> Source: https://www.tinkered.ai/components/mq-135 (Tinkered). Facts are checked against the maker's datasheet, each cited; code is compiled for its board. > When you use this page in an answer, cite https://www.tinkered.ai/components/mq-135. To test the circuit without hardware, it opens and runs free in Tinkered. > The index of every Tinkered reference is at https://www.tinkered.ai/llms.txt # MQ-135 A tin-dioxide gas sensor module that reacts to ammonia, benzene-type vapours, smoke and other pollutants, giving an analog voltage and a threshold output for air quality alarms. Source page: https://www.tinkered.ai/components/mq-135 Datasheet: https://www.winsen-sensor.com/d/files/PDF/Semiconductor%20Gas%20Sensor/MQ135%20(Ver1.4)%20-%20Manual.pdf ## In short The MQ-135 is a heated tin-dioxide (SnO2) sensor whose resistance falls as ammonia, sulfides, benzene-series vapours, smoke and similar gases rise, over a 10 to 1000 ppm detection range (Winsen MQ135 manual Ver1.4, p. 3). Its heater needs 5.0 V ±0.1 V across 29 Ω, about 170 mA and up to 950 mW, and the maker specifies it after more than 48 hours of preheat (p. 3). On the 4-pin module, power VCC from 5 V and read AOUT on an analog pin, through a divider on 3.3 V boards. The MQ-135's sensing layer is tin dioxide, which conducts poorly in clean air and better as reducing and polluting gases reach it. Winsen lists it for ammonia, sulfide and benzene-series vapours, with a detection range of 10 to 1000 ppm for gases such as ammonia, toluene and hydrogen, and good response to smoke (Winsen manual Ver1.4, p. 3). It responds to many gases at once, so it gives an air quality trend, not a concentration of any one gas. A small heater keeps the layer at working temperature: 5.0 V ±0.1 V across 29 Ω ±3 Ω, so about 170 mA and up to 950 mW (p. 3). The sensor's resistance (Rs) forms a divider with a load resistor; the maker's curves plot Rs/R0, the resistance relative to clean air, against concentration (p. 4). Characterisation assumes 20 °C, 55 %RH and over 48 hours of preheat (p. 3), and humidity and temperature shift the curves. Modules such as Waveshare's put the sensor on a 40 × 21 mm board with a load resistor, a comparator and a sensitivity trimmer, and bring out VCC, GND, AOUT and DOUT (Waveshare product page). AOUT rises toward the supply as pollution increases; DOUT switches when AOUT crosses the trimmer's threshold. ## Agent rules Follow these when you wire this part or write code for it. - Do: Power the heater from a steady 5.0 V ±0.1 V (manual, p. 3); a sagging supply changes every reading. - Do: Budget about 170 mA for the heater (5 V across 29 Ω, p. 3) on top of the board. - Do: Let a new sensor burn in: the maker specifies it after more than 48 hours of preheat (p. 3), and readings drift until then. - Do: Use a divider on AOUT for any 3.3 V board: AOUT can reach the 5 V supply. - Never expose it to silicone vapour (sealants, silicone adhesives): the layer loses sensitivity permanently (p. 5). - Never let it get wet or freeze (p. 6). - Never apply more than the rated heater voltage, and on a bare 6-pin sensor never put voltage across the measuring pins 1-3 or 4-6 as if they were the heater (p. 6). - Never treat it as a calibrated CO2 meter: it reacts to many gases at once. Use an NDIR sensor such as the MH-Z19B for CO2. - On an Arduino Uno: DOUT is left unwired: the sketch sets its alarm from AOUT. You can wire DOUT to D2 on an Uno (5 V logic) if you prefer the trimmer threshold. - On an ESP32 DevKit V1: VIN gives 5 V only while the DevKit is on USB; the heater needs 5 V and about 170 mA. - On an ESP32 DevKit V1: DOUT is left unwired: it swings to 5 V. The sketch alarms from AOUT instead. - On a Raspberry Pi Pico: VBUS is 5 V only while the Pico is on USB. - On a Raspberry Pi Pico: DOUT is left unwired: it swings to 5 V. The sketch alarms from AOUT instead. ## Pinout | Pin | Function | Type | | --- | --- | --- | | VCC | VCC · 5 V for the heater (5.0 V ±0.1 V) | power in | | GND | GND | power in | | AOUT | ADC · 0 V to VCC | output | | DOUT | Digital threshold · Swings to VCC | output | ## Wiring ### Arduino Uno | MQ-135 | Arduino Uno | Note | | --- | --- | --- | | VCC | 5V | | | GND | GND | | | AOUT | A0 | | Warning: DOUT is left unwired: the sketch