Bathroom fan that switches itself on when it gets steamy or smelly

At first we thought that [Brandon Dunson] was writing in to tell us he’s too lazy to fix his bathroom fan. What he really meant is that simply replacing the unit isn’t nearly enough fun. Instead, he developed his own bathroom fan trigger based on stinky or humid air conditions. He didn’t publish a post about the project but we’ve got his entire gallery of build images after the break.

The initial inspiration for the project came from a twitter-connected fart sensing office chair. Hiding behind the character display you can see the MQ-4 methane gas sensor which he picked up for the project. But since there’s also a shower in the bathroom he included a humidity sensor with the project. Both are monitored by an ATmega328 which averages 10 readings from each sensor before comparing the data with a set threshold. If the sensors read above this level a relay turns on the bathroom fan.

Don’t be confused by the small DC fans seen above; [Brandon] is still using a proper exhaust fan. These are just used to help circulate the air around the sensors so that low-hanging smells will still trigger the system. This has got to be the perfect thing for a heavily used restroom.


  1. just me... says:

    He jacked my idea!

  2. omegacs says:

    I’ve actually wanted to figure out if it’s practical to put a couple very thin intake vents under the hinge end of the toilet seat itself, with a variable speed exhaust fan. Keep it on very slow to pull air into the system and sense methane, and go full speed when it does. Capture things at the source….

  3. Hirudinea says:

    Not only is this the shit, it also smells it! Maybe he should consider attaching an automatic air freshener to the system, well for methane anyway.

    • ameyring says:

      Actually, people can’t smell methane (which is why they add sulfur-based compounds to natural gas). The smell of flatulence comes from hydrogen sulfide and other odorous items our body produces.

      About the hack, because I don’t have a bathroom fan, I was thinking of making a wood frame, mount some computer fans, and add a timer. When someone leaves the bathroom after a shower or crapping, press 5 or 10 min and the fan will do its job, exhausting out the window like a small window fan. Now I like the idea of using a moisture sensor so I can prevent mold from building up in the shower!

  4. Dr. DFTBA says:

    Main problem I see is that not all people produce methane.

  5. Gizmos says:


    There is a commercial product that is an exhaust fan that fits on the tank and under the lid. Draws gas out through the overflow tube. Usually wired to be on when the light is on.

  6. Sanjay says:

    Is an MQ-4 sensor rated for humid environments ?
    I think the sensor will be ruined in a day or two.

  7. fartface says:

    His methane sensor will be bugered within a week. almost all Gas sensors are damaged by high humidity… Sounds like someone did not read all his data sheets.

  8. Smonson says:

    At a backpacker’s hostel I once stayed at in Los Angeles the ensuite exhaust fan came on as soon as you turned on the light, and stayed on for about five minutes after you turned the light off. That seemed to work pretty well.

  9. Pamela says:

    Man, this guy is amazing! Come put one in my husband’s bathroom!

  10. wolfy02 says:

    this is great! was wanting to build something like this for my cats litter box. now I just have to vent it outside…

  11. Galane says:

    Only about 1/3 of humans produce methane in their colon. Almost all of us produce hydrogen sulfide. Should use a detector for that instead of methane.

    BTW, only the methane farters can light theirs on fire.

  12. wardy says:

    I know lots of circuits have brownout detectors, but this is ridiculous. :D

  13. qwerty says:

    New product: a fart detector that drives a wave player with laughs and/or cheering crowd sounds :)

  14. rasz says:

    Just wire fan to bathroom light = done.

  15. FrankenPC says:

    This may very well be the best hack ever devised.

  16. mbear says:

    Just a stream of photos with no information? This is neat, but I was hoping for more.

  17. James Newton says:

    Very nice idea! This inspired me to replace a failing bathroom fan with a computer fan. I’d been looking for and not finding the right size standard 110volt fan. This made me realize I could install a 12 volt adapter and a computer fan instead. It’s not automatic yet, but the humidity sensor and controller are on the way from

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