MQ-135 air quality sensor with NodeMCU ESP8266 — wiring and Rs/R0 code

The MQ-135 reacts to ammonia, CO2, smoke and alcohol vapour, so it is popular for “air quality monitor” projects. On a NodeMCU it needs a 5 V heater supply from VIN and a divider so its up-to-5 V output fits the A0 pin.

Instead of printing a raw number, the sketch converts A0 back to the sensor’s resistance Rs and compares it with a clean-air value R0 taken at start-up. A falling Rs/R0 means the air is getting worse — a far more honest output than a made-up ppm figure.

Wiring

Also needed: 10 kΩ resistor, 20 kΩ resistor (or 2 × 10 kΩ in series).

Every connection
NodeMCU ESP8266	MQ-135 gas sensor module	Wire	Notes
VIN	VCC	red	5 V heater, about 150 mA. VIN is USB 5 V (often 4.7 V after the board’s diode).
GND	GND	black
A0	AO	yellow	Via 10 kΩ / 20 kΩ divider — 10 kΩ from AO to A0, 20 kΩ from A0 to GND keeps A0 under 3.3 V.

5 V (heater: about 150 mA) part, 3.3 V board. The heater needs 5 V. AO can reach about 5 V, so on ESP32 and Pico feed AO through a 10 kΩ / 20 kΩ divider (or similar) to stay under 3.3 V. DO is driven by the comparator from VCC — divide it too on 3.3 V boards.

Board quirks

Two dividers are in series here: yours (10 kΩ / 20 kΩ) and the NodeMCU’s own 220 kΩ / 100 kΩ in front of the chip’s 0–1 V ADC. The board’s 320 kΩ sits in parallel with your 20 kΩ, so the sketch multiplies the A0 voltage by 1.53, not 1.5, to get back to AO.
Full scale on A0 is about 3.2 V on most NodeMCUs (1 V × 320 k / 100 k), not exactly 3.3 V. Feed A0 a known voltage, or measure it, and set ADC_FULL_SCALE if you want accurate volts.
The ESP8266 has only one analog input, so an MQ-135 and, say, a soil sensor can’t both use A0. Add an ADS1115 over I2C (D1/D2) for more channels.
The 2-minute warm-up in setup() uses delay() inside the wait loop. An empty while busy-wait that long would trip the ESP8266’s software watchdog and reset the board.
The “ppm” formulas found online assume a factory-calibrated sensor at 20 °C and 65 % RH. Treat Rs/R0 as a relative indoor-air indicator (ventilate when it drops), not a CO2 meter — use an SCD40 for real CO2.

Code

mq135-nodemcu.inoArduino C++Open in Playground Flash

// MQ-135 air-quality sensor module on a NodeMCU (ESP8266).
// VCC → VIN (5 V heater), GND → GND, AO → 10 kΩ → A0 with 20 kΩ from A0 to GND.
// Works out the sensor resistance Rs from the voltage, then the ratio Rs/R0 to a clean-air
// baseline. A falling ratio means more gas (CO2, NH3, smoke, alcohol vapour…). Not a calibrated
// ppm meter — use it to spot trends.
const float ADC_FULL_SCALE = 3.2;  // volts at A0 that read 1023 (NodeMCU divider 220k/100k; measure yours)
const float DIVIDER_RATIO = 1.53;  // 10k over (20k || the board's 320k): AO volts = A0 volts × this
const float RL_KOHM = 1.0;         // load resistor on the module (marked 102 = 1 kΩ; 103 = 10 kΩ)
const float VCC = 5.0;             // heater/sensor supply — measure VIN, it is often about 4.7 V on USB
float r0 = 0;                      // clean-air resistance, measured at start-up

float readRs() {
  long sum = 0;
  for (int i = 0; i < 16; i++) {
    sum += analogRead(A0);
    delay(5);
  }
  float vA0 = (sum / 16.0) * ADC_FULL_SCALE / 1023.0;
  float vAO = vA0 * DIVIDER_RATIO;
  if (vAO < 0.05) vAO = 0.05;  // avoid divide-by-zero with AO disconnected
  return RL_KOHM * (VCC - vAO) / vAO;
}

