Air Flow Detector

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. 

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.             

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.

ref: aaroncake.net/Circuits/airflow.asp