We found this circuit in the magazine Eletcronics Now of July 1994. The power supply is according to the relay and the transistor can be any general purpose one.
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...