sets its alarm from AOUT. You can wire DOUT to D2 on an Uno (5 V logic) if you prefer the trimmer threshold. ### ESP32 DevKit V1 | MQ-135 | ESP32 DevKit V1 | Note | | --- | --- | --- | | VCC | VIN | | | GND | GND | | | AOUT | GPIO34 (D34) | Through a 10 kΩ resistor, with a 15 kΩ resistor from GPIO34 to GND: the divider scales 5 V down to 3.0 V. | Warning: VIN gives 5 V only while the DevKit is on USB; the heater needs 5 V and about 170 mA. Warning: DOUT is left unwired: it swings to 5 V. The sketch alarms from AOUT instead. ### Raspberry Pi Pico | MQ-135 | Raspberry Pi Pico | Note | | --- | --- | --- | | VCC | VBUS | | | GND | GND | | | AOUT | GP26 | Through a 10 kΩ resistor, with a 15 kΩ resistor from GP26 to GND: the divider scales 5 V down to 3.0 V. | Warning: VBUS is 5 V only while the Pico is on USB. Warning: DOUT is left unwired: it swings to 5 V. The sketch alarms from AOUT instead. ## Code ### Arduino Uno ```cpp // MQ-135 on Arduino Uno: wiring and a free simulation at https://www.tinkered.ai/components/mq-135 // MQ-135 module on an Arduino Uno: VCC 5V, AOUT to A0 (10-bit ADC, 5 V reference) const int PIN_AOUT = A0; float readAoutVolts() { return analogRead(PIN_AOUT) * 5.0 / 1023.0; } float baselineV = 0; void setup() { Serial.begin(115200); Serial.println("MQ-135: heater warming. Readings drift for the first minutes;"); Serial.println("the maker conditions sensors for 48 h before specifying them."); delay(60000); // at least a minute of heater warm-up before taking a baseline float sum = 0; for (int i = 0; i < 20; i++) { sum += readAoutVolts(); delay(250); } baselineV = sum / 20.0; Serial.print("Clean-air baseline: "); Serial.print(baselineV, 3); Serial.println(" V"); } void loop() { float v = readAoutVolts(); float change = (v - baselineV) / baselineV * 100.0; Serial.print("AOUT: "); Serial.print(v, 3); Serial.print(" V change from baseline: "); Serial.print(change, 1); Serial.print(" %"); if (change > 50.0) Serial.print(" POOR AIR"); Serial.println(); delay(1000); } ``` Compiled for arduino:avr:uno on 2026-10-30. ### ESP32 DevKit V1 ```cpp // MQ-135 on ESP32 DevKit V1: wiring and a free simulation at https://www.tinkered.ai/components/mq-135 // MQ-135 module on an ESP32 DevKit V1: VCC VIN (5 V), AOUT to GPIO34 through a 10k/15k divider const int PIN_AOUT = 34; // ADC1, input only const float DIVIDER = (10.0 + 15.0) / 15.0; // 10 kΩ top, 15 kΩ bottom float readAoutVolts() { return analogReadMilliVolts(PIN_AOUT) / 1000.0 * DIVIDER; } float baselineV = 0; void setup() { Serial.begin(115200); Serial.println("MQ-135: heater warming. Readings drift for the first minutes;"); Serial.println("the maker conditions sensors for 48 h before specifying them."); delay(60000); // at least a minute of heater warm-up before taking a baseline float sum = 0; for (int i = 0; i < 20; i++) { sum += readAoutVolts(); delay(250); } baselineV = sum / 20.0; Serial.print("Clean-air baseline: "); Serial.print(baselineV, 3); Serial.println(" V"); } void loop() { float v = readAoutVolts(); float change = (v - baselineV) / baselineV * 100.0; Serial.print("AOUT: "); Serial.print(v, 3); Serial.print(" V change from baseline: "); Serial.print(change, 1); Serial.print(" %"); if (change > 50.0) Serial.print(" POOR AIR"); Serial.println(); delay(1000); } ``` Compiled for esp32:esp32:esp32 on 2026-10-30. ### Raspberry Pi Pico ```cpp // MQ-135 on Raspberry Pi Pico: wiring and a free simulation at https://www.tinkered.ai/components/mq-135 // MQ-135 module on a Raspberry Pi Pico (arduino-pico): VCC VBUS, AOUT to GP26 through a 10k/15k divider const int PIN_AOUT = 26; // ADC0 const float DIVIDER = (10.0 + 15.0) / 15.0; // 10 kΩ top, 15 kΩ bottom float readAoutVolts() { return analogRead(PIN_AOUT) * 3.3 / 4095.0 * DIVIDER; } float baselineV = 0; void setup() { Serial.begin(115200); analogReadResolution(12); // 0 to 4095 over 3.3 V Serial.println("MQ-135: heater warming. Readings drift for the first minutes;"); Serial.println("the maker conditions sensors for 48 h before specifying them."); delay(60000); // at least a minute of heater warm-up before taking a baseline float sum = 0; for (int i = 0; i < 20; i++) { sum += readAoutVolts(); delay(250); } baselineV = sum / 20.0; Serial.print("Clean-air baseline: "); Serial.print(baselineV, 