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:

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.