Humidity sensors...

C

cactusface

Senior Member
Hi Folks,
The 20M2 greenhouse data logger is coming along nicely, I hope to record 2 temps (in & out), light. If I get really cleaver even wind speed, etc.
While I was looking at the Bitsbox website for a few bits I spotted this Humidity sensor and @ only 80p got a couple, a bit different to others that cost about £12+
But I'm not too sure how they work?? It seems there is a change in resistance with RH and TEMP, that's OK. But then it also mentions a change in FREQ and an operating voltage of 1V max.

Or have I got it all wrong, perhaps someone can tell me how THEIR humidity sensor works.

Data sheet attached.

Any help welcome, Thanks.
Mel.
 

Attachments

Buzby

Buzby

Senior Member
The spec mentions 'impedance', not 'resistance', so it behaves like a coil or capacitor.

It has a 'resistance' dependent on frequency, i.e. an 'impedance'.

Maybe put the device as the top leg of a voltage divider and feed it at the top through a capacitor from PWM ?

The electronics wizards on this forum will tell you how to drive it, I only do digits :)
 
Buzby

Buzby

Senior Member
The device used in that app is three wire 5v powered, cactusface is using a two wire passive device.
 
Buzby

Buzby

Senior Member
eclectic said:
This is a start:
http://www.picaxeforum.co.uk/showthread.php?11174-Humidity-sensor-and-AC-output&highlight=Humidity+capaci%2A

These work:
http://www.techsupplies.co.uk/epages/Store.sf/en_GB/?ObjectPath=/Shops/Store.TechSupplies/Products/SEN008

e
Click to expand...
Doh !. Why do I always go looking elsewhere, when the answer is usually in the forum somewhere :rolleyes:

( Your second item, from techsupplies, is again a 3-wire powered device. Pay attention boy, as Captain Mainwaring would say. :p )
 
J

Jeremy Harris

Senior Member
cactusface said:
Hi Folks,
The 20M2 greenhouse data logger is coming along nicely, I hope to record 2 temps (in & out), light. If I get really cleaver even wind speed, etc.
While I was looking at the Bitsbox website for a few bits I spotted this Humidity sensor and @ only 80p got a couple, a bit different to others that cost about £12+
But I'm not too sure how they work?? It seems there is a change in resistance with RH and TEMP, that's OK. But then it also mentions a change in FREQ and an operating voltage of 1V max.

Or have I got it all wrong, perhaps someone can tell me how THEIR humidity sensor works.

Data sheet attached.

Any help welcome, Thanks.
Mel.
Click to expand...
The other links are useful, but I had a play with these earlier this year, attracted by their low price. They need to be driven with AC, or at least a 1:1 ratio square wave, as they don't tolerate DC. You then need to devise a way of measuring the change of impedance with both humidity and temperature, then performing a calibration on each sensor, as they have a wide tolerance. After playing around for a while I gave up on these, as they were originally designed as crude dew point detectors for use in things like video recorders, where condensation on the drum could mess up tapes. As such they didn't need to be accurate.

The cheapest way of reliably measuring humidity to a reasonable degree of accuracy seems to be to use the ones I used in my environmental logger: http://www.picaxeforum.co.uk/showthread.php?20562-DIY-humidity-and-temperature-sensor
 
E

eclectic

Moderator
@Buzby.
I am paying attention. :)
AFAIK, there's no working code for the cheapo jobs.
(I hope I'm proven wrong :)

The Honeywell model works reliably with hippy's code from
the link I added to post #6

e
 
M

mrburnette

Senior Member
... may I suggest simply building a 555 osc. around the device? Then use the PICAXE to monitor the frequency which will change when the RH changes...
From the table: Impedance table -- %RH VS. Temperature which has units Ohms

A breadboard, a 555, some resistors (or better, a resistor decade box) and a few surplus caps should give you some insight. Once you get the 555 frequency output to vary appropriately, it's PICAXE thereafter.
You can start from one of these vantage points:
electronic-circuits-diagrams
555-variable-frequency-square-wave-generator

Of course, you could also use a PICAXE to create a uC representation of the 555 and keep everything in the PICAXE family, but I suspect that someone else will offer that up with details.

- Ray
 
M

mrburnette

Senior Member
Buzby said:
Here is a 555 circuit for a Honeywell capacitive humidity sensor. It may give you some tips.

http://www.digikey.com/Web Export/Supplier Content/HoneywellSC_480/PDF/HCH_draft3 Rev 10 21 08.pdf?redirected=1
Click to expand...
Insight to building a discrete instrumentation amp... yep. But, also a headache ... the circuit is for a commercial grade design.

An opinion:
In a hobby/amateur design, know what performance and accuracy is required to satisfy the project at hand. Is it 30%, 20%, 10%, 5% or 1%. YES... in many cases 30% is good-e-nuff. Think about it this way, many electronic hobbyists utilize 20% parts in the design of their projects, so the mean-average is +/- 10% as the norm.

