This circuit can give a visual indication
of the rate of air flow.It can be also used
to check whether there is air flow in a
The filament of a incandescent bulb is
the sensing part of the circuit.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 variations
in the resistance will produce variation
of voltage across the filament.These
variations will be picked up by the
opamp (LM339) and the brightness of the
LED at its output will be varied
proportionally to the airflow.
Circuit diagram with Parts list.
1-The filament L1 can be made by
removing the glass of a 40W
2-The circuit can be powered from a 12 V
DC power supply.