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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kashyap Air Flow Detector Saturday, October 26, 2013 9:22:42 AM

lm 339 is connected to R1 through potentiometer? ??? plzzzzz reply

vanissa 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

Samweld 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.

haris working 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

anonymous 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.

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

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kashyap	Air Flow Detector	Saturday, October 26, 2013 9:22:42 AM
lm 339 is connected to R1 through potentiometer? ??? plzzzzz reply
vanissa	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
Samweld	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.
haris	working	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
anonymous	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.
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