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air flow monitor using thermal sensing

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naz83

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Hi

Could anyone please help me with the circuit design of an air flow monitor using thermal sensing. The sensor could be anything but i was thinkin of using two thermistors one at ambient temp and another thermistor heated and it cools when air flows. A voltage output from the electronics can be converted to digital form and displayed using an AVR

could anyone please help with the electronics design!

Thanks
 
this is a popular design using T0-92 Transistors by Steven Woodward:

**broken link removed**
I've only seen two people attesting to it's functionality, one by **broken link removed** (works good), and Liljencrants (he said the design has pretty poor quality). I built two of those but none worked, and I'd like to know if it can work for you.

Lijencrants also built several similar ones using NTC thermistors and externally heated diodes, so you can browse through his report for more info.
 
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You have to make something like this:
Air flow sensor using measurement of rate of heat loss - Patent 6763711

Its clearly a mass ariflow sensor you want to make, right?
As I remember, in a MAF the thermistor in the bridge, then from the bridge voltage you have to make a regulation of the current feed of the bridge. It it cools, the higher amps compensate the thermistor voltage change - so, the current will be proportional of the air flow.
With this and the temperature you can calculate the air flow mass, as it used in the cars.

Ohh, in the bridge there is not an NTC, its a heated wire type. I think you can use any wire (just set right the current). The simplest way to get a MAF sensor from used car parts (i dont know the word right now :).
 
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Hi

Could anyone please help me with the circuit design of an air flow monitor using thermal sensing. The sensor could be anything but i was thinkin of using two thermistors one at ambient temp and another thermistor heated and it cools when air flows. A voltage output from the electronics can be converted to digital form and displayed using an AVR

could anyone please help with the electronics design!

Thanks

hi,
The main problem with using two thermistors for accurate 'air flow' measurement is that the ambient temperature is not constant.
The reference thermistor is usually held at constant temperature above the local ambient maximum.

If you just want a switching on/off 'air flowing or not sensor', the thermistor should be ok, depending upon the local ambient range.
 
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Hi Eric, Have you constructed Steve Woodward's? It'll be good to get more feedback from people building the circuit. :)

hi Viz,
You are reading my mind.!
I have just simulated it on LTspice and I cannot get it to function as claimed, perhaps there is an error in the original posted circuit.??
The output is either 0v or 3.5V, depending upon the ZERO pot setting.
 
Hi all thanks for the suggestions...

i am thinking of building the circuit on prototype boards using actual ccomponents than using simulation.

could any1 suggest whether the transistor model or thermistor model would be a better option for the experiment?

also could you plz send me the proteus file of the design if any1 of u have it.
 
hi Viz,
You are reading my mind.!
I have just simulated it on LTspice and I cannot get it to function as claimed, perhaps there is an error in the original posted circuit.??
The output is either 0v or 3.5V, depending upon the ZERO pot setting.

Is that so? No wonder mine didn't work... But Chemelec (Gary) did attest to it, you can find the link in post #2. He said it worked fine. But I have to say, after building two of these I'm truly unimpressed. :eek:
 
I did experiment with pt1000 today.

I did use two bridges each having one pt1000 as a sensor and one of the resistors was 1.2k. as a result the pt1000 was heated to reach 1.2k.

Its like having two anemometers, one of them was subjected to air flow and the other was like a reference. and the outputs from these two was given to a differential amplifier to amplify the difference between the two. I did get it working, when no air flows the diff output is 0.13v and on air flow depending on the amount of airflow it increased to 3.0v.

could u suggest what needs to be done further to get it to a uc and the AVR part plz....
 
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