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.

Parts

• 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

• 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.