HS1101 capacitive humidity sensor; need nearly linear voltage to feed to a 1-wire ADC - Electronic Circuits Projects Diagrams Free

FirefighterBlu3 · 19th October 2007, 05:11 PM

As I have inferred and is in my profile, I'm really weak at analog

I have an HS1101 capacitive humidity sensor and I need to eventually get a digital readout from it. What I'd like to do is this:

HS1101 -> [x] -> 1-wire ADC -> distance -> 16F877A

My void exists in [x], conversion of the capacitance value to an analog voltage. I've seen a small variety of complex circuitry for doing this but I want to make the remote component count as small as possible. I am entirely ambivalent as to how I get the analog reference into the 1-wire ADC, but the 1-wire ADC -> ... is a necessary design requirement. Low power is also desired and the rate of sampling isn't crucial, about once per second is plenty fine.

I have seen multiple 555 examples fed directly into a microcontroller for counting but as just stated, this design won't suffice. I have also seen the HS1101 attached to op-amps but this output is far from linear.

It doesn't even have to be precisely linear as long as the LSB remains at least 1% within 0% to 100% accurate. So as long as I can accomplish this then I can do a table lookup for a somewhat linear slope. Of course I would prefer this 1% accuracy fit inside 8 or 16 bits from the ADC.

In summary:

1% accuracy from 0% to 100% range
small component count
low power usage
about once per second sample rate
8 or 16 bit (or in between) data output from ADC
HS1101 -> [x] -> 1-wire ADC

So, ideas?

Any sort of approach that meets these ideas should be great. For conceptual reference, this is for the http://blue-labs.org/BlueCar/ project and I will have both internal and external humidity sensors. I'll also have a similar but far more simple design for household.

-david

blueroomelectronics · 19th October 2007, 08:18 PM

The factory calibrated Humirel sensors are generally only accurate to +/-2%
A HS1101 is behaves like a very small capactior, it varies between 150pf to 210pf or so depending on humidity. The 555 solution is about the best for low parts count and accuracy as it's got sensor compensation in the design.
Humirel make voltage output versions HTG353XCH but they're probably not cheap and are +/-3% and analog voltage is not the best over any sort of distance.

Note: The 16F877A has a 10 bit DAC

What's wrong with the TLC555? Current consumption is tiny with the HS1101, I've used it in my RS485 thermostat design with a PIC

FirefighterBlu3 · 19th October 2007, 08:20 PM

A 555 doesn't talk well with others on a 1-wire network. The datasheet I have indicates 0.1% RH accuracy per deg Celsius.

blueroomelectronics · 19th October 2007, 08:24 PM

Quote:

Originally Posted by FirefighterBlu3

A 555 doesn't talk well with others on a 1-wire network.

And analog does?

If you're using a 1wire network then simply buy a 1-wire humididty sensor.
http://www.maxim-ic.com/products/ibu.../humsensor.pdf

FirefighterBlu3 · 19th October 2007, 10:54 PM

I didn't say it did. In my first post I included a 1-wire ADC in the design. I previously read the application note you point to and that document uses the Honeywell HIH-3605 which is obsolete. Places that do have it charge a pretty penny and some. I have already considered the use of a device like the battery monitor and it is one of my options. I still need to get a nearly linear voltage output interpreted from a capacitive device which I already have in my hand. I purposely chose a capacitive device because I wanted to learn a little more about analog design. That said, I don't intend to spend a few months learning a lot about analog theory just to implement this design. At present I simply need to know enough to understand how to accomplish this. Another idea popped into my head earlier today; using a tiny mcu with an A/D input and a digital I/O pin. Using a variety of components to measure the charge voltage in a given amount of time. I'm not horribly concerned if the accuracy shifts slightly by a few percent based on temperature. I am disinterested in it shifting several percent. I may need to use a paired charge conversion with a standard ~180pF capacitor with a similar drift to measure the shift. That's all beside the point however. I need a functional implementation before I worry about precision.

blueroomelectronics · 19th October 2007, 11:06 PM

I too would have been happy to find a simple voltage solution for the HS1101 but nothing worked as well as the TLC555 solution for either cost or simplicity.
I have DS1923 humidity / temperature sensor / logger which is a 1 wire (lots of PIC code out there) that will work with 1 wire sorry it's not analog though.

