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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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.

anonymous Air Flow Detector Thursday, April 10, 2008 5:30:16 AM

i think it will not work properly

anonymous Air Flow Detector Wednesday, March 12, 2008 7:44:29 PM

Can some one tell me about R4?... is it a varible resistor or a fixed resistor??.... I have built this circuit many of times but have not been able to get it to work..... Can anyone give me some feedback...thanks!!

vimal raj d Air Flow Detector Saturday, March 01, 2008 12:15:10 PM

sir i am going to take this as my project of present semster can i have some more details about this???? plz plz plz

khairul Air Flow Detector Wednesday, February 20, 2008 6:29:22 PM

hi there. nice circuit. i have some problems regarding the Incandescent lamp. i would like to implement this circuit into the pspice program but i do have problem to place the incandescent lamp. may i know, if there is a part can replace the criteria of the incandescent lamp in pspice? thank you

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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.
anonymous	Air Flow Detector	Thursday, April 10, 2008 5:30:16 AM
i think it will not work properly
anonymous	Air Flow Detector	Wednesday, March 12, 2008 7:44:29 PM
Can some one tell me about R4?... is it a varible resistor or a fixed resistor??.... I have built this circuit many of times but have not been able to get it to work..... Can anyone give me some feedback...thanks!!
vimal raj d	Air Flow Detector	Saturday, March 01, 2008 12:15:10 PM
sir i am going to take this as my project of present semster can i have some more details about this???? plz plz plz
khairul	Air Flow Detector	Wednesday, February 20, 2008 6:29:22 PM
hi there. nice circuit. i have some problems regarding the Incandescent lamp. i would like to implement this circuit into the pspice program but i do have problem to place the incandescent lamp. may i know, if there is a part can replace the criteria of the incandescent lamp in pspice? thank you