Air Flow Detector

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This simple circuit uses an incandescent lamp to detect airflow. With the filament exposed to air, a constant current source is used to slightly heat the filament. As it is heated, the resistance increases. As air flows over the filament it cools down, thus lowering it's resistance. A comparator is used to detect this difference and light an LED. With a few changes, the circuit can be connected to a meter or ADC to provide an estimation on the amount of air flow.

Schematic

Parts

Part	Total Qty.	Description	Substitutions
R1	1	100 Ohm 1/4W Resistor
R2	1	470 Ohm 1/4W Resistor
R3	1	10k 1/4W Resistor
R4	1	100K 1/4W Resistor
R5	1	1K 1/4W Resistor
C1	1	47uF Electrolytic Capacitor
U1	1	78L05 Voltage Regulator
U2	1	LM339 Op Amp
L1	1	#47 Incandescent lamp with glass removed (See "Notes")
D1	1	LED
MISC	1	Board, Wire, Sockets for ICs, etc.

Notes

The glass will have to be removed from L1 without breaking the filament. Wrap the glass in masking tape and it in a vise. Slowly crank down until the glass breaks, then remove the bulb and carefully peel back the tape. If the filament has broken, you will need another lamp.

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Comments

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Fergus Air Flow Detector Sunday, December 27, 2009 3:03:40 AM

To John - have you tried adjusting the value of R5?

anonymous Air Flow Detector Thursday, December 03, 2009 12:55:51 PM

I have never seen a schematic with a symbol like that of the arrow coming out of the 5th pin of the LM339, it points directly into the the center of Resistor 4. Does anyone know what this translates to? Thanks.
(Editor's notes: It's a pot.)

John Air Flow Detector Wednesday, November 18, 2009 3:34:46 AM

This is a simple but useful circuit, thanks for making it freely available. That said I expected the circuit to function in reverse, instead of the LED dimming when airflow is detected I assumed it would brighten. Any suggestions on how to modify the circuit so that the LED brightens with air movement? I tried reversing the input signal to the comparator and it does work but the output barely lights the LED.

anonymous Air Flow Detector Thursday, October 01, 2009 8:33:09 PM

Can i use this circuit to measure speed??

ajinkya Air Flow Detector Wednesday, September 16, 2009 9:46:07 AM

how can be the meter joined and which meter and does it need to be calibarated

ajm Air Flow Detector Friday, April 24, 2009 11:25:52 PM

R1 and R2 are parallel conected?

alrex Air Flow Detector Friday, November 21, 2008 1:23:31 AM

sir, i would like toknow if this circuit can be used to detect the flow of hydrogen gas through a process of electrolysis. i am plannig to used this circuit in my project: Hydrogen torch

RAKESH Air Flow Detector Sunday, November 16, 2008 1:15:06 AM

instead of that incandacent bulb which can be used please reply... the circuit is nice in use....

anonymous Air Flow Detector Tuesday, October 07, 2008 11:46:27 PM

This is an idea of the Hot-wire Anemometer.You've to heat L1,when air flow pass L1,it's cool down and it's resistance is changed along it's temperature. R4 is used to adjust the reference voltage,you've to adjust it to the highest velocity or at the position that make the LED just blind or use lower resistance,I think value not over 1k may be used.

anonymous Air Flow Detector Wednesday, August 27, 2008 10:39:00 PM

How about substituting a tiny positive temperature coefficient (PTC) thermistor in place of L1? That would be a lot safer than breaking light bulbs, and would achive the same result. Many electronic components dealers offer them very cheap too.

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Fergus	Air Flow Detector	Sunday, December 27, 2009 3:03:40 AM
To John - have you tried adjusting the value of R5?
anonymous	Air Flow Detector	Thursday, December 03, 2009 12:55:51 PM
I have never seen a schematic with a symbol like that of the arrow coming out of the 5th pin of the LM339, it points directly into the the center of Resistor 4. Does anyone know what this translates to? Thanks. (Editor's notes: It's a pot.)
John	Air Flow Detector	Wednesday, November 18, 2009 3:34:46 AM
This is a simple but useful circuit, thanks for making it freely available. That said I expected the circuit to function in reverse, instead of the LED dimming when airflow is detected I assumed it would brighten. Any suggestions on how to modify the circuit so that the LED brightens with air movement? I tried reversing the input signal to the comparator and it does work but the output barely lights the LED.
anonymous	Air Flow Detector	Thursday, October 01, 2009 8:33:09 PM
Can i use this circuit to measure speed??
ajinkya	Air Flow Detector	Wednesday, September 16, 2009 9:46:07 AM
how can be the meter joined and which meter and does it need to be calibarated
ajm	Air Flow Detector	Friday, April 24, 2009 11:25:52 PM
R1 and R2 are parallel conected?
alrex	Air Flow Detector	Friday, November 21, 2008 1:23:31 AM
sir, i would like toknow if this circuit can be used to detect the flow of hydrogen gas through a process of electrolysis. i am plannig to used this circuit in my project: Hydrogen torch
RAKESH	Air Flow Detector	Sunday, November 16, 2008 1:15:06 AM
instead of that incandacent bulb which can be used please reply... the circuit is nice in use....
anonymous	Air Flow Detector	Tuesday, October 07, 2008 11:46:27 PM
This is an idea of the Hot-wire Anemometer.You've to heat L1,when air flow pass L1,it's cool down and it's resistance is changed along it's temperature. R4 is used to adjust the reference voltage,you've to adjust it to the highest velocity or at the position that make the LED just blind or use lower resistance,I think value not over 1k may be used.
anonymous	Air Flow Detector	Wednesday, August 27, 2008 10:39:00 PM
How about substituting a tiny positive temperature coefficient (PTC) thermistor in place of L1? That would be a lot safer than breaking light bulbs, and would achive the same result. Many electronic components dealers offer them very cheap too.