arduino thermal anemometer

Forum 2005-2010 (read only) Hardware Interfacing
1

Hello everyone,

I'm new to the forum and electronics so apologies for any faux pas I might make! I've been trying to use my arduino as a thermal anemometer based on the excellent and tiny IST FS5 thermal mass flow sensor; there is more information and a datasheet here - uk.farnell DOT com/ist/fs5-0-1l-195/sensor-flow-gas/dp/1778049. I've set up the circuit as recommended in the data sheet, albeit using a different op-amp and transistor (and excluding the calibration resistor) and it seems to work nicely. The only problem is the output is between 5 and 10V, and obviously the arduino likes something in the 0 - 5V range. I've used a simple voltage divider to scale the voltage between 2.5 and 5V, but it would be nice to use the full range... unfortunately my electronics knowledge is far too limited for this! I've read some similar forum posts and tried to use a 5V voltage regulator and another opamp to produce something useful but failed miserably!

I'd really appreciate it if anyone could give me some hints on how to use an op-amp to achieve this. I feel like it also must be possible to change the wheatstone bridge arrangment so as to produce this voltage range in the first place, but haven't a clue how!

Thanks all,

Horatio.

p.s. There is a schematic and a photo of the setup here, was not allowed too embed them as images as it's my first go!

dl.dropbox DOT com/u/7549930/aaSchematic1.png
dl.dropbox DOT com/u/7549930/arduino_anemometer.JPG

2

I would use an op-amp to offset the voltage. You could do this by using a difference amplifier:

If you have a potentiometer I would replace R2/R3 with it; that way you can more easily control the offset. As shown, it is set for about 3.6V offset (30K/100K*12V)

The one cavet with this circuit is that since you are only using one supply, you can't get the output to go all the way down to 0 volts.

I just picked 10K via rectal extraction so you will want to look at signal loading, 100K is probably better.

3

Thanks Chris,

The difference amplifier works perfectly. If anyone is interested the final schematic I used is here : http://dl.dropbox.com/u/7549930/aaSchematic2-diffAmp.png. I ended up using a 7805 as the 5V reference and a LT1635 rail-to-rail op amp as recommended by the folk at chiphacker (thanks!). I'll stick the calibration stuff on the playground in the next few weeks should anyone be interested.

Horatio.

4

Horatio,

Great work!
I'm planning doing almost the same and curious about your results.
Any success on getting it work as a wind anemometer in real situation yet?

/Toca