We found this circuit in the September 1971 Radio Electronics magazine. The SCR can be any of the 106 series and the buzzer can be replaced by a relay according to the supply voltage.
Found in an American publication, this circuit applies to almost any operational amplifier. The power...
This circuit was found in a 1980s instrumentation publication and can be applied to virtually any...
Found in an American 80's documentation, this circuit can use common FETs like the BF245 and cover...