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"The Chemistry of the Smell of Toilets & Human Waste" has a really nice overview of the types of gases you want to detected if you're after building a toilet-clean-air detector.

Hydrogen sulfide (H2S) sensors like MQ-136 are available for as little as $20 but what about the other gases? Isn't there a sensor that detects several of those faeces gases?

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  • 4
    \$\begingroup\$ For ease and cheapness you can't beat the human nose (well you can but it gets a bit bloody after a while) but if you wanted a quantitaive and qualitative analysis go with a hand held gas chromatograph \$\endgroup\$ – JIm Dearden Jul 31 '16 at 14:54
  • \$\begingroup\$ Cheapness, really? I've never seen those come cheap.. \$\endgroup\$ – Alex Meiburg Jul 31 '16 at 21:37
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According to this paper in 2000 (could have changed in 15 years!) there were no commercial sensors on the market specially built for this. However, the following were tested:

Figaro sensor type    most sensitive to: 
TGS2600               general air contaminants 
TGS2610               general hydrocarbons                         
TGS2611               methane natural gas 
TGS2620               alcohol and organic solvent vapors 
TGS825                hydrogen sulfide                           
TGS826                ammonia

Overall, though, they concluded: high sensitivity to humidity proved so vexing that the practical prospects for this approach were deemed discouraging. An alternative approach using the differential response of a matched pairs of sensors, with one of the pair equipped with a filter that traps fecal component gases and vapors, is now under investigation

I would think though that in 15 years time the sensor capabilities/stability/immunity to humidity may have improved. So you could check the Figaro website for sensors detecting the same chemicals as the paper and maybe get better results.

Instead of checking all of these individually, you might be able to just check for general VOC's.

Another slightly hokey report on baby diapers

And a patent for a fecal incontinence system

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  • \$\begingroup\$ The link to Figaro is helpful, thanks. TGS2606 looks like a good fit. The downside is that operates at 5V - good if you're on Arduino, a little less ok if you're on an ESP8266. \$\endgroup\$ – Marcel Stör Jul 31 '16 at 19:57
  • \$\begingroup\$ @MarcelStör: True. You could always just use a 1:2 resistive voltage divider if you don't need much current. \$\endgroup\$ – jbord39 Jul 31 '16 at 20:03
  • \$\begingroup\$ Did this really work? Could you reliably detect the smell with the TGS2606? \$\endgroup\$ – Eggi Nov 8 '18 at 6:33
  • \$\begingroup\$ @Eggi take a look at this H2S project \$\endgroup\$ – WhiskerBiscuit Nov 9 '18 at 4:05

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