Personally, I usually go for 10% since my bench parts are usually 5% but I'm known to only stock standard parts values in the bin. So, I don't go doing series and parallel capacitors or resistors just to get a perfect value... I deviate and calibrate/correct later. It's a hobby, not heart surgery.

So... approach every project from the obtainable... if you can get a free $20 instrumentation amplifier ... for goodness sake, use it! But if you have to make do with the parts at hand... I suggest trying to keep it simple and focus more on the principles of theory implementation rather than microscopic accuracy. You will learn more, be less frustrated, have more working-complete projects, and live longer... kind of a Zen thing.

Whatever you do folks... if you do not understand the basics of what you are trying to accomplish... take an hour and go to Wikipedia and review the basic, underlying principles that govern what you wish to accomplish. What do you really need: temperature and humidity? temperature and relative humidity? temperature and due point? Are you aware of the underlying math? can you manage the math in the PICAXE limited integer environment. Each question and each answer improves your knowledge and moves you one point closer to completing the check-list for successfully building your project. And the next project will be even more fun because you will know more and seek deeper answers to your questions. The quest never ends... every answer begets a subsequent question.


- Ray
 
J

Jeremy Harris

Senior Member
mrburnette said:
... may I suggest simply building a 555 osc. around the device? Then use the PICAXE to monitor the frequency which will change when the RH changes...
From the table: Impedance table -- %RH VS. Temperature which has units Ohms

A breadboard, a 555, some resistors (or better, a resistor decade box) and a few surplus caps should give you some insight. Once you get the 555 frequency output to vary appropriately, it's PICAXE thereafter.
You can start from one of these vantage points:
electronic-circuits-diagrams
555-variable-frequency-square-wave-generator

Of course, you could also use a PICAXE to create a uC representation of the 555 and keep everything in the PICAXE family, but I suspect that someone else will offer that up with details.

- Ray
Click to expand...
I already posted a link to a thread where I did just this with a cheap purely capacitive sensor, earlier in this thread. Here is the direct link to the circuit: http://www.picaxeforum.co.uk/attachment.php?attachmentid=10479&d=1329590870 and here is the direct link to the original code (which can be improved): http://www.picaxeforum.co.uk/attachment.php?attachmentid=10480&d=1329590871

I doubt there's a cheaper way to make a reasonably good 5 V humidity sensor. The raw sensors I used were about £2, the cmos 555 is a few pence, as are the small handful of passive components. The end result has been working well for a few months now, I have around 5 of these sensors scattered around the place feeding a central logging system.

No matter how I played with the impedance varying sensors I couldn't get a good, repeatable and reasonably accurate result.
 
M

mrburnette

Senior Member
@Jeremy,

Yes sir, you did. I even read your post
The cheapest way of reliably measuring humidity to a reasonable degree of accuracy seems to be to use the ones I used in my environmental logger: http://www.picaxeforum.co.uk/showthr...erature-sensor
Click to expand...
and did not realize that you were implementing an outboard 555 VFO approach which is a dead-giveaway that I did not follow-the-link to conclusion. My apologies for doubling-up on your already detailed 555 usage.

Repeatability is the key to any outboard effort in my opinion since accuracy can be programmed into logic. If it is not repeatable, it certainly is not good. I have no information or experimental data to fault your conclusion.

- Ray
 
C

cactusface

Senior Member
Hi All,
Thanks for your many replies, suggestions and links. Well I now know why these things only cost 80p, there's a lot of work and time needed to get it to do anything! The data-sheet say's a sine wave but I think, I'll try PWM at 1KHz thro an R & C. I think I'll just try it and see? If it works? I'll get back to you and let you know, but don't let that stop you.....
Regards
Mel.
 
G

Goeytex

Senior Member
This device can be used easily with just about any Picaxe. Adjust the pause in the code to produce a 50% duty square wave at ~ 1Khz.

Schematic and code below.

Note: Extra resistors may need to be added with some low voltage "resistive type" sensors. I think this one is rated at 1V RMS max so design accordingly.

Humidity readings will need to be temperature compensated.
 

Attachments

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C

cactusface

Senior Member
Hi,
Again many thanks to all who came up with good ideas and advice! I have just spotted this on the web. http://dx.com/p/diy-hr-202-humidity-detection-sensor-module-140725 So I ordered a couple and a few other bargains? they seem to have for the experimenter. If I recall right I ordered a couple of tiny cars from them (18 months ago) that had very tiny motors which is what I really needed, most of it I'm sure comes from china.

The actual sensor on this looks just like the 80p job I stated with. For all you shield fans they do quite a range and cheap!!

Thanks
Mel.
 
W

westaust55

Moderator
The HopeRF HH10D humidity sensor as available from Futurlec uses a CMOS 555 as the basis of the circuit. The only other chips is an EEPROM which holds some calibration constants to provide the calibrationa nd linearity for scaling the incoming signal to RH.

The above link to the datasheet provides the HopeRF schematic.
This forum link provides some maths based upon the calibraiton constants: http://www.picaxeforum.co.uk/showthread.php?11749-HOPE-RF-HH10D-Humidity-Sensor

Edit: futurlec link fixed
 
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