Air Model Finder

This simple circuit produces a loud trilling noise, if no valid transmitter signal is received. This frequency must correspond with the resonant frequency of the piezo element, otherwise the sound level will be very low.
Air Model Finder - schematic

When you switch off the transmitter, the IC1, pin 5 provides a 2-KHz signal and pin 7 goes "high". If you use a buzzer with internal electronics producing the signal, R2 must be connected to pin 7 instead of pin 6.

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