This circuit is suggested by General Electric in 1966 documentation. The Triac must match the engine. The resistors depend on the triac's sensitivity to firing.
This simple metal detector was found in a 1960 Sylvania transistor manual. The phone must be high...
This is one of the many versions of this type of indicator that the reader can find on our...
Found in an English publication from the 1980s, this circuit can be built with equivalent...