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. [Editor from the future: link rot, seems to be here now.]

[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.

52 thoughts on “A Smart Clothes Dryer

    1. Popularity of clothes driers varies regionally. They’re very popular in parts of America where the climate is so humid that clothes hung outside mildew before they dry.

      1. I kinda wish I’d just said what I was thinking instead of dancing around the point: getting smug about your dryer-free lifestyle is about as obnoxious as going around telling everyone how you don’t own a TV. Also, 10 feet of line is not a hack. Can we skip the argument over the utility, efficiency, or desirability of electric close driers?

        1. No need to be snippy, I own a TV (with my XBMC box to get the free movies off the interweb) but I do love the smell of clothes fresh off the line, nothing like it, but I have to admit I never thought of areas where it was to humid to dry clothes before they mildew, that’s wild, and sticky! Oh and if you don’t think 10 feet of line is a hack imagine how it must have blown peoples minds when it was first tried “Now I can dry my pants when I wash them, once a year!”

      2. Nowadays, the way to make an efficient dryer (assuming a clothesline isn’t an option) is to use a dehumidifier to extract the moisture. It’s easily 2-3 times as efficient as a regualr dryer for the solid state (Peltier) versions, probably even more for the ones that use compressors.

          1. Ah, sensible, many launderettes (laundromats to non-English speakers) have these. Use them between washing and drying, saves a lot of time and money on the tumble dryer, if your clothes can take the pressure.

            They don’t usually do a complete dry, must be some extreme G to do that.

  1. Shatner-ized text alert. ;^D

    How well do these sensors tolerate the fine lint that will eventually build up on them?
    Assuming you are able to install with reasonable access, Is there a practical way to clean them/
    Due to air flow demands there’s not really a practical way to avoid the fines that will get past the lint screen of the dry exhaust.
    I actually set a kitchen timer at roughly 16 min first stop
    then 12 min intervals when I run the dryer as the lint trap/screen is
    typically about 60 percent covered with approximately 2~3 mm
    of lint.
    Some folks may find that inconvenient to do But I’ve found that this helps a lot with the drying efficiency.
    But some fabrics shed less than something like a load of 100% cotton towels does, so a pressure differential switch in the exhaust plumbing would be handy for setting a beep to tell me when the screen is getting restricted.
    The timer and an educated guess about the laundry works well enough But aren’t we here largely because we like our gadgets?!
    But I’m still thinking of the slow, fine buildup on the diaphragm.
    Too bad an automotive type filtration setup wouldn’t be practical
    for this (would clog terribly fast)

  2. Wow this is smart. Seems obvious when you think about it.

    I have a cheap dryer that came with my apartment which only does timed drying. I often find my clothes still damp if I do a heavy load even after the end of an 80 minute drying period. Maybe it’s time to do something like this. Definately needs the ability to text when the clothes are done.

  3. Last SEVERAL driers I’ve owned (moving etc) have had moisture sensors in them. Chose level of dryness and walk away and it turns off when it reaches that point. Some times it misses the mark and they’re still a tad damp but close enough. These were pretty cheap units too.

    1. Most of them do, interestingly enough, before they had the moisture sensor, they had a thermostat that would register above a specific set point. The logic was that since water had a large capacity for heat, the temperature of the exhaust would stay low until most of the moisture left the cloths, then as the vent temperature went up, the sensor would start the timer and that would finish it of, no programing required, just a good knowledge of psychrometric charts.

  4. There are two things every homeowner should do regularly to keep their dryer operating at its best (besides -not- overloading it):

    1) Clean the lint trap before drying any clothes (do this every time to use the dryer).

    2) If your dryer vents to the outside, make sure to clean the vent and any tubing on a regular basis (once a month or so).

    It’s that last one that most people forget about. The tubing or ductwork, and the vent leading outside, both can build up lint over a long period of time, leading to a clogged air path, which impedes airflow out of the dryer, and will leave your clothes “less dry” (sometimes fairly moist). Not only that, but by having such a build up, you can actually incur a “dryer vent fire” – which as you can imagine would cause a real problem for you.

