Thursday, September 17, 2009

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.


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