Search these forums for Cricket (my thermostat with humidity) for more info, I finally used the 555 solution but an analog temperature sensor LM34 with a precision VREF (TL431)

http://www.maxim-ic.com/products/ibu...ons.cfm#sensor

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19th October 2007, 05:11 PM	(permalink)
FirefighterBlu3	HS1101 capacitive humidity sensor; need nearly linear voltage to feed to a 1-wire ADC As I have inferred and is in my profile, I'm really weak at analog I have an HS1101 capacitive humidity sensor and I need to eventually get a digital readout from it. What I'd like to do is this: HS1101 -> [x] -> 1-wire ADC -> distance -> 16F877A My void exists in [x], conversion of the capacitance value to an analog voltage. I've seen a small variety of complex circuitry for doing this but I want to make the remote component count as small as possible. I am entirely ambivalent as to how I get the analog reference into the 1-wire ADC, but the 1-wire ADC -> ... is a necessary design requirement. Low power is also desired and the rate of sampling isn't crucial, about once per second is plenty fine. I have seen multiple 555 examples fed directly into a microcontroller for counting but as just stated, this design won't suffice. I have also seen the HS1101 attached to op-amps but this output is far from linear. It doesn't even have to be precisely linear as long as the LSB remains at least 1% within 0% to 100% accurate. So as long as I can accomplish this then I can do a table lookup for a somewhat linear slope. Of course I would prefer this 1% accuracy fit inside 8 or 16 bits from the ADC. In summary: 1% accuracy from 0% to 100% range small component count low power usage about once per second sample rate 8 or 16 bit (or in between) data output from ADC HS1101 -> [x] -> 1-wire ADC So, ideas? Any sort of approach that meets these ideas should be great. For conceptual reference, this is for the http://blue-labs.org/BlueCar/ project and I will have both internal and external humidity sensors. I'll also have a similar but far more simple design for household. -david __________________ Gay/geek/firefighter/mechanic/gardner/<insert many things>. Linux software developer/user - Kernel, Apache, PostgreSQL, Sendmail, etc. mostly into digital toys, my analog clue is quite lacking http://blue-labs.org/ - http://southmeriden-vfd.org/

19th October 2007, 08:18 PM	(permalink)
blueroomelectronics	The factory calibrated Humirel sensors are generally only accurate to +/-2% A HS1101 is behaves like a very small capactior, it varies between 150pf to 210pf or so depending on humidity. The 555 solution is about the best for low parts count and accuracy as it's got sensor compensation in the design. Humirel make voltage output versions HTG353XCH but they're probably not cheap and are +/-3% and analog voltage is not the best over any sort of distance. Note: The 16F877A has a 10 bit DAC What's wrong with the TLC555? Current consumption is tiny with the HS1101, I've used it in my RS485 thermostat design with a PIC __________________ Bill Smart Kits build Smart People http://www.blueroomelectronics.com Last edited by blueroomelectronics; 19th October 2007 at 08:20 PM.

19th October 2007, 08:20 PM	(permalink)
FirefighterBlu3	A 555 doesn't talk well with others on a 1-wire network. The datasheet I have indicates 0.1% RH accuracy per deg Celsius. __________________ Gay/geek/firefighter/mechanic/gardner/<insert many things>. Linux software developer/user - Kernel, Apache, PostgreSQL, Sendmail, etc. mostly into digital toys, my analog clue is quite lacking http://blue-labs.org/ - http://southmeriden-vfd.org/ Last edited by FirefighterBlu3; 19th October 2007 at 08:24 PM.

19th October 2007, 10:54 PM	(permalink)
FirefighterBlu3	I didn't say it did. In my first post I included a 1-wire ADC in the design. I previously read the application note you point to and that document uses the Honeywell HIH-3605 which is obsolete. Places that do have it charge a pretty penny and some. I have already considered the use of a device like the battery monitor and it is one of my options. I still need to get a nearly linear voltage output interpreted from a capacitive device which I already have in my hand. I purposely chose a capacitive device because I wanted to learn a little more about analog design. That said, I don't intend to spend a few months learning a lot about analog theory just to implement this design. At present I simply need to know enough to understand how to accomplish this. Another idea popped into my head earlier today; using a tiny mcu with an A/D input and a digital I/O pin. Using a variety of components to measure the charge voltage in a given amount of time. I'm not horribly concerned if the accuracy shifts slightly by a few percent based on temperature. I am disinterested in it shifting several percent. I may need to use a paired charge conversion with a standard ~180pF capacitor with a similar drift to measure the shift. That's all beside the point however. I need a functional implementation before I worry about precision. __________________ Gay/geek/firefighter/mechanic/gardner/<insert many things>. Linux software developer/user - Kernel, Apache, PostgreSQL, Sendmail, etc. mostly into digital toys, my analog clue is quite lacking http://blue-labs.org/ - http://southmeriden-vfd.org/

19th October 2007, 11:06 PM	(permalink)
blueroomelectronics	I too would have been happy to find a simple voltage solution for the HS1101 but nothing worked as well as the TLC555 solution for either cost or simplicity. I have DS1923 humidity / temperature sensor / logger which is a 1 wire (lots of PIC code out there) that will work with 1 wire sorry it's not analog though. Search these forums for Cricket (my thermostat with humidity) for more info, I finally used the 555 solution but an analog temperature sensor LM34 with a precision VREF (TL431) http://www.maxim-ic.com/products/ibu...ons.cfm#sensor __________________ Bill Smart Kits build Smart People http://www.blueroomelectronics.com