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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") | |
| D1 | 1 | LED | |
| 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.