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Author Topic: Need help on choosing temperature & humidity sensor  (Read 4782 times)
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blacknight72
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« on: January 05, 2010, 04:43:04 04:43 »

My application needs to measure both temperature and humidity. I am looking for a suitable sensor for this.
Do you have any suggestion? (I am currently choosing SHT75 of Sensirion, but don't know it's good or not)
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kayvee
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« Reply #1 on: January 05, 2010, 07:46:59 07:46 »

It looks to be a good device, however at 35 bucks a pop it's not low cost.

If it fits your budget for the project, great, it's a nice simple interface without getting involved with all the analog signal pre-conditioning circuitry and accuracy headaches.  That's what you will be paying the premium for.  It's also very compact.
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tAhm1D
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« Reply #2 on: January 05, 2010, 10:32:56 10:32 »

Hi,
If price is an issue, you can look at HS1101, costs around $7 each.
What are you using for temperature, DS1820, LM35, LM335, TC72 or something else? Easiest would be LM35/LM335 as they give linear output.
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Sepiroth
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« Reply #3 on: January 05, 2010, 11:46:16 11:46 »

hai there, imo first thing you should do is write down your requirements, i.e budget, temp & humidity range to be measured, how you will interface it and to what (i.e uC, any other DAQ device etc.) But idd, a digital output sensor saves a lot from extra hardware. for temperature sensor, if you are fine with +/-1degree precision (which i suppose you need more accuracy than that), go for LM35. DS1820 is another nice choice with 0.5degree precision.
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sherm
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« Reply #4 on: January 05, 2010, 03:32:41 15:32 »

You haven't told us how you want to read the data being output in your circuit so any response is going to be based on a reasonable amount of speculation.

Since you state you're already using a SHT75 I'll speculate that you're using a microcontroller of some sort to process the data.

Firstly, the SHT75 is very good. It's a top quality unit and it's reasonably easy to work with using a simple two wire interface. There's a lot of information for different microcontrollers on how to interface using two wire. The programming platform you're using will also probably have some information in the help files on how to use it.

Pros: It's all housed in a single, reasonably compact, unit that can be easily used on a prototyping breadboard for rapid integration into your project.
Cons: It's expensive compared to some other avenues and has a slow response time (around 8 seconds for RH)

You could look at the (slightly) cheaper SHT15. It still uses the two wire interface, but is specced a little differently.

Pros: Small footprint IC with fast response time (typically less than 4 secs for RH.) Cheaper than the SHT75 (about US$28.)
Cons: Small footprint IC. You'll need to add it to an adaptor to use with a prototyping breadboard.

If money is an issue then you can reduce your prototyping project costs by using a separate RH and temperature sensors. The caveat here is more wiring (usually) along with a little more coding for the microcontroller as a general rule.

Depending on the accuracy you need, you can cover the temperature side of things with a Dallas DS1820 series device. There's a fair few different models in this range so you'll need to do some homework to find which is right for your project. I think Dallas still give away free samples of this device once you register with them for a samples account. Otherwise you're looking at about $1.50 each. These use a 1-wire interface and you can find plenty of example code on how to use them for a plethora of different platforms.

For the RH side of things you can use capacitive response sensor like a HS1101LF. This will need additional components (a 555 timer) in circuit however to deliver a result. Still, that shouldn't break the bank (about US$14 for the HS1101LF and under a buck for the TLC555.) Now you will require a lot more code using this method. You'll also need the necessary algorythm to calculate the RH in your circuit.

Personally, I prefer splitting the sensors. It gives you a lot more control over how you derive your results. The best location for the temperature reading is not always the best place for the humidity reading. Splitting them apart gives you the best of both worlds and the most accurate results.

In brief, if you already have the SHT75 in stock then continue to use it. It's a good device. Basically, if it's doing what you want then you may as well stick with it. If you haven't committed to it yet however you might want to experiment with a few other devices until you find the best solution for your needs. 
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blacknight72
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« Reply #5 on: January 07, 2010, 04:42:35 04:42 »

Quote
Firstly, the SHT75 is very good. It's a top quality unit and it's reasonably easy to work with using a simple two wire interface. There's a lot of information for different microcontrollers on how to interface using two wire. The programming platform you're using will also probably have some information in the help files on how to use it.

Pros: It's all housed in a single, reasonably compact, unit that can be easily used on a prototyping breadboard for rapid integration into your project.
Cons: It's expensive compared to some other avenues and has a slow response time (around 8 seconds for RH)

You could look at the (slightly) cheaper SHT15. It still uses the two wire interface, but is specced a little differently.

Pros: Small footprint IC with fast response time (typically less than 4 secs for RH.) Cheaper than the SHT75 (about US$28.)
Cons: Small footprint IC. You'll need to add it to an adaptor to use with a prototyping breadboard.

According to the information on Sensirion website, I find that SHT15 & SHT75 nearly have the same specifications, except for size, mount type and price. (SHT15 also needs 8s for RH response).
At first, I'd like to use SHT15 but like you said, it needs an adaptor, and it's difficult to find a place for it. My board will be in a metal box, and the temp & humi values need to be measured belong outside (environment). So I think SHT75 is better choice when I can connect it to my board by 4 wires.


