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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.
|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")|
|MISC||1||Board, 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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