This configuration was found in a publication for radio amateurs in 1979. The two integrated circuits are still common today and the speaker can be replaced by a piezoelectric transducer, without the need for a 470 ohms resistor.

This configuration was found in a publication for radio amateurs in 1979. The two integrated circuits are still common today and the speaker can be replaced by a piezoelectric transducer, without the need for a 470 ohms resistor.