MQTT: Arduino Light Sensor

Code:

#include <SPI.h>
#include <PubSubClient.h>
#include <Ethernet.h>

/*
 * LightSensorMqttDemo
 *
 * A simple m2m.io platform demo for Arduino.
 */

#define MQTT_SERVER "192.168.0.100"  

// MAC Address of Arduino Ethernet Sheild (on sticker on shield)
byte MAC_ADDRESS[] = { 0x90, 0xA2, 0xDA, 0x0D, 0x31, 0xB8 };
IPAddress ip(192, 168, 0, 4);
IPAddress server(192, 168, 0, 100);

EthernetClient ethClient;
PubSubClient client(server, 1883, callback, ethClient);

// Pin 8 is the LED output pin
int ledPin = 8;
// Analog 2 is the input pin
int lightPinIn = 2;

// defines and variable for sensor/control mode
#define MODE_OFF    0  // not sensing light, LED off
#define MODE_ON     1  // not sensing light, LED on
#define MODE_SENSE  2  // sensing light, LED controlled by software
int senseMode = 0;

unsigned long time;

char message_buff[100];

void setup()
{
  // initialize the digital pin as an output.
  pinMode(ledPin, OUTPUT);
  
  // init serial link for debugging
  Serial.begin(9600);
  
  if (Ethernet.begin(MAC_ADDRESS) == 0)
  {
      Serial.println("Failed to configure Ethernet using DHCP");
      return;
  }
}

void loop()
{
  if (!client.connected())
  {
      // clientID, username, MD5 encoded password
      // arduino-mqtt, mqtt-spt, e10adc3949ba59abbe56e057f20f883e (123456)
      client.connect("arduino-mqtt");
      client.publish("arduino/lightsensor", "I'm alive!");
      client.subscribe("arduino/lightsensor");
  }
  
  // read from light sensor (photocell)
  int lightRead = analogRead(lightPinIn);

  // if there is light in the room, turn off LED
  // else, if it is "dark", turn it on
  // scale of light in this circit is roughly 0 - 900
  // 900 is a "magic number" for "dark"
  if (lightRead > 900) {
     digitalWrite(ledPin, LOW);
  } else {
    digitalWrite(ledPin, HIGH);
  }
       
  // publish light reading every 5 seconds
  if (millis() > (time + 5000)) {
    time = millis();
    String pubString = "{\"report\":{\"light\": \"" + String(lightRead) + "\"}}";
    pubString.toCharArray(message_buff, pubString.length()+1);
    //Serial.println(pubString);
    client.publish("arduino/lightsensor", message_buff);
  }
 
  // MQTT client loop processing
  client.loop();
}

// handles message arrived on subscribed topic(s)
void callback(char* topic, byte* payload, unsigned int length) { 

  int i = 0;  

  //Serial.println("Message arrived:  topic: " + String(topic));
  //Serial.println("Length: " + String(length,DEC));
  
  // create character buffer with ending null terminator (string)
  for(i=0; i<length; i++) {
    message_buff[i] = payload[i];
  }
  message_buff[i] = '\0';
  
  String msgString = String(message_buff);
  
  //Serial.println("Payload: " + msgString);
  
  if (msgString.equals("{\"command\":{\"lightmode\": \"OFF\"}}")) {
    senseMode = MODE_OFF;
  } else if (msgString.equals("{\"command\":{\"lightmode\": \"ON\"}}")) {
    senseMode = MODE_ON;
  } else if (msgString.equals("{\"command\":{\"lightmode\": \"SENSE\"}}")) {
    senseMode = MODE_SENSE;
  }
}