This circuit was found in American documentation from the 1980s. The integrated circuit used is a voltage comparator of the time. Modern equivalents can be tested.
In the original 1992 application, this passive circuit was intended for use in the car. At that...
This 1995 circuit uses an audio amplifier that is no longer common today. However, other...
This circuit is from an 80's documentation. The integrated circuit is from the time and the...