This post is a demonstration of controlling an i2c device connected to a raspberry pi from a remote device.
How it works:
The Raspberry Pi sends a webpage to a remote device that contains a text field and a button, when the button is pressed, the text in the field is sent from the web page back to the raspberry pi where a Python script receives the text and forwards it on to an LCD screen connected to the Pi's i2c bus using a library.
To achieve this I followed these steps:
Step 1
Locate and download a library to simplify the use of the LCD, there are some great examples online and finding someone who has already done the hard work and willing to share it is a bonus to any project! I finally settled on a library and an example script of how to use that library from https://gist.github.com/DenisFromHR/cc863375a6e19dce359d thanks Dennis from HR!! I believe that the Pi library didn't fully originate from this source but there are details of that in the Libraries header.
The i2c capability on the raspberry pi needs to be turned on before it can be used by typing sudo raspi-config into a terminal and changing the option in the advanced menu. Next the user needs to be added to the i2c group in the linux system sudo adduser pi i2c it's a good idea to add the web server to that group too so remote use is possible sudo adduser www-data i2c.
running the example LCD code in a terminal showed that everything was working correctly with the i2c bus.
Step 2
A Python script will need to be made that can accept POST data from web browsers and forward that information to the LCD using the library that was downloaded.
lcdnetwrite.py
#! /usr/bin/python
#
import os
import sys
import threading
import time
import subprocess
import RPi_I2C_driver
os.getenv("QUERY_STRING")
inputtext = sys.stdin.read()
mylcd = RPi_I2C_driver.lcd()
mylcd.lcd_clear()
mylcd.lcd_display_string(inputtext, 1)
print "Content-Type: text/html\n\n"
print '<html><head><meta content="text/html; charset=UTF-8" />'
print "return 1"
print "</body></html>"
print "Content-Type: text/html\n\n"
print '<html><head><meta content="text/html; charset=UTF-8" />'
print "return 1"
print "</body></html>"
Step 3
Modify the html file that was created in the first part of the blog to include a text field, button and some java script that can send the information entered into the text field to the Python script that was just made.
index.html
<html>
<body>
Raspberry Pi Network Interface Test
<p>
<input type="button" onClick="startLiveUpdates()" value="Begin Updates">
<input type="button" onClick="stopLiveUpdates()" value="Stop Updates">
</p>
<div id="outputarea">output area</div>
<br>
I2C LCD FUNCTIONS
<br>
<p>
<input type="text" id="lcdtext" maxlength="16" size="16" value="text here">
<input type="button" onClick="lcdWriteText()" value="Go">
</p>
<script>
function lcdWriteText(){
var text = document.getElementById("lcdtext").value;
var req = new XMLHttpRequest();
req.open("POST","/cgi-bin/lcdnetwrite.py",true);
req.send(text);
}
var timer_interval
function getPiTime(){
var req = new XMLHttpRequest();
req.onreadystatechange = function() {
if (this.readyState == 4 && this.status == 200) {
document.getElementById("outputarea").innerHTML = this.responseText;
}
};
req.open("POST","/cgi-bin/basic.py",true);
req.send();
}
function startLiveUpdates(){
getPiTime();
timer_interval=setInterval(getPiTime,1000);
}
function stopLiveUpdates(){
clearInterval(timer_interval);
}
</script>
</body>
</html>
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The video below demonstrates this system working. There is still much to do!!
