This circuit was found in the July 1992 issue of Radio Electronics magazine. The circuit is based on a 741 that activates a buzzer with oscillator. R5 adjusts the sensitivity.
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...