Selecting a humidity and temperature sensor

[ombadei]

humidity sensor ic

Hi,

Currently, i am doing a data acquisition project where we need to measure humidity electronically.

Our choice is to use the below capacitative humidity sensor.
**broken link removed**

My question is: Practically, how do i convert it to read a voltage value for the ADC so that the controller can process the data?

 

[Siniša]

humidity sensor digital ic

There are various methods of measuring capacitance that differ a lot in perfomanse and complexity. Most of them are not suited for ADC and is easier to have capacitance to time conversion than to voltage.
You could set pin as output to charge capacitor through resistor for fixed time and then read voltage, but because non linear curve it would be difficult to implement and it wont be acurate.
Just buy capacitance to digital converter IC and use micro to collect and iterpret data.
Other option is build integrator with opamp where your sensor is integrating capacitor and feed input with pulse of known length, then output will be analog value proportional to capacitance.
There are many more methods... so google is your friend. Be aware that any method of conversion that involves longer times would be succeptible to leakage currents, and there are many pitfalls in design of such sircuit. CDC IC is easiest solution.

 

[Kripton2035]

how to make a digital humidity sensor

this one is more expensive, but you have NO more circuit to add to your microcontroller as it is fully digital.
this can be far easier.

https://www.farnell.com/datasheets/103625.pdf

 

[ombadei]

Re: Humidity Sensor

Thanks.. Very much appreciate on your insights... the CDC sounds like a good choice..

Btw, do you have any insights on temperature sensing? We are veering towards enviromental temperal range of 0 to 40++ degrees Celcius with a 0.1deg resolution.

My current design is a thermister to be linearised with a resistor and fed into an instrumentation amplifier. And an ADC connected to the micro..

Added after 5 minutes:

this one is more expensive, but you have NO more circuit to add to your microcontroller as it is fully digital.
this can be far easier.

https://www.farnell.com/datasheets/103625.pdf

Yea that's the sensiron one.. I came across that before but it will overshoot my project budget.. thanks..

 

[Siniša]

Re: Humidity Sensor

You can select maybe I²C humidity/temperature sensor from Sensirion? Even less headache, all digital, both measurements in single sensor...

 

[kender]

Re: Humidity Sensor

My question is: Practically, how do i convert it to read a voltage value for the ADC so that the controller can process the data?

Here's one cheap way of connecting the capacitive sensor to a microcontroler (uC). Build an RC oscillator with an inverter gate, where the sensor is the C and R is a fixed resistor. As the sensed parameter changes, so will the capacitance and the frequency of the oscillator. Then you can connect the square wave from the oscillator to a frequency-to-voltage converter (FVC) and then to the ADC. Alternatively, you can connect the square wave directly to the uC and measure the frequency directly.

 

[ombadei]

Re: Humidity Sensor

I am currently evaluating the Qt300, CDC.

Was ploughing through the datasheets...Is the cap (Cs) meant for the variable cap? supposing that i am using it as the humidity cap sensor. And does anyone know wat (Cx) is meant for?

Maybe the main question is how should i connect my capacitative sensor cap to the CDC?

Added after 6 minutes:

Build an RC oscillator with an inverter gate, where the sensor is the C and R is a fixed resistor. As the sensed parameter changes, so will the capacitance and the frequency of the oscillator. Then you can connect the square wave from the oscillator to a frequency-to-voltage converter (FVC) and then to the ADC. Alternatively, you can connect the square wave directly to the uC and measure the frequency directly.

How can i achieve this with the RC oscillator? I am using an FPGA for this project. So, my only resources are digital outputs.

Pardon me, my circuit understanding is rather horrendous.

 

[Kripton2035]

Humidity Sensor

well .. if you're starting an fpga project without knowing how to make an rc oscillator well well...you're not arrived at the end of your project !!!!!

 

[Siniša]

Re: Humidity Sensor

Pardon me, my circuit understanding is rather horrendous.

Yea that's the sensiron one.. I came across that before but it will overshoot my project budget.. thanks..

You should think about your time budget as well. I would agree with you about your circuit understanding, that is why I would recommend you only as much initegrated solution as possible. Least ammount of chance to screw up.

