This circuit was obtained in an Electronics Now from the 90s. The circuit uses operational amplifiers from the time and the indicator is of the Sonalert type with internal oscillator.
This circuit is from a 1976 Motorola CMOS manual. It can be implemented with modern Darlingtons like...
In the figure we have the way of modulating or controlling the frequency of a relaxation...
The purpose of this circuit is to monitor the operation of an alternator in cars with a 12 V...