This circuit diagram is used to give a visual indication of the rate of air flow. It used a filament L1 for the sensor, the filament L1 can be made by removing the glass of a 40W incandescent bulb. When there is no air flow, the resistance of the filament will be low. When there is a air flow, the resistance will be drop because the moving air will remove some of the heat generated in that filament. This variation of resistance will produce variation of voltage across the filament. These variations will be picked up by the opamp LM339 and the LED brightness as its output will be varied proportionally to the air flow. R4 POT is used as voltage reference in opamp, set its value to set the sensitivity of the circuit.
source : www.circuitstoday.com