This circuit was found in the September 1992 issue of Radio Electronics magazine. The circuit does not use a transistor to drive the relay and has no adjustments.
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...