I made a set of experiments using this sensor and wrote an instructable with schematics, code and calibration procedure that can be, with some complications, performed at home: https://www.instructables.com/id/Arduino-CO-Monitor-Using-MQ-7-Sensor/ . Code is relatively complicated since we need to drive transistor with PWM in order to create 1.4V on the sensor - we must use 5V / 1.4V cycles for heating and measuring data. But my results are quite similar to manufacturer's datasheet, so I believe I did it mostly right

Kind of frustrating how many places say "just hook it up to 5V", when you can't do that with this sensor. You really don't want the breakout they're selling with this sensor either. If you look at the test conditions in their datasheet, they gave the sensor itself 5V, and had a separate lead for the heater, giving it a 60s@5V, 90s@1.5V while-taking-readings cycle. You'll get lousy results if you "just give it 5V", and you'll get very low voltages if you use the breakout and take readings while only giving it 1.5V. The gas monitor tutorial does a much better job describing that. For my circuit, I have a 5V rail, a separate 1.5V output buck converter, and a relay switching between the two. The sensor itself always gets 5V, the heater on the sensor gets switched between the 5V and 1.5V as specified, I wait 30s into the 90s read cycle to start taking readings then take them every ten seconds from 40 to 80. If you read the datasheet, that's how this is supposed to work.

i have to sense the carbon monoxide level of vehicle.could this CO sensor sense that concentration .because according to my knowledge,exhaust co concentration of vehicle is about 30,000 ppm ou of new fresh vehicle.so give me solution

We made a cool project with this sensor, which we connected with Arduino Uno to an RGB LED that changes color according to the amount of carbon monoxide in the air. The sensor itself is connected to a helium balloon. You can find the project here >> http://www.instructables.com/id/Red-Balloon/

I'm unclear on how to operate this. I hooked it up to 5V and an Analog pin and am getting readings just fine, but then I'm reading that it's better to cycle the Power using a PWM pin. Can somebody clearly outline a best practice for operating the sensor? Will keeping it on 5V damage the sensor?

Is the accuracy of the meter dependent on atmospheric density? Like really low pressures. Also anybody know a good (and affordable) CO2 sensor?

"Description:...and can be used with the breakout board below." There is no breakout board shown below. Please update the description then delete this comment.

how do you convert the analog read to a pollution value? does anyone have an example with some threshold values? thanks

I brought this sensor about a year ago and I haven't figured out what to do with it, is a complex piece of hardware, the procedure to calibrate and adjust this device are just ridiculous complex, you would require to generate fixed and measurable amounts of C0 in a test chamber to effectively calibrate this module, not only that but you'll need additional sensor data to properly calculate deviations caused by humidity and temperature.
Buying this product as is makes no sense at all, and I warn everyone please use proper CO alarms in your home/office ! Sadly I know a lot of people who have died because of CO intoxication.

It looks like the manufacturer, Hanwei, also makes many other sensors. Could you carry ones for O2 and H2S specifically? I'd like to make a personal air-quality monitor.

Pay attention to the datasheet for this sensor. When it says 5k load resistor minimum, it means it. Also, you need to switch this unit from a low heat voltage(1.4v) to a high heat voltage(5v) cyclically to achieve the desired results. In my testing I found that just leaving the heater on high gave diminishing sensitivity until I turned off the heater for a few minutes.

Is this sensor safe enough to detect CO in car exhaust fumes ? If not, what could be an alternative ? I am asking this because, the exhaust temperature is probably beyond operational limits as mentioned in the datasheet.

Has anyone attempted a project to text exhaust fumes from a car ?

Used the Arduino Uno and an adjustable voltage regulator with an optical solid state relay to drive the MQ7 and read its output. More in my blog at blog . Enjoy!

upto what temperature it can operate? does it can be used for continuous monitoring

What does that “preheat” for 24h mean? - Do I leave it 24h with 1.4v? (28% PWM) - Do I cycle it for 24h (1.4/5v) and WHY? Is preheat period needed only for the so called calibration or for usage also? (these are the same questions as some post above)...any answers ? Thanks for your time

is there a wiring example for MQ-7??

