This sensitive circuit was found in the August 1971 issue of Radio Electronics magazine. The SCR is equivalent, and the transistor may be a BC548. The load may be a relay.
This circuit, found in a 1988 documentation consisting of an audio switch that turns off the load...
This stabilized source can supply currents up to 200 mA with the indicated transistor or 500 mA...
With the shadow over the sensor, it is possible to interrupt the sound of your amplifier using this...