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.
The TDA2613 is the mono version of the TDA1521A and has pinouts compatible with an integrated...
This circuit from an old 1978 publication uses germanium transistors but can be implemented with...
This project shows how to use an SCR in a starter circuit for a common fluorescent lamp. The SCR...