I haven't had SHT75 yet. I am trying to buy it from foreign countries. So I will find out more about other devices as you recommended.
Thanks.
« Last Edit: January 07, 2010, 04:44:47 04:44 by blacknight72 » Logged
janakfun
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« Reply #6 on: January 08, 2010, 01:10:34 13:10 »

Hi,

As per your application and requirment choose the sensor.
For low range of temperatures youcan go for LM35 as suggested by other friends..
If you want to measure high temperature than go for PT100 temp sensor
or you can use NTC type thermistor JRS 7392 of GE.

Well aal depends on how much you want to measure..
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Regards,

Janak
sherm
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« Reply #7 on: January 09, 2010, 10:25:00 10:25 »

According to the information on Sensirion website, I find that SHT15 & SHT75 nearly have the same specifications, except for size, mount type and price. (SHT15 also needs 8s for RH response).

You're quite right about the response time. Sorry for giving you a bum steer there!

Be careful about the box you choose too. There's a whole section in the data sheet about how metals and sealants effect the response times of the devices:

"1.8 Materials Used for Sealing / Mounting
Many materials absorb humidity and will act as a buffer
increasing response times and hysteresis. Materials in the
vicinity of the sensor must therefore be carefully chosen.
Recommended materials are: Any metals, LCP, POM
(Delrin), PTFE (Teflon), PE, PEEK, PP, PB, PPS, PSU,
PVDF, PVF.
For sealing and gluing (use sparingly): High filled epoxy for
electronic packaging (e.g. glob top, underfill), and Silicone.
Out-gassing of these materials may also contaminate the
SHT1x (see Section 1.3). Therefore try to add the sensor
as a last manufacturing step to the assembly, store the
assembly well ventilated after manufacturing or bake at
>50C for 24h to outgas contaminants before packing."

Check out Futurlec for parts. They don't carry the SHT's, but they have a HH10D there which is a pretty cheap unit fo experimenting with. You'll need a 555 timer with that one too. The data sheet seems reasonably detailed with an example cicuit diagram.

http://www.futurlec.com/Temperature_Sensors.shtml

I've bought stuff from them before and they shipped pretty quickly to Australia.

Hope this is of use to you Smiley
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blacknight72
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« Reply #8 on: January 09, 2010, 02:18:28 14:18 »

Thanks you very much!

I've bought a SHT75 and waiting for delivering. Hope it's will be okay.
After testing it, I will try another devices and make a comparison.
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gregger2k
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« Reply #9 on: January 09, 2010, 05:29:11 17:29 »

Maybe this module will work for you  http://www.sureelectronics.net/goods.php?id=904
It comes with a PIC micro and outputs temp and humidity as analog or serial data.
Greg
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blacknight72
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« Reply #10 on: January 10, 2010, 12:13:14 00:13 »

Quote
Maybe this module will work for you  http://www.sureelectronics.net/goods.php?id=904
It comes with a PIC micro and outputs temp and humidity as analog or serial data.
Greg

I will consider it when I need a low-cost & low accuracy application. In fact, I prefer using LM35 or DS1820 for temp & above device (introduced by others) for humi. We can save a PIC16F690, then^^

Thanks and welcome to Sonsivri Smiley
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sherm
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« Reply #11 on: February 18, 2010, 01:39:44 13:39 »

How did you go with the SHT75? What are your thoughts on it? Did it yield the results you needed for your project, or have you had to try something else since then?
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blacknight72
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« Reply #12 on: February 19, 2010, 02:26:46 02:26 »

I've been having the Tet holiday (Lunar New Year) in my hometown for 2 weeks. I'll be back to work next week. I will post the result to Sonsivri, then.

Thanks for your mention.
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majimbu
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« Reply #13 on: April 09, 2010, 10:09:53 22:09 »

Hello, i have both applications recently and i used LM35 temperature sensor and H23K5A resistive humidity sensor, and it works fine, very cheap price also...
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rentau
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« Reply #14 on: June 18, 2010, 08:30:27 08:30 »

Hello, i have both applications recently and i used LM35 temperature sensor and H23K5A resistive humidity sensor, and it works fine, very cheap price also...

hi majimbu, about H23K5A, can you tell me the price? and base on its table, it has very few data, which i think not that accurate? also do you have some application note on how to drive it? a driving circuit?


thanks
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majimbu
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« Reply #15 on: June 22, 2010, 08:04:42 20:04 »

hello rentau,

here is the site where i bought the resistive humidity sensor, http://e-gizmo.com/PRODUCT/ELECTRO/SENSOR.HTM
about $4.13, but later in there post they call this sensor CM-R, not H23K5A, but i think the mistake is mine, its look alike. Grin
i easily use it as like using LDR, i series a 0.1uf ceramic capacitor to one end of its lead frame to ground and the other direct
to mcu (i used PIC16F877A). i really don't know how accurate it is because its not required on my later work so i only test it
a couple of times (by directly blowing into the sensor) thru digital tester and sample program i made, and works fine for me
its good if you have an accurate instrument to test the actual humidity of weather to base its accuracy. i hope this help...

regards  
 
« Last Edit: June 22, 2010, 11:07:17 23:07 by majimbu » Logged
rentau
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« Reply #16 on: June 25, 2010, 02:15:42 02:15 »

i series a 0.1uf ceramic capacitor to one end of its lead frame to ground and the other direct
to mcu (i used PIC16F877A).

uhm, sorry but not that clear to me

a series of .1uF from ground? or from other PIC pin? and other on ADC.,

 i was thinking to protect sensor from DC, a capacitor needed from a pin that would generate 1 VAC at 1kHz, and then feed to ADC.  and from there data sheet, a thermistor connected to ground for some temperature drift.


thanks
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