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 for air flow detector


Total Qty.
R11100 Ohm 1/4W Resistor
R21470 Ohm 1/4W Resistor
R3110k 1/4W Resistor
R41100K 1/4W Resistor
R511K 1/4W Resistor
C1147uF Electrolytic Capacitor
U1178L05 Voltage Regulator
U21LM339 Op Amp
L11#47 Incandescent lamp with glass removed (See "Notes")
MISC1Board, Wire, Sockets for ICs, etc.


  1. 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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Air Flow Detector
Saturday, October 26, 2013 9:22:42 AM
lm 339 is connected to R1 through potentiometer? ??? plzzzzz reply
Air Flow Detector
Friday, February 17, 2012 9:13:30 AM
sir ,i want to know witch sensor used in a spirometre who detecte the unspiratory and the expiratory air and please give the the circuit
Air Flow Detector
Thursday, September 09, 2010 12:42:23 PM
Quite simple to implement but I'll try it out before I can comment. But then, I'll like to say.. nice job sir.
Thursday, May 13, 2010 6:17:44 AM
Hiz is this R4 resistor a variable resistor also instead of the bulb can i use a variable resistor, if yes what should be its resistance?? also my circuit is not working properly when i ground the pin 12 of the U2
Air Flow Detector
Saturday, March 27, 2010 11:41:59 AM
Was wondering if anybody could help me out, by assisting me with adding the ADC and a 7-segment counter to the circuit.
Air Flow Detector
Sunday, December 27, 2009 3:03:40 AM
To John - have you tried adjusting the value of R5?
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.)

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.
Air Flow Detector
Thursday, October 01, 2009 8:33:09 PM
Can i use this circuit to measure speed??
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
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