Does anyone know how to interpret the output of this sensor? I follow the connection in the Wiring example. And the output is integer values I believe ranging from 0 to 1023. However, how to convert this reading to the PPM values? Thanks!

Any one had used it with netduino?

What happens if moisture gets in the screen? Will the heater coil take care of it?

Is it me or did the price go up from $4.95 to $7.25?????

I just got this baby from the Greek sparksfun partner. One question only : what does that "preheat" for 24h mean?
- Do I leave it 24h with 1.4v? (28% PWM)
- Do I cycle it for 24h (1.4/5v)
and WHY?
Is preheat period needed only for the so called calibration or for usage also?
Thank you for yout time in advance for answering this

I would like to see a breakout board for this sensor that provides a calibrated value via I2C or SPI or something.

I have the MQ-7 and I only see a wiring diagram for the MQ-135. Looks similar. Also, I can't really see any indicator that shows A vs. B sides -- they look symmetrical -- can they be wired either way. What does it mean to heat for 48 hours? Is this for calibration? Appears that the sensitivity is 200ppm, which is pretty high -- for firefighting, we measure in the single digits (I'm making my own CO detector for our department). Is there a better sensor out there?

What about giving it a lower DC voltage through a linear regulator instead of the PWM?
And what about the settling times?
How much variation is to be expected between devices?

Driving the heater with the suggested 5v/1.4v rectangular produces odd spikes in the sensor output, making the gas response signal hard to identify. I've found that a slow sine heater drive, via PWM, yields a much more sane output. Here is a 120-second heater cycle between 2v and 5v, and a very small Hydrogen release. 10k load.
http://rightime.com/images/Misc/CO_Sensor_Hydrogen_3.GIF

Here is a better Hydrogen test, two small releases (no guess in PPM), which I then allowed to dissipate quickly. This is with a 10k load. http://rightime.com/images/Misc/CO_Sensor_Hydrogen_2.GIF

CO sensor response to rubber cement (apparently n-Heptane): http://rightime.com/images/Misc/CO_Sensor_RubberCement.gif

dim CycleHeaterTaskStack(1 to 40) as byte  
const pinGas as byte = 13  
const pinPWM2 as byte = 25  
sub Main()  
 dim sV as single, sX as single  
 callTask "CycleHeaterTask", CycleHeaterTaskStack ' start heater cycling  
 call PutPin(pinGas, bxInputTriState) ' set pin for ADC  
 do  
  do until (register.rtctick mod 512) < 5 loop ' 1Hz sensor sample rate  
   sV = csng(register.OCR2) / 255.0 * 5.0 ' heater voltage  
   sX = csng(GetADC(pinGas)) / 1024.0 ' get sensor  
   debug.print fmt(sX, 3) &" "& fmt(sV, 2) ' display  
  loop  
 end sub  
sub CycleHeaterTask()  
 const iHighVolts as integer = 255 ' 100% PWM = 5v  
 const iLowVolts as integer = 71 ' ~0.28 * 5v = 1.4v  
 const Rate as byte = 1 ' PWM 14400Hz  
 call PutPin(pinPWM2, bxOutputLow) ' PWM pin to output  
 register.TCCR2 = Rate or bx0110_0000 ' init PWM  
 do register.OCR2 = cbyte(iHighVolts) ' heater 5v  
  register.rtctick = 0 ' for 60 seconds  
   do while Timer < 60.0  
   loop  
  register.OCR2 = cbyte(iLowVolts) ' heater 1.4v  
  register.rtctick = 0 ' for 90 seconds  
   do while Timer < 90.0  
   loop  
 loop  
end sub  

Not surprisingly, this device runs warm. The documentation specifies that the heater should be cycled between 1.4v (drawing ~30mA) for 90 seconds and 5v (140mA) for 60 seconds. The internal temperature is only slightly above room ambient at 1.4v but quickly climbs to 150F(66C), then settles at ~175F(80C), at 5v; the plastic body temperature reaches 117F(47C) in ~75F free air. This is not a low-power part.
I've found that cycling the effective heater voltage is easy via 28% and 100% PWM and a VN2222 FET from 5v.

If I read the datasheet correctly, this device is actually a better H2 sensor than it is a CO detector! The heater gives me pause, though, for that application.