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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.


Schematic for air flow detector


Total Qty.
R11100 Ohm 1/4W Resistor
R21470 Ohm 1/4W Resistor
R3110k 1/4W Resistor
R41100K 1/4W Resistor
R511K 1/4W Resistor
C1147uF Electrolytic Capacitor
U1178L05 Voltage Regulator
U21LM339 Op Amp
L11#47 Incandescent lamp with glass removed (See "Notes")
MISC1Board, Wire, Sockets for ICs, etc.


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.

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