Gas detector based on MQ-2 without microcontroller

Nowadays people are using microcontrollers even for blinking an LED. And that is no problem, since they are cheap enough, have low power consumption and are easy to program. But there was a time when microcontrollers were expensive and hard to find. And even then, engineers were building working devices, just smart enough to do the job they were designed for. Let's try to build a "microcontroller-less" gas detector using one of the MQ sensors.

Gas sensors from MQ family are analog tin dioxide detectors which change their resistance in the presence of volatile compounds like gases or smoke. Except MQ-7 and MQ-9 which are designed for carbon monoxide detection and require alternating heater voltage, with analog output being read at the end of each heater voltage cycle, every other sensor may be used in the following circuit.

Built analog gas detector with 3D printed enclosure

CO and LPG gas sensor with Arduino and LCD

In a previous post I looked at a MQ-9 sensor module. Unfortunately, although the sensor can detect CO and LPG, it cannot be used as it is wired in the module. After analyzing the datasheet I figured the best thing to do is remove it from existing PCB and build my own. In short, like other sensors from MQ family, MQ-9 has a heater resistor inside. In order to get any useful reading from it, this resistor must be heated at 5 V for 60 seconds, then cooled at 1.4 V for 90 seconds. The same is true for MQ-7. The issue with modules is that all sensors from MQ family are fitted on the same PCB design.

In this post, I'll share two other methods of powering the heater resistor and I will design a PCB. Sensor readings will be displayed on an alphanumeric LCD powered by Arduino. Since real ppm is temperature and humidity dependent, I will provide a PCB header for DHT sensor. I already tested the sensor with the LM317 power supply I built in the previous post, and I did some measurements.

Attempts at reading data from MQ-2 gas sensor

MQ-2 is a gas leakage detecting sensor with good sensitivity to a wide range of gases. Since you can get most MQ sensor on ready-made modules, people are interfacing those with development boards. However, the modules are far from perfect. Some of the sensors require variable heater voltages. This is not the case for MQ-2. Since I own a module with this sensor and it can probably be used as is, I decided to make some tests while I'm waiting a PCB for MQ-9 to be manufactured and shipped.

In the previous post I explained why modules with MQ-7 and MQ-9 are no good. Now, I'm about to discover the same for MQ-2. I thought I could use the module as is, since I am more interested in finding a method of computing useful data from the analog output of the sensor. With an Arduino compatible board and an MQ-2 module I will attempt to get ppm values. But not before some parts swapping.

MQ-2 test fixture, with sensor exposed to alcohol

Useless sensor modules based on MQ-7 and MQ-9

When shopping for electronics parts and modules, I oftentimes add to cart things I didn't plan to buy, since most suppliers offer free shipping when total order amount is above a threshold. This was the case with a module I bought recently, a carbon monoxide and LPG detector based on MQ-9 sensor. When I got the time to build a breadboard circuit to test it, I came across a problem. As with most modules and devices, I started with MQ-9 datasheet. And at first I did not quite understand what they were saying about high and low heater voltage.

And the internet is full of examples regarding such modules interfaced to Arduino. And almost everybody seems to be powering it from 5 V, while some even developed code with advanced calculations to get real ppm value from the sensor. Throughout reading of the datasheets of both MQ-7 and MQ-9 reveals a "detail" almost everybody seems to have missed. In this post I will show you the correct way of using MQ carbon monoxide sensors. Keep in mind that CO and LPG are dangerous gases and if you need a detector, you should always buy a professionally manufactured one which is also properly calibrated.

MQ-9 ready for testing