Control a separate running script from a Web Server (python)- RPi

Let’s say that you have a running Flask server and you want your user to control the state (on/off) of a motion sensor via the GPIO. The most complex way is to create a multi-threading script which handles the server and GPIO code.

Another approach is to separate the server script from the GPIO script. Thus having two layers of scripts. This has the advantage to debug in isolation. For this technique one has to use Unix Signals which can be used to send signals from one process to another.

When you execute a script on your UNIX system, the system creates a process id (pid) which is different every time. A signal is a software interrupt which notifies a process with a significant event or request.

The following table gives a list of the most common signals:

SIGHUP 1 Linux sends a process this signal when it becomes disconnected from a terminal.
SIGINT 2 Linux sends a process this signal when the user tries to end it by

pressing CTRL+C.

SIGILL 4 Linux sends a process this signal when it attempts to execute an illegal instruction.
SIGABRT 6 Linux sends a process this signal to the process when the process calls the ‘abort ()’ function
SIGFPE 8 Linux sends a process this signal when it has executed an invalid floating-point math instruction
SIGKILL 9 Linux sends a process this signal to end it immediately
SIGUSR1 10 User programs can send this signal to other process
SIGUSR2 12 User programs can send this signal to other process
SIGSEGV 11 Linux sends a process this signal when the program has attempted an invalid memory access
SIGPIPE 13 Linux sends a process this signal when the program has attempted to access a broken data stream, such as a socket connection that has been already closed
SIGALRM 14 A process can receive this signal from the Linux using the function alarm (), after a time period mentioned in its argument.
SIGTERM 15 Linux sends a process this signal requesting it to terminate
SIGCHLD 17 Linux sends a process this signal when a child process exits
SIGXCPU 24 Linux sends a process this signal when it exceeds the limit of

CPU time that it can consume.

SIGVTALRM 26 A process can receive this signal from the Linux using the function setitimer (), after a time period mentioned in its argument.

We are interested in SIGUSR1 and SIGUSR2 which can be used to send user signals.

Step 1:

First create the server layer ( Documentation about Flask server can be found here.

from flask import *
import os
import signal

app = Flask(__name__)

def index():
    return render_template('index.html')

def process1():
    #read process id from text file
    processId = int(

    #send signal to process id
    os.kill(processId, signal.SIGUSR1)

    return render_template('trigger1.html')

def process2():
    #read process id from text file
    processId = int(

    #send signal to process id
    os.kill(processId, signal.SIGUSR2)

    return render_template('trigger2.html')

if __name__ == '__main__':,host='')

Step 2:

Create the script (you can modify this to control the GPIOs)

import os
import signal
import time

fh.write(str(os.getpid())) #get current process id and store in file

def handUSR1(signum,frame):

def handUSR2(signum,frame):

signal.signal(signal.SIGUSR1,handUSR1) #callback function for SIGUSR1 signal
signal.signal(signal.SIGUSR2,handUSR2) #callback function for SIGUSR2 signal

    print("Waiting for signal")

Step 3: Run Server


Step 4: Run


Step 5: 

In the browser, click Trigger 1 and Trigger 2 hyperlink buttons. Notice that outputs ‘triggered 10′ and ‘triggered 12′. This means that both signals were sent and received correctly.


Link to project:

PIXEL(Raspbian)-Virtual Box-Persistence Drive

Please note that this is not officially approved by the PI team but the MagPi magazine offers a similar tutorial using a pendrive.

Step 1

Download the Pixel (Jessie Raspbian) image file:

Step 2:

Open Virtual Box, press new and create the following settings:


Press Create and enter the following settings


Press Start



Step 3:

Browse to the downloaded ISO file and press Start


Pixel is loaded


Step 4:

To test persistence, create a new folder on your desktop and restart your OS.



Once your OS is restarted, you will notice that the folder you created is lost. This is because there is no persistence drive present.


Step 5 (to create persistence drive):

Open terminal window and enter sudo apt install gparted


When installation is finished, enter sudo gparted


Click on Device > Create Partition Table and press Apply


Right Click on the unallocated partition and press New


Enter label name persistence with the following default settings and press Add


Press the green very good sign and press Apply to any warning messages


Press Close and close everything


Now we are going to copy  the whole operating system from the virtual optical drive to
the hard drive.

Open the terminal window and enter sudo dd if=/dev/sr0 of=/dev/sda bs=1M


Close the virtual machine (shutdown the OS)

Step 6:

Create a new Virtual Machine with the following settings. It is very important that you use your previously created virtual hard disk file. Do no create a new virtual hard disk.


Step 7:

Start your new machine and create a new python script to test persistence. I saved my file in the desktop. Restart your OS.


Step 8:

Test file is still there. Persistence worked 🙂



Screen Sky – Android App

Download my new Android App – Screen Sky



– This app changes your wallpaper automatically every 60 minutes according to the weather. Weather data is collected from by using your GPS location.

