This circuit was obtained from a Radio Electronics issue from January 1989. The indicator is a ceramic transducer, and the frequency of the emitted sound is given by C3.
The LM1468L can also be used. The circuit is from Motorola's Linear Interface ICs and the...
We found this light-effect circuit with LEDs using the well-known 4017 in a 1987 documentation. All...
This circuit was found at Popular Electronics. The speaker is of the greater than 100-ohm...