This circuit is built using LM339 IC. The filament of an incandescent bulb is the sensing part of the circuit. The filament L1 can be made by removing the glass of a 40W incandescent bulb. The circuit can be powered from a 12 V DC power supply. Operation of the circuit is when there is no air flow the resistance of the filament will be low. When there is air flow the resistance drops, because the moving air will remove some of the heat generated in the filament. This variation in the resistance will produce variation of voltage across the filament. These variations will be picked up by the op amp (LM339) and the brightness of the LED at its output will be varied proportionally to the airflow.
