This circuit has visual and audible status indication using an oscillator. The circuit is from a Radio Electronics magazine from June 1991.
This circuit causes an LED to flash if the monitored voltage exceeds the value set in the pot. The...
Found in an Electronics World of 1966 this circuit can be made with current power transistors. It causes...
Found in a 1970 documentation, this SCR bistable can be implemented with modern equivalent SCRs. Observe...