    They make a brush on a long flexible piece of steel spring, called aptly a “dryer vent brush”; they are very cheap. Just clean your regular lint trap, then turn your dryer on to “cool dry” (no heat needed for cleaning). Run the brush from the outside dryer vent to as far toward the dryer as you can (you might have to twist and turn it to navigate bends and such), occasionally pulling it back and out; you won’t believe the crap that will come out with it. Sometimes you have to work as well from the “dryer side” (which means you’ll have to disconnect the vent from the dryer).

    Do these two things regularly, and your dryer should work properly and as designed. While I applaud the above hack for the hack it is, for the most part, dryers tend to work properly, provided they are kept clean and maintained, and aren’t overloaded. Generally, if your dryer isn’t working properly despite the above maintenance, it means there is something wrong with either the blower and/or the heating element (generally the latter).

    1. Has there been a documented “Dryer Vent Fire” in this century?

      How exactly can the air exiting a dryer reach flash point for “lint” and not first catch the clothes on fire first?

      I think this is one of those “stories” that hasn’t happened for a very very long time (like since the invention of temperature sensors).

        1. Used responsibly, no.

          Left on a charger, unmonitored by the owner and without proper fire shielding…?

          Well, I don’t go into a rage over it, but I have seen the aftermath reported more than once by somebody who didn’t take the necessary precautions, and it leaves me shaking my head. We’re talking homes burnt to the ground in some cases…

          We already know about the earlier LiPo incidents involving the Boeing Dreamliner – though that was likely due to manufacturing defects or something similar.

          Lithium chemistry batteries (not just LiPo, though they seem to be the more susceptible chemistry of all – but that’s just from my layman’s perspective) seem to have a bad habit of dying in a catastrophic manner when you least expect them to. Overall I’d say they are pretty safe, when used with a proper and well-designed charger (otherwise more laptops, cell phones, and tablets would be catching fire or exploding – though there are more than a few cases of swelling that have happened).

          The problem is many people play fast and loose with them, with the chargers they use, and don’t think about the potential consequences (for some reason, despite all of the issues being well documented). Most of the danger comes from R/C LiPo batteries and chargers, given that they tend to be cycled often, charge cycles don’t tend to be documented (so that you know when it is too old to charge), and batteries aren’t well matched to chargers (plus el-cheapo batteries with el-cheapo chargers). Some hobbyists do pay close attention, but some don’t.

          Then there are the crazy people who decide to cut into or otherwise mutilate a LiPo pack or cell – either in an attempt to make the pack or cell smaller, or for who knows what reason. These are the same kind of people who’d drill into a mostly empty steel gas tank with a carbon-steel drill, without a fire extinguisher or a friend nearby. Those guys are not to be trusted with tools of any kind…

        1. So it seems my suggestion of a rotating filter with a scrapper to remove lint build up not only keeps the dryer at max efficiency it also will help prevent fires… didn’t realise they were this common or that lint build up would cause them! Just need to a few more round tuits and a dryer so I can build a prototype.

    2. Is there anything wrong with, to my mind, the sensible and efficient solution, of having the vent tube end up in a bucket of cold water? You can buy attachments especially for this. The air bubble up through the water, and any vapour immediately condenses out and stays in the bucket.

  5. Monitoring the temperature of the exhaust may be worthwhile also. Since the clothes are wet, they will cool the hot air from the heating element due to evaporative cooling. As the clothes dry, the exhaust temperature should gradually increase to some fixed value.

  6. Hmm, every clothes dryer I’ve seen in the last 30 years or so – even very dumb low-end ones – has a moisture sensor that speeds up timer rotation when the clothes are dry. Of course I’ve only owned a few and in the US, so that may not be universal. But check the inside back wall of the dryer. See two metal contact strips surrounded by an insulator? It’s my understanding that’s the sensor.

    Assuming [Andy]’s dryer really doesn’t have a sensor, then this is a great idea. However, I have one concern. Fabric softeners and dryer sheets are loaded with hydrophobic (water repelling) agents. And these eventually coat everything exposed to the air inside a dryer, especially with dryer sheets.