void setup() {
  Serial.begin(115200);
  Serial.println();
  Serial.println("MQ-135: warming up 2 min in clean air, then measuring R0");
  unsigned long t0 = millis();
  while (millis() - t0 < 120000) delay(100);  // delay() feeds the watchdog; an empty busy-wait would not
  float sum = 0;
  for (int i = 0; i < 10; i++) sum += readRs();
  r0 = (sum / 10) / 3.6;  // datasheet curve: Rs/R0 is about 3.6 in clean air
  Serial.printf("R0 = %.2f kOhm\n", r0);
}

void loop() {
  float rs = readRs();
  float ratio = rs / r0;
  Serial.printf("Rs %.2f kOhm  Rs/R0 %.2f  %s\n", rs, ratio, ratio < 2.5 ? "air worse than at start-up" : "similar to start-up");
  delay(2000);
}

// MQ-135 air-quality sensor module on a NodeMCU (ESP8266).
// VCC → VIN (5 V heater), GND → GND, AO → 10 kΩ → A0 with 20 kΩ from A0 to GND.
// Works out the sensor resistance Rs from the voltage, then the ratio Rs/R0 to a clean-air
// baseline. A falling ratio means more gas (CO2, NH3, smoke, alcohol vapour…). Not a calibrated
// ppm meter — use it to spot trends.
const float ADC_FULL_SCALE = 3.2;  // volts at A0 that read 1023 (NodeMCU divider 220k/100k; measure yours)
const float DIVIDER_RATIO = 1.53;  // 10k over (20k || the board's 320k): AO volts = A0 volts × this
const float RL_KOHM = 1.0;         // load resistor on the module (marked 102 = 1 kΩ; 103 = 10 kΩ)
const float VCC = 5.0;             // heater/sensor supply — measure VIN, it is often about 4.7 V on USB
float r0 = 0;                      // clean-air resistance, measured at start-up

float readRs() {
  long sum = 0;
  for (int i = 0; i < 16; i++) {
    sum += analogRead(A0);
    delay(5);
  }
  float vA0 = (sum / 16.0) * ADC_FULL_SCALE / 1023.0;
  float vAO = vA0 * DIVIDER_RATIO;
  if (vAO < 0.05) vAO = 0.05;  // avoid divide-by-zero with AO disconnected
  return RL_KOHM * (VCC - vAO) / vAO;
}

void setup() {
  Serial.begin(115200);
  Serial.println();
  Serial.println("MQ-135: warming up 2 min in clean air, then measuring R0");
  unsigned long t0 = millis();
  while (millis() - t0 < 120000) delay(100);  // delay() feeds the watchdog; an empty busy-wait would not
  float sum = 0;
  for (int i = 0; i < 10; i++) sum += readRs();
  r0 = (sum / 10) / 3.6;  // datasheet curve: Rs/R0 is about 3.6 in clean air
  Serial.printf("R0 = %.2f kOhm\n", r0);
}

void loop() {
  float rs = readRs();
  float ratio = rs / r0;
  Serial.printf("Rs %.2f kOhm  Rs/R0 %.2f  %s\n", rs, ratio, ratio < 2.5 ? "air worse than at start-up" : "similar to start-up");
  delay(2000);
}

Expected serial output

Serial Monitor · 115200Output

MQ-135: warming up 2 min in clean air, then measuring R0
R0 = 4.87 kOhm
Rs 17.21 kOhm  Rs/R0 3.53  similar to start-up
Rs 10.94 kOhm  Rs/R0 2.25  air worse than at start-up

MQ-135: warming up 2 min in clean air, then measuring R0
R0 = 4.87 kOhm
Rs 17.21 kOhm  Rs/R0 3.53  similar to start-up
Rs 10.94 kOhm  Rs/R0 2.25  air worse than at start-up

Common errors

R0 comes out as a tiny or huge number

Check RL_KOHM against the resistor marked on your module (102 = 1 kΩ, 103 = 10 kΩ), and that the divider is wired as described.

Board resets during the warm-up

Usually a supply that can’t hold 150 mA of heater current. See rst cause 4 if the reset reason says watchdog.