– You can also upload your pictures or take photos and tag them with weather and time conditions.

– Default app pictures and uploaded photos can be deleted by a long click on the respective image.

– Notifications are pushed when no wallpapers are found under respective weather and time status.

Maltese Online Radio – Raspberry Pi

So here’s the story:

  1. I woke up to Mum complaining that she can’t listen any longer to the radio from our kitchen due to noise interferance
  2. My brother suggested of buying an online radio
  3. I said “No..I can build one using the Raspberry Pi 2 and Music Player Daemon”
  4. I expected some excitement from their end but they just replied “ok”. Sad.
  5. Using the terminal I installed the player:
sudo apt-get install mpd mpc
  1. Changed the audio output to the headphone jack:
  2. In the terminal I added some local radio stations using the command mpc add. I used Chrome’s Developer Tools(f12) to get these links:
Radio RTK mpc add
Radio Malta 1 mpc add
One Radio mpc add
Radio Malta 2 mpc add
Magic Malta mpc add
Radio 101 mpc add
Radju Marija mpc add
bay radio mpc add
bay easy mpc add
bay retro mpc add
Campus FM mpc add mms://
Classic FM mpc add
Vibe FM mpc add
  1. From Ebay, I bought: 16 x 2 LCD Keypad Kit + DIY Transparent Acrylic Case For Raspberry Pi. I selected this product because it comes with 5 buttons and a case.
  1. When the product arrived, I attached the LCD on the Pi. No soldering is required. It’s very easy. Just place the LCD board in the respective GPIOs.
  2. Followed this tutorial to enable I2C (with Raspi Config) –
  3. Continued the setup :×2-character-lcd-plus-keypad-for-raspberry-pi/usage
  4. Installed the following library (can be done in step 10):
  5. Programmed the following code (Buttons Up&Down = Volume Control; Buttons Left/Right = Skip radio; Button Select – Turn off Pi) :
# Example using a character LCD plate.
import time
import Adafruit_CharLCD as LCD
import traceback
import subprocess
import os.path

def saveChannelInFile(index):
    channel_txt = open("/home/pi/Desktop/RVellaFM/channel.txt","w")

def isConnected():
        remote_server = ""
        status =['ping','-c','1',remote_server])#test connection with google
        if(status == 0):
            return True
            return False
    return False

def Main():

    # Initialize the LCD using the pins
    lcd = LCD.Adafruit_CharLCDPlate()

    #keep repeating until internet connection is found
            lcd.message("connecting to\n internet")


        radio = ['RTK','Malta1','One Radio','Malta2','Magic','101','Radju Marija','Bay','Bay Easy','Bay Retro','Campus FM','Classic FM','Vibe']
        #every element in radionLinks list is respectively mapped to the radio name found in radio list. example: RTK radio is mapped to etc
        index = 1

        #replace index with saved station
        channel_txt = open("/home/pi/Desktop/RVellaFM/channel.txt","r")
        index = int(channel_txt.readline())
        # create some custom characters
        lcd.create_char(1, [2, 3, 2, 2, 14, 30, 12, 0])
        lcd.create_char(2, [0, 1, 3, 22, 28, 8, 0, 0])
        lcd.create_char(3, [0, 14, 21, 23, 17, 14, 0, 0])
        lcd.create_char(4, [31, 17, 10, 4, 10, 17, 31, 0])
        lcd.create_char(5, [8, 12, 10, 9, 10, 12, 8, 0])
        lcd.create_char(6, [2, 6, 10, 18, 10, 6, 2, 0])
        lcd.create_char(7, [31, 17, 21, 21, 21, 21, 17, 31])

        #turn on green light 
        lcd.set_color(0.0, 0.0, 1.0)

        # Make list of button value, text, and backlight color.
        buttons = ( (LCD.SELECT, 'Select', (1,1,1)),
                    (LCD.LEFT,   'Left'  , (1,0,0)),
                    (LCD.UP,     'Up'    , (0,0,1)),
                    (LCD.DOWN,   'Down'  , (0,1,0)),
                    (LCD.RIGHT,  'Right' , (1,0,1)) )

