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Air Flow Detector circuit Schematic with explanation

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 its 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.

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.