Electronic Circuits and Projects Forum

Need a schematic of your setup.

the concept you're using is similar to the differential heating anemometer concept in this article:
http://electronicdesign.com/Roadbloc...nd-measurement

The circuit:
http://archive.electronicdesign.com/.../figure_01.gif

However, thermal sensors are quite lousy in detecting airflow (in my opinion), since it detects ambient temperature. When wind blows on a thermal sensor, the temperature goes down, which looks promising, but this also means the ambient temperature has reduced.

This is why my LM35 doesn't shoot back into the nominal room temperature when I shut the fan off. It goes back down only after.... like eons. I remedied this condition using some ideas from the article provided.

That circuit's lousy, by the way, I've tried it, and another guy from the 'net also attested to it. Just an idea.

Originally Posted by steaky

Hi,
However, the output has a very small swing of 0.1V - when measured through my 10bit ADC is only 30points. Given that the line could be noisy this isnt acceptable.
I tried amplifying the signal through a non-inverting op-amp but I need to reduce my input voltage by around 1.6V before I can amplify it - any thoughts?

If audioguru's here, he'd bash at you using a 741. It's an extremely lousy IC.
What is it that you're trying to amplify? The 0.1V output? A single stage differential amplifier is enough to obtain the desired output. See the datasheet of LM358, they're better op-amps, not the best though.

Audioguru!!! Where are you?!!
steaky, I'm no expert but I have built my own anemometer, and trust me, airflow sensors using thermal properties is very tricky. Primarily, what is the maximum change in the thermistor from still air to a full fan blast? That is VERY important. Even a designated temperature sensor LM35 doesn't detect airflow properly, I've tested it feverishly.

Thermistors are very slow to measure airflow.

Here is a solution published by Steve Woodward, university of North Carolina. It uses transistors as sensing elements. It's output is linearized and digitized.

Boncuk

Originally Posted by Vizier87

Audioguru!!! Where are you?!!
steaky, I'm no expert but I have built my own anemometer, and trust me, airflow sensors using thermal properties is very tricky. Primarily, what is the maximum change in the thermistor from still air to a full fan blast? That is VERY important. Even a designated temperature sensor LM35 doesn't detect airflow properly, I've tested it feverishly.

Hi Vizier87,

you might also want to take a look at the circuit designed by Steve Woodward. It heats one transistor for a constant temperature difference of both to be 50deg/C. So the airspeed doesn't really matter. Measuring airflow with a single LM35 doesn't take ambient temperature into account, and therefor can't be precise.

I wish I had an anemometer of his design when crawling through air channels and pipes measuring airspeed. The hot wire anemometers just need a little kick to become unusable, and replacement and calibration was awefully expensive.

Two transistors, mounted side by side don't take much more space than the hot wire pen, but they certainly can stand a lot more mechanical stress.

I also designed a single sided PCB layout for the anemometer. There are four wire jumps - necessary only for a 100% ground fill. The sensor board is the small one on the right side.

Boncuk

I did build it myself (from the article I provided as the reply, the design which is from Steven Woodward in ElectronicDesign.com http://electronicdesign.com/Roadbloc...nd-measurement), and I have to say I'm not very impressed with the outcome.

How about yours, Hans?