The other week at work we launched a petition to support lifesaving Aid using a Jumbotron display, so I’ve been inspired to have a go at building a (very) mini version using my Raspberry Pi and an Arduino.

This is a handy way of getting your Pi to display information from any Python app you may have written – so you could use it to make a new email notifier, flash up messages from twitter, or if you’re using your Pi as a server without a monitor – status update messages. I’m interested in using it as a ‘now playing’ message board for an email controlled spotify jukebox I’m working on (more updates to follow).

There are lots of ways you could achieve this – it’s certainly possible to drive LED displays directly from the Raspberry Pi GPIO pins, but this is method is easy to do as it uses off the shelf kits, provides a bit of soldering practice, and an opportunity to learn a bit about Arduinos, Python and serial communications. As with a lot of Arduino projects once you’ve got the hand of how things work you could easily build a cheaper version using only the parts you need. Plus these are my first steps getting an Arduino to do something more interesting than flash a single LED on and off.

I’ve been using a standard Arduino Uno which is available for around £20, and the LED Matrix Shield v.1 from Ciseco which is £5 – this is a Arduino add on board that sits on top of the Arduino. The Matrix shield is fun to put together and good for practising soldering, as a lot of the parts aren’t particularly heat sensitive.

Setting up the Arduino

The instructions that come with the Matix shield are a bit basic – for a much better step by step how to check out this tutorial which I found in the forums.

Next step is to prepare the Arduino with the code you need it to run. Unlike the Nerdkit microcontroller I mentioned in a previous post the Arduino has a nice user friendly graphical front end – the downside to this is that it runs quite slowly on the Raspberry Pi. I used my Mac to upload the code to the Arduino – just remember to connect it directly to the USB ports on your Mac and not via a USB hub.

First you’ll need to add frequencytimer.h to your Arduino Library – there are instructions on how to install libraries here and you can download frequency timer here.

Then you’ll need to download 8x8LEDMatrix written by Andy Lindsay who has all sorts of other interesting Arduino / Pi stuff on his blog.

The script includes a 5×7 font file which you can experiment with to get the display to show different characters. Compile and upload this to your Arduino to check that everything’s working properly.

Now you’ll need to modify Andy’s code to send the text you want your Arduino to display over the serial port. Using a tilda character clears the script (although I discovered you don’t actually need this as the Pi resets the Arduino when it communicates with it on the serial port)

/* * Show messages on an 8x8 led matrix, * scrolling from right to left. * * Uses FrequencyTimer2 library to * constantly run an interrupt routine * at a specified frequency. This * refreshes the display without the * main loop having to do anything. * */ #include #include &amp;quot;font5x7.h&amp;quot; #define DELAY 80 int incomingByte = 0; // for incoming serial data String incomingString; // for the incomming serial data string char message[] = &amp;quot;&amp;quot;; // define the message byte col = 0; byte leds[8][8]; // pin[xx] on led matrix connected to nn on Arduino (-1 is dummy to make array start at pos 1) int pins[17]= { -1, 17, 8, 7, 19, 9, 13, 4, 10, 6, 5, 18, 3, 2, 11, 12, 16}; // col[xx] of leds = pin yy on led matrix int cols[8] = { pins[13], pins[3], pins[4], pins[10], pins[06], pins[11], pins[15], pins[16]}; int rows[8] = { pins[9], pins[14], pins[8], pins[12], pins[1], pins[7], pins[2], pins[5]}; void setup() { // sets the pins as output for (int i = 1; i 0) { // read the incoming byte: char incomingByte = (char)Serial.read(); incomingString += incomingByte; if (incomingByte == &amp;#039;~&amp;#039;) { // using a tilda ~ clears the string incomingString = &amp;quot;&amp;quot;; } // check string for command variables incomingString.toCharArray(message, 160); } scrollMsg( message ); } void clearLeds() { // Clear display array for (int i = 0; i < 8; i++) { for (int j = 0; j < 8; j++) { leds[i][j] = 0; } } } void setChar(char ch) { for (int i = 0; i < 5; i++) { unsigned char bt = pgm_read_byte(&amp;amp;(smallFont [(ch-32)*5 + i] )); for (int j = 0; j < 8; j++) { leds[j][i+1] = (bt &amp;amp; 0x01); bt = bt >> 1; } } } void slideChar(char ch, int del) { for (int l = 0; l < 5; l++) { for (int i = 0; i < 7; i++) { for (int j = 0; j < 8; j++) { leds[j][i] = leds[j][i+1]; } } unsigned char bt = pgm_read_byte(&amp;amp;(smallFont [(ch-32)*5 + l] )); for (int j = 0; j < 8; j++) { leds[j][7] = (bt &amp;amp; 0x01); bt = bt >> 1; } delay(del); } for (int i = 0; i < 7; i++) { for (int j = 0; j < 8; j++) { leds[j][i] = leds[j][i+1]; } } for (int j = 0; j < 8; j++) { leds[j][7] = 0; } delay(del); } // Interrupt routine void display() { digitalWrite(cols[col], LOW); // Turn whole previous column off col++; if (col == 8) { col = 0; } for (int row = 0; row < 8; row++) { // if (leds[col][7 - row] == 1) { if (leds[col][ row] == 1) { digitalWrite(rows[row], LOW); // Turn on this led } else { digitalWrite(rows[row], HIGH); // Turn off this led } } digitalWrite(cols[col], HIGH); // Turn whole column on at once (for equal lighting times) }

Once you’ve uploaded this you can test its working properly by clicking on the serial monitor icon (the magnifier glass on the right hand side) and typing some text – if everything is working OK you’ll see the text appear on your Arduino’s matrix display.

Setting up the Raspberry Pi

Now you need to connect the Arduino to the Pi. I’ve used one of the Pi’s USB ports rather than via a USB hub – you could also use the serial pin on the GPIO port to do this as well. Make sure the Arduino is connected before you power up the Raspberry Pi.

You can test the Pi -> Arduino control with minicom (thankyou http://codeandlife.com/2012/07/29/arduino-and-raspberry-pi-serial-communication/)

Install minicom on the pi via the console:

sudo apt-get install minicom

then run minicom and connect it to the Arduino:

minicom -b 9600 -o -D /dev/ttyAMA0

Hopefully everything you type into the console will be appearing on the Arduino Matrix.

Next is to get Python to send data to the Arduino over the serial port. Install the python serial libraries:

sudo apt-get install python-serial

Now you’re ready to write some Python code to control the Arduino’s LED display. I’ve been using the IDLE app on the Raspberry Pi. The following code is a super simple python script that can send some text to the LED Matrix:

import time import serial #configure the serial connection ser = serial.Serial("/dev/ttyACM0",9600) ser.open() # add some time delays to stop the Arduino resetting time.sleep(1.5) ser.isOpen() time.sleetp(1.5) ser.write("Raspberry Pi")

Hit f5 and you should see this:

And that’s it – you now have a neat way of controlling an LED display with Python. The LED display always shows the last message it received until it gets a new one.

Update: Cisco – the company who made the lovely shield kit, are now doing a kickstarter for a Pi version with a lot more red or white LEDs that plugs directly into the GPIO.

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