    Try pulling out the lint filter and remove the lint by hand. Then while holding the filter horizontally, put a few drops of water on top of it. Instead of slipping through the holes as you’d expect, they remain beaded up on top. That’s the hydrophobic agents at work. This demonstration is often used to “prove” that dryer sheets ruin dryers to people who don’t understand basic science, claiming if that the water can’t pass through, then neither can air! Which is nonsense, yet many appliance repairmen and even Snopes believe this. You can remove the harmless coating by washing with soap and water if you want.

    The DHT11 is eventually going to get coated too. It’s more delicate than the sensors intended for use in a dryer, and eventually this may cause it to stop reading properly. And I doubt washing it with soap and water is an option…

  7. All these comments about cleaning filters every use made me think why isn’t the filter circular and rotating with a scrapper to remove the lint? Seems like another simple automation upgrade. I don’t own a drier, so won’t make this, but the first person to document this build online gets +10 internets ;)

        1. Short of implementing the genius Autohatic-5000 deluxe the lint bin would still need to be emptied, although less often. The benefit was that the drier would work closer to maximum efficiency throughout the whole lint build up/bin emptying cycle. You could also use a beam break to know when the bin was 80% full. Detecting the filter being clogged is harder, as although the exhaust air flow rate would change, that would also happen with different loads so not as good data.

  8. I’ve tested those humidity sensors in the bathroom with a shower running and no fan. It takes only a few minutes for the hot water to envelop the bathroom in fog, but it took almost 20 minutes for the sensor to slowly catch up.

    1. Unfortunately so are housing associations and apartment buildings that forbid line drying. Fortunately, we can get some fine devices that cost very little in terms of energy. My washer/dryer hybrid costs something like $14 a year in electricity. My computer uses more than that.

  9. The existing commercial “smart” clothes dryers come in two basic types – the most common have a temperature sensor in the exhaust (160°F in the U.S., if it matters) that starts a timer when the exhaust temperature climbs past its trigger point (indicating an increasingly dry load, per @Kevin). The “More Dry”/”Less Dry” setting on these is just a variation of the timer once it starts. These are essentially indestructible – the temperature sensors are fixed “Klixon” types that don’t suffer much from contamination unless your dryer is completely choked solid.

    The second type are “smarter” in that they monitor actual humidity and do effectively the same thing, looking for a shutoff level of RH rather than a simple trigger temperature. These can be (and the DHT 11 almost certainly is) susceptible to contamination problems, particularly from too many dryer sheets’ waxy goo.

    Learning to use a psychrometric chart can be very helpful if you’re trying to design something like this – we use the basic principles in many types of industrial product drying.

  10. If doing a retrofit to an existing dryer, cut the power to the heating element first, not the whole appliance, or go for an alert instead. It seems that you need to do that cold cycle that comes at the end of the program; I’ve been told this has to do with fire risk? Certainly the (Indesit) instructions make a big deal of it…

    Complete each program with the relative cold drying phase. (sic)

    Do not switch the tumble dryer off if warm items are still inside it.

    !WARNING Never stop the dryer before the end of the drying cycle unless all the items have been quickly removed and hung in order to dispel heat.

    (I drop this here just in case there is any truth in what I was told, could be a unwelcome unintended consequence of this hack.)

  11. Humidity sensors in clothes dryers is supposed to be old hat, available for many decades, slightly post WWII, I think. It’s possible you have one that fails to sense dryness.

  12. The smart clothes dryer, really seems to be smarter. It has become more essential for everyone to have an alternate to dry the clothes. And this one, could one such choices to go with for people who wish to be economical, safe and inventive.

  13. When writing an article that links to another article, it would be really helpful to save them in the internet archive and link to that version alongside the live link to prevent link rot.

    E.g. “A DIY Clothes Dryer (Internet Archive Link)”

    Saving is dead easy. Just go to web.archive.org/save and paste in the link. If you’re logged in, you can additionally choose “Save Outlinks” and capture all the places *that* site links to and prevent link rot even further.

    Not only does this help keep links working in an age where link rot is rife, but is also crucial when linking to e.g. code projects on GitHub. At time of writing, you can verify the code works and is not malicious, but this may not be true at any given stage in the project’s future. Linking to the IA copy gives you a trusted set of code to refer and link to.

    E.g. link to this page:


    E.g. link to the new copy of the previously-rotted link, in case it rots again:


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