This circuit uses a General Electric coupler from the time of the publication in the 1980s. Equivalent SCR can be used.
Suggested by Texas Instruments in 1980s documentation, this circuit uses a component of the era....
The idea of this simple circuit is to use a tape recorder to key a transmitter in code emission....
This relay circuit with a dual channel MOSFET (not very common today) was found in an old American...