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 circuit allows the use of headphones and microphones with diverse characteristics in audio...
We found this circuit in the 1994 Linear Applications Handbook from National Semiconductor. The...
This circuit was found in Radio Electronics from the 90s. The transistor is for general use. The...