The MQ-4 breakout board uses this schematic:Īs you can see, Aout connects to one of the electrodes and in parallel to a resistor RL. The value of RS would have to be known through a sketch. My MQ-4 electrodes give out 945 ohms of resistance for both 1-6 and 4-3 electrodes. We measure the resistance of the electrodes 1-6 or 4-3 (see figure below) using an ohmmeter. Of these two, R0 would be easier to determine. Methane PPM Output Arduino SketchĪs mentioned, RS is the sensor resistance in the presence of Methane while R0 is the sensor resistance in clean air. But before that, we need to determine the resistance ratio RS/R0. We can now use this formula in our sketch. Substituting these values in the equation above:Ĭhanging Y to RS/R0 and X to PPM and solving for PPM: Here, we will assign Y1 = 1, X1 = 1000 from the first point and Y2 = 0.58 and X2 = 5000 from the second point. The second point is when RS/R0 is somewhere around 0.58 and PPM = 5000. The most obvious point is when RS/R0 = 1 and PPM = 1000. This formula will then be used in our Arduino sketch later on. We take two points on this graph to derive a formula. Hence, by knowing RS/R0, we can determine the concentration of the gas in PPM. RS/R0 is the ratio of sensor resistance at target gas (RS) and resistance in clean air (R0). The curve is a log-log scale and shows the relationship between RS/R0 and gas concentration in parts-per-million (PPM).

Absent in the curve are propane and butane gases although both are known components of LPG (which is second to methane in this curve). As seen here, it’s most sensitive to CH4, the chemical name of methane. This curve is from the device datasheet and shows the sensitivity of the MQ-4 to gases. The sensitivity curve of the MQ-4 is shown below: Using Analog Output Determining PPM Equation If however you need to determine methane concentration, you’ll need the A0 pin. Serial.println('Methane detected!') // Output to serial monitor * MQ4 Sensor - Digital Output ExampleĪttachInterrupt(digitalPinToInterrupt(MQ4_Pin), sensor_triggered, CHANGE) //attach interrupt on MQ4 pin The Arduino sketch below uses an interrupt so that the microcontroller always detects the MQ-4 first. Here, the D0 pin connects to digital pin 2 of the Arduino. Here’s a circuit diagram with an Arduino UNO: If your project only requires detection of methane then reading the D0 pin will do. You can calibrate this “digital” output through the trimmer pot on the board. The D0 pin generates a high (equal to VCC) when in the presence of methane gas and low (equal to around 0.1 V) otherwise. The power pins VCC and GND can be connected directly to an Arduino’s 5 V pin and GND respectively. This breakout board has four output pins, namely A0, D0, GND and VCC.

How to calculate ppm from mq4