A0_to_ThingSpeak.ino

/*
 ESP8266 --> ThingSpeak Channel via MKR1000 Wi-Fi
 
 This sketch sends the value of Analog Input (A0) to a ThingSpeak channel
 using the ThingSpeak API (https://www.mathworks.com/help/thingspeak).
 
 Requirements:
 
   * ESP8266 Wi-Fi Device
   * Arduino 1.6.9+ IDE
   * Additional Boards URL: http://arduino.esp8266.com/stable/package_esp8266com_index.json
   * Library: esp8266 by ESP8266 Community
 
 ThingSpeak Setup:
 
   * Sign Up for New User Account - https://thingspeak.com/users/sign_up
   * Create a new Channel by selecting Channels, My Channels, and then New Channel
   * Enable one field
   * Note the Channel ID and Write API Key
    
 Setup Wi-Fi:
  * Enter SSID
  * Enter Password
  
 Tutorial: http://nothans.com/measure-wi-fi-signal-levels-with-the-esp8266-and-thingspeak
   
 Created: Feb 3, 2017 by Hans Scharler (http://nothans.com)
*/

#include <ESP8266WiFi.h>

// Wi-Fi Settings
const char* ssid = "XXX"; // your wireless network name (SSID)
const char* password = "XXXXXXXX"; // your Wi-Fi network password

WiFiClient client;

// ThingSpeak Settings
const int channelID = XXX;
String writeAPIKey = "XXXXXXXXXXXXXXXX"; // write API key for your ThingSpeak Channel
const char* server = "api.thingspeak.com";
const int postingInterval = 20 * 1000; // post data every 20 seconds

void setup() {
  Serial.begin(115200);
  WiFi.begin(ssid, password);

  while (WiFi.status() != WL_CONNECTED) {
    delay(500);
  }
}

void loop() {
  if (client.connect(server, 80)) {
    
    // Measure Analog Input (A0)
    int valueA0 = analogRead(A0);

    // Construct API request body
    String body = "field1=";
           body += String(valueA0);
    
    Serial.print("A0: ");
    Serial.println(valueA0); 

    client.println("POST /update HTTP/1.1");
    client.println("Host: api.thingspeak.com");
    client.println("User-Agent: ESP8266 (nothans)/1.0");
    client.println("Connection: close");
    client.println("X-THINGSPEAKAPIKEY: " + writeAPIKey);
    client.println("Content-Type: application/x-www-form-urlencoded");
    client.println("Content-Length: " + body.length());
    client.println("");
    client.print(body);

  }
  client.stop();

  // wait and then post again
  delay(postingInterval);
}