The circuit is from a June 1995 Electronics Now. The relay is power-dependent and both R1 and R2 should have close resistances in operation.
This circuit, found in a 1994 documentation, uses 20 A 2N5448 triacs or equivalent and should be...
This traditional amplifier configuration is from the National Semiconductor's Linear Applications Handbook...
This circuit is from GE technical documentation from the 1970s. The circuit can be implemented with SCRs...