When the voltage supplying the circuit falls below the zener value, the LED lights up, and if a low current buzzer is used, it is activated. The circuit is from a 1999 American publication.
This circuit was suggested by Popular Elecronics from the 90's. The circuit has high input...
Figure 1 shows a circuit than can be used as time stage in timers, and other applications....
This circuit is from a Popular Electronics from 1993. The magazine is no longer printed, but the...