Help with thermal conductivity sensor

Other Hardware Sensors
1

Firstly I think I better point out that I have pretty much zero experience with electronics so I might be trying to run before I can walk with this

I want to hook up the following thermal conductivity sensor to my Arduino:

http://sgx.cdistore.com/datasheets/e2v/VQ5000_2013.pdf

I've cobbled together the following theoretical circuit based on various datasheets but I have no idea if it will actually work.

Right click on the image and choose "View Image" to see the diagram full size.

Any comments or help would be greatly appreciated

Components:

2

You miss the zero calibration, but your circuit might just work.
Be sure u hook up the ICs correctly

3

Thanks Knut... That’s great.

I think I just wanted a second opinion that there wasn’t something fundamentally wrong with it before attempting it for real.

One thing I’ve just realized though is that both of the IC’s are surface mount which is going make bread boarding it a bit of a pain.

Again thanks for taking the time to look at it. This all new to me.

*** edit ***

Just found a similiar ADC to the ADS1232 but in a DIP pacakge:
http://www.mouser.com/ds/2/609/AD7730_7730L-278980.pdf

*** edit ***

4

Google an HX711 module. The chip is basically the same as the ADS1232 but already on a board ... and it's inexpensive. Arduino libraries are available as well.

For your resistors you'll want .1% precision and metal film; you want them to be accurate and stable with temperature.

5

Hi Chagrin...

I was originally looking at the HX711 Sparkfun board but the excitation voltage was 5v i think and not the 4.2v that datasheet said the sensor needed. I was toying with the idea of trying to replace the two resistors on the vfb pin but i wasn’t sure what to replace them with (if I could get them off at all). It would definitely be easier / cheaper if i could use the Sparkfun board.

Thanks for the resistor tip.

6

You can power the HX711 module with 4.2V. Pedantically you would use a level converter between the HX711 and your 5V Arduino but I highly doubt that would cause any issues.

7

Hi Chagrin..

OK maybe there is something I’m missing here. I thought the purpose of the voltage regulator was to maintain a fixed voltage regardless of the input? so if I try and under drive the board I would have expected the regulator to bump the voltage back up again.

I think my problem is that I don’t understand how the values of r1 and r2 regulate the output voltage. I see that one side connects to the reference bypass (1.25v), the other side connects to the actual line that is being regulated and the midpoint feeds back into the regulator.

Does that mean that I can get any voltage between 0 and 1.25? (obviously this is wrong as i read somewhere that the excitation voltage was more than this) or does connecting to the line under regulation do something funky?

The manual says VAVDD = VBG * (R1+R2) / R1. Maybe I should stop trying to understand how it works and just accept the formula at face value substituting in appropriate values to get 4.2v

4.2v = 1.25 * ((1 + 2.36) / 1)

confused :slight_smile:

8

To be honest I never read the datasheet to that level of detail. I used it for a load cell application, I got good results ... then I moved on with my life. I just assumed it was using a 5V excitation voltage (same as VCC).

I have one of the eBay modules and testing it right now, VCC of 5V, I see (with a multimeter) the excitation voltage at 4.12V. And yes this module has R1 and R2 at 20K and 8.2K just like the Sparkfun board, and this was tested with a load cell attached. So comparing that to the VAVDD formula which comes out to 1.7625V for 20K and 8.2K resistors I really have no idea what's going on.