Was ploughing through the datasheets...Is the cap (Cs) meant for the variable cap? supposing that i am using it as the humidity cap sensor. And does anyone know wat (Cx) is meant for?

You were not ploughing enough. It is all explained in data sheet you mentioned. Again, even with this relativelly integrated solution you have problems. This IC is not best suited for sensor you want to use. There are ways it could be used, but I beleive it is beyond your capabilities for now.

How can i achieve this with the RC oscillator? I am using an FPGA for this project. So, my only resources are digital outputs.

I never heard of FPGA with only outputs. You should first learn how to crawl before you try to fly. If you don't have idea how to make RC oscillator, you are in big trouble. Listen to my advise, fork out money for integrated sensor solution, get I²C IP and program your FPGA if you know how.

Another advise for you is try to find evaluation kit that would have sensors built in so you do only programming task.

Good luck.

 

[ombadei]

Re: Humidity Sensor

You were not ploughing enough. It is all explained in data sheet you mentioned. Again, even with this relativelly integrated solution you have problems. This IC is not best suited for sensor you want to use. There are ways it could be used, but I beleive it is beyond your capabilities for now.

Well, i am a student so that's why i will have problems. But i am trying to learn something from here. If i may ask, what is the reason that it is not suited? I know that Cs is fixed and Cx is the variable cap(the humidity sensor is in pico-range). Shouldn't it be fine?

I never heard of FPGA with only outputs. You should first learn how to crawl before you try to fly. If you don't have idea how to make RC oscillator, you are in big trouble. Listen to my advise, fork out money for integrated sensor solution, get I²C IP and program your FPGA if you know how.

On the FPGA's o/p, perhaps i shouldn't mention 'only resources', what i meant was on the idea of controlling something. Of course, there are i/p where i can time operations when triggered.

Thanks very much on your opinions.

 

[Siniša]

Re: Humidity Sensor

Well you found out what Cs and Cx is by reading. You could find out why this IC is not suitable for this sensor by further reading of both data sheets.
If you figure out how to go arround this obstacle, there might be hope for you yet.

 

[ombadei]

Re: Humidity Sensor

Well you found out what Cs and Cx is by reading. You could find out why this IC is not suitable for this sensor by further reading of both data sheets.
If you figure out how to go arround this obsticle, there might be hope for you yet.

Hmm.. It is baffling me about whether this is feasible: Somewhere along the line of putting a fixed cap value in series with the variable cap to 'step down' to a equivalent Cx? However, i was doing the maths.. and i realise it 'compounds' the range. And moreover, the graph in the CDC datasheet doesn't depict the range of Cx of 48pF and above, eventhough spec says that its range is 0-100pF.

Do you have any ideas on the work around solution? Is it the digital value that corresponds?

At this stage, i can only hope..hehe

Added after 5 minutes:

Might sound like a silly question, what does it mean by resolution per count?

 

[kender]

Re: Humidity Sensor

Might sound like a silly question, what does it mean by resolution per count?

"Resolution per count" is voltage per one count of an A/D. Here's an example. Consider an A/D, which has 8 bits and measures voltages between 0 and +3.3V. Its digital output value ranges from 0 to 255, i.e. it has 255 counts. Its resolution per count is therefore 3.3/255 = 13 mV/count .

P.S. What school do you go to?

 

[ombadei]

Re: Humidity Sensor

"Resolution per count" is voltage per one count of an A/D. Here's an example. Consider an A/D, which has 8 bits and measures voltages between 0 and +3.3V. Its digital output value ranges from 0 to 255, i.e. it has 255 counts. Its resolution per count is therefore 3.3/255 = 13 mV/count .

So, it is the equivalent of Δvolt/bit.. Thanks for the elaboration. Trying to get familiar to so many terminology.. especially after scanning thru many datasheets.

P.S. What school do you go to?

Uni in Sydney...

Added after 10 minutes:

Currently, i am considering the monostable multivibrator. Is there any unknown practicalities i should be wary of?

 

[kender]

Re: Humidity Sensor

So, it is the equivalent of Δvolt/bit.

Probably not. I've never heard anyone use Δvolt/bit. I doubt that they are equivalent. bit ≠ count , because when you add one bit, you double the number of counts. I would suggest using Δvolt/count to avoid any confusion.