Humidity Sensor Shootout

If you want to measure humidity (and temperature, and maybe even barometric pressure) in a device that you’re building, have a look at this comprehensive test of seven different options. We’re going to summarize the results here, but you’ll really want to read up on the testing methodology — it’s great science hacking. Did you know about using saturated salt solutions to produce constant humidity levels for calibration? We didn’t.

The eBay hacker favorite, the so-called DHT22 module, doesn’t fare all that well, with one of six that [Robert] tested being basically horrible, and three of them breaking within two years of use. The one that works well, however, is pretty good. Feeling lucky?

The Bosch BME280 looks great. It costs a bit more as a bare part, and a few times more than that when it is mounted on a friendly module, but it seems to be very reliable. And you get a barometer thrown in for the extra work. Indeed, it performed so well that Hackaday contributor [Nava Whiteford] put the part under a scanning electron microscope to figure out what’s going on.

The other sensors were fine, with the HTU21D and SHT71 being standouts for their ultra-fast response. For the full details, go click on that link at the top. Having just installed a sextet of DHT22s in our house last year, we’re left with that sinking feeling that we may have gotten what we paid for, which wasn’t much. At least they’re all still running.

Thanks to [Dodutils] and [mac012345] via comments in another thread.

A Smart Clothes Dryer

dryer6 Here’s a question that will rack your brain: does your clothes dryer stop when the clothes are dry? It seems if you have a machine that guzzles power for one single purpose, you’d like it to stop when its job is done, or for the sake of convenience, keep going until the clothes are dry. Temperature and humidity sensors are cheap, and if you don’t have an auto sensing clothes dryer, a DIY smart clothes dryer seems both efficient and convenient.

[Andy] figured when clothes are dry, they stop emitting moisture. Based on that premise, he could monitor the operation of a clothes dryer and either shut off the machine or send a message that it’s time to take the clothes out. It’s a simple enough idea, and with an Arduino and a DHT11 temperature and humidity sensor, it was pretty easy to put together.

The clothes dryer used for this experiment was a self-ventilating model that doesn’t vent to the outside. Instead, it condenses the water in your towels and jeans into a tub to be emptied by hand later. This might introduce a little error into tests, but [Andy] did come up with a way to mount the temperature sensor without modifying his dryer in any way. From the initial data, the ventless dryer might be introducing a little experimental error, but it’s still too good of an idea to not try out with a traditional dryer that vents to the outside. Here’s the code should you want to try this yourself.

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.

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