for combining button presses with camera shots and servo action:

#!/usr/bin/env python

from time import sleep
import os
import RPi.GPIO as GPIO

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

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

def setServo(angle):
    set('servo', str(angle))

set("delayed", "0")
set("mode", "servo")
set("servo_max", "180")
set("active", "1")

delay_period = 3 

while True:
    if ( GPIO.input(25)== False ):
        print "switch connected"
        setServo(45)
        sleep(1)
        os.system('fswebcam test.jpeg')
        sleep(2)
        setServo(90)
        sleep(1)
        os.system('fswebcam test.jpeg')
            sleep(2)
    else:
        print "waiting ..."        
    sleep(0.1);

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