These sensors, made by Hanwei in China, are super popular online – Sparkfun carries them, as does Polulu and several others. They retail for about $5 by themselves, and a breakout can cost up to $10. The MQ gas sensor series are great – they generally require 5v input and output an analog voltage – making them super easy to use. There are models that measure Methane (MQ-4), Liquefied Petroleum Gas (MQ-5), and even Alcohol (MQ-3) among several others.
However, the Carbon Monoxide model (MQ-7) is unique in that it does not simply require a 5v heater voltage. Reading the datasheet, you’ll see it requires a power cycle: 1.4v at 90s, and 5v at 60s and repeat. From the datasheet:
There are a lot of ways to go about accomplishing this. However, the datasheet makes no mention of current – just voltage. After a lot of testing, it became apparent that this guy needs quite a bit of current to take readings at all. This is because of the nature of the sensor. Inside, a small filament is stretched between all pins. This filament is coated in a chemical that is sensitive to a particular gas, in this case CO. When it is exposed to the target gas, the resistance of the filament itself changes, and so does the output voltage (Signal).
I made two different boards to accomplish this in the easiest way possible. In one version, an adjustable voltage regulator is used to get a 1.4v power line. A really tiny relay toggles between that voltage and the 5v from the Arduino circuit. The other version uses 4 diodes, in series, to drop the voltage down (it went about 1.2v each time). These are 4148 300mv diodes. In both boards, a single digital pin can be used to toggle between the voltages – just send the pin LOW, and you get 1.4v, send it HIGH, and 5v is pumped to the sensor. I will be releasing timer-based sketches with the board so that you can be doing whatever you’d like in the sketch, and don’t need to worry about the toggling – it will happen in the background. Check out the completed boards — these are barebones PCB prototypes.
You can download these prototype schematic and EAGLE PCB board files here.