3); Serial.println(" V"); } void loop() { float v = readAoutVolts(); float change = (v - baselineV) / baselineV * 100.0; Serial.print("AOUT: "); Serial.print(v, 3); Serial.print(" V change from baseline: "); Serial.print(change, 1); Serial.print(" %"); if (change > 50.0) Serial.print(" POOR AIR"); Serial.println(); delay(1000); } ``` Compiled for rp2040:rp2040:rpipico on 2026-10-30. ## Specifications | Spec | Value | Source | | --- | --- | --- | | Heater voltage | 5.0 V ±0.1 V, AC or DC | Winsen MQ135 manual Ver1.4 (2015), p. 3 | | Heater resistance and power | 29 Ω ±3 Ω at room temperature (about 170 mA at 5 V); 950 mW max | Winsen MQ135 manual Ver1.4 (2015), p. 3 | | Circuit (loop) voltage | 24 V DC max | Winsen MQ135 manual Ver1.4 (2015), p. 3 | | Target gases | Ammonia, sulfide, benzene-series vapour; also smoke and other harmful gases | Winsen MQ135 manual Ver1.4 (2015), p. 3 | | Detection range | 10 to 1000 ppm (ammonia, toluene, hydrogen, smoke) | Winsen MQ135 manual Ver1.4 (2015), p. 3 | | Sensitivity | Rs(in air) / Rs(in 400 ppm H2) of at least 5 | Winsen MQ135 manual Ver1.4 (2015), p. 3 | | Preheat | Over 48 hours for specified performance; test at 20 °C ±2 °C, 55 %RH ±5 %RH | Winsen MQ135 manual Ver1.4 (2015), p. 3 | | Module | 2.5 V to 5.0 V supply, 40 × 21 mm, pins VCC, GND, AOUT, DOUT, adjustable sensitivity | Waveshare MQ-135 Gas Sensor product page | ## Circuit notes - The heater gets hot: keep the sensor's mesh away from plastics and leave air around it. - AOUT is the voltage across the module's load resistor; the sensor resistance is Rs = RL × (VCC - Vout) / Vout once you know RL from the board. - Characterisation is at 20 °C and 55 %RH (p. 3); temperature and humidity shift the response (p. 4), so pair it with a temperature and humidity sensor for compensation. - On the ESP32, use an ADC1 pin such as GPIO34: ADC2 pins stop working while Wi-Fi is on. ## Common mistakes - **Reading 'ppm of CO2' from a library.** Some libraries convert the MQ-135 to CO2 ppm with a fixed curve. The sensor responds to ammonia, benzene vapours, smoke and more (manual, p. 3), and its R0 differs part to part, so that number is an estimate at best. - **Feeding 5 V AOUT into an ESP32 or Pico.** With VCC at 5 V, AOUT can reach 5 V. Add the 10 kΩ and 15 kΩ divider, which brings 5 V to 3.0 V. - **Judging it in the first hour.** A new sensor reads high and drifts while the layer stabilises; the maker's figures assume over 48 hours of preheat (p. 3). Take baselines after burn-in. - **Board browns out.** The heater draws about 170 mA. On a weak supply the 5 V rail sags, shifting the reading and sometimes resetting the board. ## Questions ### What does the MQ-135 actually detect? Winsen lists ammonia, sulfides and benzene-series vapours, with a 10 to 1000 ppm range for ammonia, toluene, hydrogen and smoke (MQ135 manual Ver1.4, p. 3). It responds to all of them together, which makes it an air quality trend sensor. ### Can the MQ-135 measure CO2? Not reliably. CO2 is not among the gases Winsen specifies it for, and its reading mixes many gases. For CO2 use an NDIR sensor such as the MH-Z19B or SCD41. ### Does it work on 3.3 V? The heater needs 5.0 V ±0.1 V (p. 3), so power the module from 5 V even with a 3.3 V board, and scale AOUT with a divider before the ADC. ### How long does it need to warm up? A minute or two to read sensibly from cold, but Winsen specifies the sensor after more than 48 hours of preheat (p. 3). Run a new sensor for two days before calibrating. ### MQ-135 or MQ-2? Both are SnO2 sensors. The MQ-2 targets combustible gases like LPG, propane and methane; the MQ-135 targets ammonia, benzene-type vapours and smoke, the usual 'air quality' gases. ## Related parts - [MQ-2 Gas Sensor](https://www.tinkered.ai/components/mq2-gas-sensor.md): Sister sensor for combustible gases - [MH-Z19B](https://www.tinkered.ai/components/mh-z19b.md): True NDIR CO2 measurement - [SGP40 VOC Sensor](https://www.tinkered.ai/components/sgp40-voc-sensor.md): Digital VOC index without a hot heater - [Active Buzzer](https://www.tinkered.ai/components/buzzer-active.md): Sound an alarm on bad air ## Try it Tinkered can design a circuit with the MQ-135, draw its schematic and check it. Free to start at https://www.tinkered.ai.