        lcd.message("connecting: \n" + radio[index-1])   "mpc play " + str(index),shell=True) #start player
        lcd.message("\x01 " + radio[index-1])        
        cache_oldmessage = "\x01 " + radio[index-1]
        while True:
            message = ""
            #bug fixed - mpc sometimes switch channel automatically due to channel error. Now index is updated constantly for such cases
            currentSongLink = subprocess.check_output('mpc current -f ["%file%"]',shell=True)
            tmp_index =1
            for channel in radioLinks:
                if(channel == currentSongLink):
                    index = tmp_index
            message = ("\x01 " + radio[index-1])
            # Loop through each button and check if it is pressed.
            for button in buttons:
                if lcd.is_pressed(button[0]):
                    if(button[0] == LCD.LEFT):
						print("left button")
						index = index - 1
						if(index < 1):
							index = len(radio)
						lcd.message("connecting:\n " + radio[index-1])"mpc play " + str(index),shell=True)
						message = ("\x01 " + radio[index-1])
                    elif(button[0] == LCD.RIGHT):
                        print("right button")
                        index = index + 1
                        if(index > len(radio)):
                            index = 1
                        lcd.message("connecting: \n" + radio[index-1])
              "mpc play " + str(index),shell=True)
                        message = ("\x01 " + radio[index-1])
                    elif(button[0] == LCD.UP):
              "mpc volume +5" ,shell=True)
                        print("volume increased")
                    elif(button[0] == LCD.DOWN):
              "mpc volume -5" ,shell=True)
                        print("volume decreased")
                    elif(button[0] == LCD.SELECT):
                        lcd.set_color(0.0, 1.0, 1.0)
                        lcd.set_color(0.0, 0.0, 0.0)

              ['poweroff'], shell=True, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
                    time.sleep(0.75) #button click delay

            if(cache_oldmessage != message): 
                cache_oldmessage = message
    except Exception as ex:
        lcd.set_color(1.0, 0.0, 0.0)


  1. Followed the following tutorial to launch the python script automatically on start-up:
  2. If you are having mpc connection timeouts with newest raspbian versions, just follow the fix by user csrlima in the following forum:
  3. Tested the radio. Like a boss 🙂


Raspberry Pi GPIO Emulator

This Raspberry Pi emulator simulates some of the functions used in the RPi.GPIO library (using python). The intention of this library is educational.



The easiest way is to download the zip file and extract the files in the same working environment of your script. To use the emulator just type the following at the beginning of your script.

from EmulatorGUI import GPIO


This library simulates the following functions which are used in the RPi.GPIO library.

  • GPIO.setmode()
  • GPIO.setwarnings()
  • GPIO.setup()
  • GPIO.input()
  • GPIO.output()

Test Example

from EmulatorGUI import GPIO
#import RPi.GPIO as GPIO
import time
import traceback

def Main():


        GPIO.setup(4, GPIO.OUT)
        GPIO.setup(17, GPIO.OUT, initial = GPIO.LOW)
        GPIO.setup(18, GPIO.OUT, initial = GPIO.LOW)
        GPIO.setup(21, GPIO.OUT, initial = GPIO.LOW)
        GPIO.setup(23, GPIO.IN, pull_up_down = GPIO.PUD_UP)
        GPIO.setup(15, GPIO.IN, pull_up_down = GPIO.PUD_DOWN)
        GPIO.setup(24, GPIO.IN, pull_up_down = GPIO.PUD_DOWN)
        GPIO.setup(26, GPIO.IN)

            if (GPIO.input(23) == False):

            if (GPIO.input(15) == True):

            if (GPIO.input(24) == True):

            if (GPIO.input(26) == True):

    except Exception as ex:
        GPIO.cleanup() #this ensures a clean exit



Click here to download

Christmas Tree Controller

This is a brief explanation how to create a Christmas tree that its LEDs are controlled by the end user from a website. The concept is pretty simple. We need to have a server, a website, a database, a relay, LEDs and of course a Netduino. Obviously this concept can be applied on a RaspberryPi.

Video Demonstration

General Setup

For this project Node.JS was chosen because it has a popular library called Socket.IO and can be easily connected with MongoDB. Socket.IO is a JavaScript library for real time web applications. It enables real-time communication between web clients and servers. MongoDB is a cross-platform document-oriented database which is classified as a NoSQL database. Socket.IO was used to display the Christmas tree’s animation state (controlled by netduino) in real time to all connected users via Node.JS. MongoDB was used to store the user’s LEDs animation.

Here is a step-by-step breakdown structure of what was done:

Step 1 – User enters animation sequence: Selected LED colours with duration in seconds.

Step 2 – Upon submission, the user’s data is stored in MongoDB and emitted via Socket.IO to all connected clients in JSON.

Step 3 – Netduino checks every two seconds with Node.JS if there’s a new animation for playing. If there is, Node.JS gets the data from the database and returns it to Netduino as JSON string. Then Netduino parses the JSON string and plays the animation. Once it finishes, it instructs Node.js to get the next animation.

Step 4 – Another challenging part was how to setup the relay with the batteries, LEDs and Netduino. The following tutorial was followed and implemented:




Digital Game Based Learning – Learn Binary & Hexadecimal Conversions



I created this game using Unity 3D for a teaching course. The game takes place in World War 2 and contains one single mission. A number of planes are inbound to destroy a town and the objective of the player is to stop the enemy. The player has to translate incoming messages by using ASCII encoding and destroy respective enemy planes.

This game supports assessment for:

  • Decimal to ASCII Encoding
  • Binary to ASCII Encoding
  • Hexadecimal to ASCII Encoding



Click here to download (Runs only on Windows)


Step 1 – First Screen

Step 1