MQ136 Gas sensor

Hi everyone, I am going to do a project detector to detect bad breath from children. I found that bad breath was generally caused by Volatile Sulphide Compounds (VSC). I would like to ask can I use MQ136 gas sensor to detect the sulphide gas?

The sensor’s data sheet says it detects H2S gas. A quick Google search tells me that VSCs are mostly hydrogen sulphide (H 2 S), methyl mercaptan (CH3SH) and dimethyl sulphide ((CH3)2S).

This publication about VSC in breath give a relation between the various compounds, not sure if that applies to your breath samples as well. H2S could be a reference for the whole shebang.

Point to note: those researchers measured concentrations of 100-300 ppb. The sensor you’re looking at measures 1-100 ppm, so the minimum of that sensor is an order of magnitude above the typical concentration of H2S in morning breath.

So based on this quick research (really, why I’m doing your homework?) the answer is no.

Thanks for the reply. I have searched for another sensor to detect halitosis, which is 3SP_H2S_50. Can anyone

give your valuable opinion? I not really know how to integrate the sensor with Arduino. Thanks.

Complete documentation on this family of sensors here. How to read it and driver circuit you can find in this document.

A quick search on “3SP_H2S_50 breakout” gave no useful results so I’m afraid you have to build your own PCB for this sensor. The driver circuit is based around a dual OpAmp with a few resistors and filtering caps. It should be pretty straightforward to build a driver for this sensor. This can then be read from the analog port of your Arduino.

According to the spec sheet it has a range of 0 to 50 ppm, and a resolution of < 5 ppb - instrumentation dependent. 250 nA/ppm so in your case 100 ppb = 0.1 ppm = 25 nA of current. So now we have a problem - the input bias current of the LM358 is 20 nA. So you need a better opamp for that - such as the TLC072 which has a typical input bias current of 1.5 pA, max 50 pA.

All that’s left now is some close study of the data sheet & suggested driver circuit to choose good resistor values, and to start building.

Of note: the 10-bit resolution of the Arduino’s analog input is not enough for this sensor to get to the 5 ppb resolution (which you have to aim for, as you expect to measure 100-300 ppb ranges) so you will have to add an external ADC such as the 16-bit ADS1113.

Thanks for the reply once again. So I have contacted the seller and they suggested me to use this driver circuit…Any idea?

driver_circuit.pdf (25.8 KB)

Did they also give part numbers for U1-U4? Those appear to be missing from the schematic. Without that, can't say anything sensible about it. Also several resistor values are not given. There are a few 0 Ohm resistors such as the R12 bypass resistor at the regulator. This looks like a schematic for a generic production board where the user can remove that resistor to enable the regulator.

U1 is a regulator, best guess to provide 3.3V but need specs.
U2/U3 are OpAmps, at least U3 I guess will be a high impedance input one.
U4 seems to be an ADC but it's not connected to anything, really doesn't make any sense to have that part there at all.
What is the value of R2 and what are switches XY1 and XY2 doing there? R1 an R3 are unusually high values for a voltage divider.
What is the purpose of J11, J12, J13?

At a glance the main difference with the reference circuit as given in the spec sheet for the sensor is that they use a double OpAmp stage on both the reference/counter and output sides instead of a single stage. I don't know why that is.

May I know where the driver circuit given in the datasheet? I referred to this http://www.spec-sensors.com/wp-content/uploads/2016/10/3SP_H2S_50-C-Package-110-304.pdf , but did not see any driver circuit inside.

Typo for the link in #3, hereby the correct one (and I also edited #3 to have it work as intended). On the product page one link below the one you looked at.

Well, I was suggested by few parties now to use sensor with driver circuit board built in such as:
DGS-H2S-968-036 , a sensor with Simple Digital UART Interface OR
ULPSM-H2S-968-003 , a sensor with 0 to 3 V Analog Signal Output

(of course higher price for these sensor). Do you suggest me to use the built in board or develop my own driver circuit? (just seeking for opinion).

I would go for a breakout board any time. Much easier.
Why bother designing and building and testing your own circuit, when you can buy a tested, built, and known good one. Costs a bit more now, but saves lots of time (and cost) in the long run, and allows you to concentrate on what you actually need to do: use the sensor to measure bad breath.

I have found another sensor with cheaper price, which is Figaro TGS 2602 sensor. But the datasheet says that it detect 1 to 30 ppm of EtOH…I am not really sure whether this sensor can be used or not. I found this link which explain how the detection of H2S. Also, I found someone used this sensor to build bad breath detector…here…So I am confused now.

It seems you have narrowed it down quite well - why don't you just buy one or two of those sensors and see whether it works for your project?

The seller just emailed me and said TGS2602 cannot be used for bad breath...only for indoor air quality control...

I wouldn't take a seller's word for it - they probably don't know much about the sensor. Ask the manufacturer, and indeed experiences posted online are another good source of information - as long as you realise that this may be wrong of course, be critical.

Indoor air contamination I expect to have lower H2S levels than bad breath for reason of dilution in the air.

In the product info sheet of the sensor the manufacturer shows a chart showing a curve for H2S starting at 100 ppb (without a clear lower limit), which would be well within range of what you expect in bad breath.

I think best you can do - as I said before - is to buy one of each of the sensors, and try them out. For what I can see from the specs and the expected concentration they both should work. See what works best for you. At least I do assume you have a bit of a budget to tinker and try things out!

I bought the sensor...Btw...While waiting for the arrival, I just found that my topic is to build a wall mounted breath odor detector for children to check their mouth after brushing teeth, and give interactive feedback... Actually this is my final year project as degree student...I am thinking to find few more sensor to sense content found in toothpaste ( fluoride for example ). Do you think is it relevant to use temperature and humidity sensor to sense mouth water vapour concentration as well?

sheldon:
Actually this is my final year project as degree student...I am thinking to find few more sensor to sense content found in toothpaste ( fluoride for example ).

That's not a gas. At least I'd hope it's not, or you may run out of students rather quickly. So you shouldn't be able to detect it in their breath; except maybe through droplets that have the fluoride dissolved.

Do you think is it relevant to use temperature and humidity sensor to sense mouth water vapour concentration as well?

You expect it to be much different from 37°C and 100%? It's your project...

So I have decided to use 5 different Figaro Sensor, including TGS2602, TGS2600, TGS2620, TGS825 and TGS2180 to detect the breath odor... I am not sure whether it works or not...at least hoping for some results...just afraid these 5 sensors not suitable to sense human breath...

Sheldon, can you share your results with those sensors? I'm thinking to build one for our family members.