Circuit Documentation

Summary

The circuit in question is designed to monitor air quality using an MQ135 sensor and display the readings on a 16x2 LCD. The NodeMCU V3 ESP8266 microcontroller is used to process the sensor data and control the display. The circuit also includes a buzzer for alerting when air quality levels are dangerous, and a potentiometer to adjust the LCD contrast. A resistor is used to provide backlight for the LCD. The NodeMCU V3 ESP8266 is also capable of sending the air quality data to a remote server via Wi-Fi.

Component List

MQ135 Air Quality Sensor

• Pins: VCC, GND, A0, D0
• Description: Used to measure the air quality by detecting various gases in the air.

NodeMCU V3 ESP8266

• Pins: A0, GND, VU, S3, S2, S1, SC, S0, SK, 3V3, EN, RST, Vin, D0, D1, D2, D3, D4, D5, D6, D7, D8, RX, TX
• Description: A microcontroller with Wi-Fi capability for processing sensor data and controlling other components in the circuit.

16X2 LCD

• Pins: VSS, VDD, V0, RS, RW, E, D0, D1, D2, D3, D4, D5, D6, D7, A, K
• Description: A display used to show the air quality readings and status messages.

Resistor

• Pins: pin1, pin2
• Description: A 200 Ohm resistor used to provide backlight for the LCD.

Potentiometer

• Pins: GND, Output, VCC
• Description: Used to adjust the contrast of the LCD display.

Buzzer

• Pins: POSITIVE, NEGATIVE
• Description: An audible alert device that is activated when air quality reaches dangerous levels.

Wiring Details

MQ135 Air Quality Sensor

• VCC: Connected to the power supply net.
• GND: Connected to the ground net.
• A0: Connected to A0 on the NodeMCU V3 ESP8266.

NodeMCU V3 ESP8266

• Vin: Connected to the power supply net.
• GND: Connected to the ground net.
• A0: Connected to A0 on the MQ135 sensor.
• D0-D7: Connected to D7-D0 on the 16X2 LCD respectively.
• D8: Connected to the POSITIVE pin of the Buzzer.

16X2 LCD

• VDD: Connected to the power supply net.
• VSS, K: Connected to the ground net.
• V0: Connected to the Output of the Potentiometer.
• RS, E, D4-D7: Connected to D5, D4, D3-D0 on the NodeMCU V3 ESP8266 respectively.
• A: Connected to pin1 of the Resistor.

Resistor

• pin1: Connected to the A pin of the 16X2 LCD.
• pin2: Connected to the power supply net.

Potentiometer

• VCC: Connected to the power supply net.
• GND: Connected to the ground net.
• Output: Connected to V0 on the 16X2 LCD.

Buzzer

• POSITIVE: Connected to D8 on the NodeMCU V3 ESP8266.
• NEGATIVE: Connected to the ground net.

Documented Code

#include <ESP8266WiFi.h>
#include <LiquidCrystal.h>  // include the library code:

// LCD interface pin association with the NodeMCU pin number
const int rs = D5, en = D4, d4 = D3, d5 = D2, d6 = D1, d7 = D0;
LiquidCrystal lcd(rs, en, d4, d5, d6, d7);

const int aqsensor = A0;  // MQ135 connected to A0 pin of NodeMCU

int gled = D6;  // Green LED connected to pin D6
int rled = D7;  // Red LED connected to pin D7
int buz = D8;   // Buzzer connected to pin D8

String apiKey = "YOUR_API_KEY";     // Enter your Write API key here
const char *ssid =  "YOUR_WIFI_SSID";     // Enter your WiFi Name
const char *pass =  "YOUR_WIFI_PASSWORD"; // Enter your WiFi Password
const char* server = "api.thingspeak.com";
WiFiClient client;

void setup() {
  pinMode(gled, OUTPUT);
  pinMode(aqsensor, INPUT);
  pinMode(rled, OUTPUT);
  pinMode(buz, OUTPUT);
  
  Serial.begin(115200);
  lcd.clear();
  lcd.begin(16,2);

  Serial.println("Connecting to ");
  lcd.print("Connecting.... ");
  Serial.println(ssid);
  WiFi.begin(ssid, pass);
  while (WiFi.status() != WL_CONNECTED) {
    delay(1000);
    Serial.print(".");
    lcd.print(".");
  }
  Serial.println("");
  Serial.println("WiFi connected");
  Serial.print("IP Address: ");
  delay(5000);
  Serial.println(WiFi.localIP());
  delay(5000);
}

void loop() {
  int ppm = analogRead(aqsensor); // Read MQ135 output

  Serial.print("Air Quality: ");
  Serial.println(ppm);

  lcd.setCursor(0,0);
  lcd.print("Air Quality: ");
  lcd.print(ppm);
  delay(1000);

  if (client.connect(server,80)) {
    String postStr = apiKey;
    postStr +="&field1=";
    postStr += String(ppm);
    postStr += "\r\n\r\n";

    client.print("POST /update HTTP/1.1\n");
    client.print("Host: api.thingspeak.com\n");
    client.print("Connection: close\n");
    client.print("X-THINGSPEAKAPIKEY: "+apiKey+"\n");
    client.print("Content-Type: application/x-www-form-urlencoded\n");
    client.print("Content-Length: ");
    client.print(postStr.length());
    client.print("\n\n");
    client.print(postStr);
    Serial.print("Air Quality: ");
    Serial.print(ppm);
    Serial.println(" PPM. Sent to Thingspeak.");
  }
  if(ppm <= 200) {
    digitalWrite(gled, LOW);
    digitalWrite(rled, LOW);
    digitalWrite(buz, LOW);
    lcd.setCursor(1,1);
    lcd.print("AQ Level Normal");
    Serial.println("AQ Level Normal");
  } else if (ppm > 200 && ppm < 350) {
    digitalWrite(gled, HIGH);
    digitalWrite(rled, LOW);
    digitalWrite(buz, LOW);
    lcd.setCursor(1,1);
    lcd.print("AQ Level Medium");
    Serial.println("AQ Level Medium");
  } else {
    lcd.setCursor(1,1);
    lcd.print("AQ Level Danger!");
    Serial.println("AQ Level Danger!");
    digitalWrite(gled, LOW);
    digitalWrite(rled, HIGH);
    digitalWrite(buz, HIGH);
  }

  client.stop();
  Serial.println("Waiting...");
  delay(1000);
}

Note: Replace YOUR_API_KEY, YOUR_WIFI_SSID, and YOUR_WIFI_PASSWORD with your actual API key, Wi-Fi SSID, and password before uploading the code to the NodeMCU.