Temperature, humidity and air

MQ-2 gas sensor with ESP32

Do not use an MQ-2 as a safety device. It is a cheap heated sensor that reacts to several gases and cannot tell them apart, so a reading says that something has changed, not what is in the room or how much. For gas leaks and fire, use a certified alarm. For a workshop experiment or a chart of air changes, the MQ-2 is fine.

Its heater needs a steady 5 V, which means a real 5 V supply and not the 3.3 V pin. The module warms up noticeably, which is normal. New or long-stored sensors need preheating: one reference gives at least 24 hours for the best stability, and calibration can drift during storage. Even after that, allow a few minutes after every power-on before you trust the value.

The analog output can rise towards 5 V, and that is too much for an ESP32 pin. Use a divider, such as 10 kΩ from AO to the pin and 20 kΩ from the pin to ground, which limits 5 V to about 3.3 V. The ESP32 ADC is not perfectly linear, so compare readings with each other instead of trusting one number. Converting to ppm needs a clean-air baseline and a curve for one gas, and the result stays an estimate. LCD Ultra Pro can read an analog pin in a rule and scale it for a bar or label without code, which suits this kind of relative display.

Power and logic levels

5 V signal: needs a divider or level shifter

Supply	5 V heater
Signal level	AO and DO can reach about 5 V
On an ESP32	Use a divider on AO. Do not connect DO without one.

ESP32 pins, including those on a display board's connectors, work at 3.3 V and are not 5 V tolerant. Modules differ, so if in doubt, measure the signal before you connect it.

const int AO_PIN = 4;             // an ADC1 pin, so WiFi can stay on
const float DIVIDER = 1.5;        // 10k + 20k divider

void setup() {
  Serial.begin(115200);
  Serial.println("Warming up for 2 minutes");
  delay(120000);
}

void loop() {
  int mv = analogReadMilliVolts(AO_PIN);
  Serial.printf("Sensor output %.2f V\n", mv * DIVIDER / 1000.0);
  delay(1000);
}

Use this information at your own risk. It is for general guidance only: the example code compiles, but it has not been tested on every part, and modules differ between sellers. Check the manufacturer's data before you build. Product names belong to their owners, and LCD Ultra Pro is not affiliated with them. See our Terms.