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Topic: Temp/Humidity Sensor - Best value for this case? (Read 6436 times) previous topic - next topic


I want to track temperature and humidity in several locations around my house. I've seen the SHT15 and the DHT22 over at sparkfun. The DHT22 is clearly less expensive and as I understand it a little less accurate, but since I may end up with 5-10 sensors it is significant.

I'd like to have these sensors strung out around the house with Cat5e run to each of the sensors None of the runs would be over 50'. I'd like to minimize the number of pins I use on the Arduino as I'd like to also use it for other duties around the house including a control panel which is likely to demand a lot of pins with LEDs and an LCD screen, along with ethernet network access.

Can the DHT22's unique 1-wire-ish interface be daisy chained (like the Dallas 1-wire stuff)?
Will the DHT22's accuracy remain constant or drift and will it be consistent across a variety of DHT22s?

Should I expect problems with the SHT15's 2-wire interface across 50' of twisted pair?
Any chance the SHT15's permit daisy chaining?

Is their any amplification inherent in the daisy chaining process? In other words, do I need to consider the longest run between sensors or the run from the furthest sensor to the Arduino?

Is there another temp/humidity sensor available that would work better without breaking the budget, perhaps a Dallas 1-wire sensor?
What does it take to weatherproof these for outdoor use?
Are there other questions I should be asking of these sensors?


(Don't have SHT15's)

I've never used DHT22's with cables longer than 1 feet. You probably need pullup resistors to improve the signal.

Can the DHT22's unique 1-wire-ish interface be daisy chained (like the Dallas 1-wire stuff)?

No, you need one wire per sensor (and two for GND and +5V but that can be shared), the Dallas thingies have an unique address. An UTP cable with 8 wires could in theory support 6 devices. => place the Arduino in the middle to minimize cable length?

Will the DHT22's accuracy remain constant or drift and will it be consistent across a variety of DHT22s?

I have two of them and I recall they were pretty close when I tested them (for DOA) when they arrived. Never did a range test.
Most important to know is that the DHT22 are relative slow. Think in the order of 1 sample per 2 seconds, and they take some time to recover.

Is there another temp/humidity sensor available that would work better without breaking the budget, perhaps a Dallas 1-wire sensor?

For temperature the Dallas DS18B20's are quite good. I'm not aware of a Dallas 1wire humidity sensor.

What does it take to weatherproof these for outdoor use?

You need a roof on a pole so rain (even with wind) cannot  touch it, somethingh like

          ||  S

The S is the sensors

Are there other questions I should be asking of these sensors?

Do you expect the humidity to be much different in the various locations? Do you really need 10 of them? I can imagine that 1 outside and maybe 2 inside (living & kitchen) may be enough.   And use temperature-only for the other locations, as temperature may vary more than humidity (I assume)

Rob Tillaart

Nederlandse sectie - http://arduino.cc/forum/index.php/board,77.0.html -
(Please do not PM for private consultancy)


Hi Flickerfly,

I don't have experience with either of the humidity sensors you're talking about. I went with the HIH-4030 breakout from Sparkfun. I just wanted to let you know that 5-10 sensors is probably overkill and a potential headache. Everything I've read about humidity sensors is that you shouldn't *ever* expect more than two sensors to agree about what the humidity is, even if their right next to each other. So having 5+ would be worse obviously. That is, if you want to get an "actual" humidity percentage reading. If you only care about reading that are relative then you might be fine with more than one or two. And by relative, I'm meaning that if you only care about humidity trends of up or down, and by how much. Hmm, maybe I'm still not being clear. Say sensor one is 50%, and sensor two is 54% and they're sitting right next to each other. If you put a cup of steaming water exactly between them they might both jump up to 70% and 74% respectively.

The other reason more than one sensor might be overkill is that my experience with this single sensor is that it's very sensitive. My sensor is through three doorways, down a hallway, and around four corners from my shower (about 40 feet total). But it still easily shows an obvious increase in humidity after every shower we take. It also shows an obvious drop in humidity any time the HVAC (heating or cooling) is active. It will even show an increase in humidity when it's raining outside.

For reference, my home is only 7 years old and pretty well insulated. I'm in Northern California, near Sacramento. And I've got about 2000 square feet, and the home is single story.

I do have 8 DS18B20 sensors throughout the house, and one outside. They seem to be the sensors I pay more attention to. But to each his own. I certainly wouldn't *mind* having multiple humidity sensors (including one outside), but it's currently something I wouldn't spend my money on.

I can't speak to the wire length question you have for the humidity sensors since mine is only a couple inches away from the Arduino. But the DS18B20 sensors work pretty nicely over the CAT-5E wiring in my house with distances up to about 60 feet. I wired them up using the existing wiring that the home builder put in for phone/network. Most of my rooms had a spare pair of wires in the CAT-5 so I used them in parasite power mode. The topology is star, aka. home run. I did have to create two busses for all the sensors to make it more reliable though. I think you'll be fine with your 50' run.

I've bought a second HIH-4030 to use outside, but haven't put it in yet. My plan was to put it inside a plastic outdoor electrical box to protect it from the elements, then put it under a roof eave. I thought that having a hole in the plastic box would allow moisture in, but also bugs and dust. So I thought I'd get a small bit of Tyvek house wrap (or envelopes) over the hole. Tyvek is designed to let water vapor through, but not water drops, or hopefully bugs or dust. A small bit of metal screen over the hole would keep out bugs that might be able to bite through the Tyvek. My box is also supposed to hold an Arduino clone (the Dorkboard) in it, so I want that extra protection. Two holes, or a larger hole might give better air flow.

I hope this helps. Have fun in what ever you end up doing!


I don't see the point in checking humidity around the entire house, unless you have more than one level. Although, one of those things in a bathroom if you don't have a big opening to let the steam out would be quite nice since you could measure how long it takes for the humidity to drop and if not, how could you improve it to stop any kind of humidity related problems to happen.

So cost wise, two or three humidity sensors (one would be outside the house so you could have a reference to know how much the weather afects your house) seems about enough. One thing you could think about is creating wiring for the humidity sensor in more locations so you could log the humidity in different areas of the house with the same sensor. Chances are that the, let's call it, humidity profile of the house isn't going to change over night, so you could still get a pretty accurate idea of how your house behaves humidity wise in different conditions. And, later on, if you see fit, you could increase the sensor network.

On the other hand, the DS18B20s are pretty good and quite resistant too... not that it matters but they can stand quite a beating. I had a few inside drying concrete and they managed to survive for about 8 hours or so. LOL
And for the purpose needed, they are faster and cheaper.
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The sht15 are nice humidity sensors, and the temperature from them seems accurate also.  But they are not cheap.  I also use the dht11 at about 1/10 the cost.  I don't use them for temperature, but the humidity seems accurate. I think the dht22 gives you a more accurate decimal point than the dht11, but I wouldn't make any decisions based on fractional humidity readings.



you may want to check out this product, if you plan on buying a bunch:


(I guess its an older version, but its also only a quarter of the price...)

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